WindowOrWorkerGlobalScope mixinVarious mechanisms can cause author-provided executable code to run in the context of a document. These mechanisms include, but are probably not limited to:
script elements.javascript: URLs.addEventListener(), by explicit event handler content attributes, by
event handler IDL attributes, or otherwise.Scripting is enabled in a browsing context when all of the following conditions are true:
Scripting is disabled in a browsing context when any of the above conditions are false (i.e. when scripting is not enabled).
Scripting is enabled for a node if the node's node document has a browsing context, and scripting is enabled in that browsing context.
Scripting is disabled for a node if there is no such browsing context, or if scripting is disabled in that browsing context.
A script is one of two possible structs. All scripts have:
An environment settings object, containing various settings that are shared with other scripts in the same context.
Either a Script Record, for classic scripts; a Source Text Module Record, for module scripts; or null. In the former two cases, it represents a parsed script; null represents a failure parsing.
A JavaScript value, which has meaning only if the record is null, indicating that the corresponding script source text could not be parsed.
A JavaScript value representing an error that will prevent evaluation from succeeding. It will be re-thrown by any attempts to run the script.
Since this exception value is provided by the JavaScript specification, we know that it is never null, so we use null to signal that no error has occurred.
A base URL used for resolving module specifiers. This will either be the URL from which the script was obtained, for external scripts, or the document base URL of the containing document, for inline scripts.
A classic script is a type of script that has the following additional item:
A boolean which, if true, means that error information will not be provided for errors in this script. This is used to mute errors for cross-origin scripts, since that can leak private information.
A module script is another type of script. It has no additional items.
An environment is an object that identifies the settings of a current or potential execution environment. An environment has the following fields:
An opaque string that uniquely identifies the environment.
A URL record that represents the location of the resource with which the environment is associated.
In the case of an environment settings object, this URL might be
distinct from the environment settings object's responsible
document's URL, due to mechanisms such as
history.pushState().
Null or a target browsing context for a navigation request.
Null or a service worker that controls the environment.
A flag that indicates whether the environment setup is done. It is initially unset.
An environment settings object is an environment that additionally specifies algorithms for:
A JavaScript execution context shared by all scripts that use this settings object, i.e. all scripts in a given JavaScript realm. When we run a classic script or run a module script, this execution context becomes the top of the JavaScript execution context stack, on top of which another execution context specific to the script in question is pushed. (This setup ensures ParseScript and Source Text Module Record's Evaluate know which Realm to use.)
A module map that is used when importing JavaScript modules.
A browsing context that is assigned responsibility for actions taken by the scripts that use this environment settings object.
When a script creates and navigates a new
top-level browsing context, the opener attribute
of the new browsing context's Window object will be set to the
responsible browsing context's WindowProxy object.
An event loop that is used when it would not be immediately clear what event loop to use.
A Document that is assigned responsibility for actions taken by the scripts that
use this environment settings object.
For example, the URL of the
responsible document is used to set the URL of the Document after it has been reset
using document.open().
If the responsible event loop is not a browsing context event loop, then the environment settings object has no responsible document.
A character encoding used to encode URLs by APIs called by scripts that use this environment settings object.
A URL used by APIs called by scripts that use this environment settings object to parse URLs.
An origin used in security checks.
An HTTPS state value representing the security properties of the network channel used to deliver the resource with which the environment settings object is associated.
The default referrer policy for fetches performed using this environment settings object as a request client. [REFERRERPOLICY]
An environment settings object also has an outstanding rejected promises weak set and an about-to-be-notified rejected promises list, used to track unhandled promise rejections. The outstanding rejected promises weak set must not create strong references to any of its members, and implementations are free to limit its size, e.g. by removing old entries from it when new ones are added.
This section introduces a number of algorithms for fetching scripts, taking various necessary inputs and resulting in classic or module scripts.
Script fetch options is a struct with the following items:
The cryptographic nonce metadata used for the initial fetch and for fetching any imported modules
The integrity metadata used for the initial fetch
The parser metadata used for the initial fetch and for fetching any imported modules
The credentials mode used for the initial fetch (for module scripts) and for fetching any imported modules (for both module scripts and classic scripts)
Recall that via the import() feature, classic scripts can import module scripts.
The default classic script fetch options are a script fetch options
whose cryptographic nonce is the empty
string, integrity metadata is the
empty string, parser metadata is "not-parser-inserted", and credentials mode is "omit".
Given a request request and a script fetch options options, we define:
Set request's cryptographic nonce metadata to options's cryptographic nonce, its integrity metadata to options's integrity metadata, and its parser metadata to options's parser metadata.
Set request's cryptographic nonce metadata to options's cryptographic nonce, its integrity metadata to options's integrity metadata, its parser metadata to options's parser metadata, and its credentials mode to options's credentials mode.
For any given script fetch options options, the descendant script fetch options are a new script fetch options whose items all have the same values, except for the integrity metadata, which is instead the empty string.
The algorithms below can be customized by optionally supplying a custom perform the
fetch hook, which takes a request and an is
top-level flag. The algorithm must complete with a response (which may be a network error), either
synchronously (when using fetch a classic worker-imported script) or asynchronously
(otherwise). The is top-level flag will be set
for all classic script fetches, and for the initial fetch when fetching a module script graph or fetching a module worker script graph, but not for the fetches resulting from
import statements encountered throughout the graph.
By default, not supplying the perform the fetch will cause the below algorithms to simply fetch the given request, with algorithm-specific customizations to the request and validations of the resulting response.
To layer your own customizations on top of these algorithm-specific ones, supply a perform the fetch hook that modifies the given request, fetches it, and then performs specific validations of the resulting response (completing with a network error if the validations fail).
The hook can also be used to perform more subtle customizations, such as keeping a cache of responses and avoiding performing a fetch at all.
Service Workers is an example of a specification that runs these algorithms with its own options for the hook. [SW]
Now for the algorithms themselves.
To fetch a classic script given a url, a settings object, some options, a CORS setting, and a character encoding, run these steps. The algorithm will asynchronously complete with either null (on failure) or a new classic script (on success).
Let request be the result of creating a potential-CORS request given url, "script", and CORS setting.
Set request's client to settings object.
Set up the classic script request given request and options.
If the caller specified custom steps to perform the fetch, perform them on request, with the is top-level flag set. Return from this algorithm, and when the custom perform the fetch steps complete with response response, run the remaining steps.
Otherwise, fetch request. Return from this algorithm, and run the remaining steps as part of the fetch's process response for the response response.
response can be either CORS-same-origin or CORS-cross-origin. This only affects how error reporting happens.
Let response be response's unsafe response.
If response's type is "error", or response's status is not an ok status, asynchronously
complete this algorithm with null, and abort these steps.
If response's Content Type metadata, if any, specifies a character encoding, and the user agent supports that encoding, then set character encoding to that encoding (ignoring the passed-in value).
Let source text be the result of decoding response's body to Unicode, using character encoding as the fallback encoding.
The decode algorithm overrides character encoding if the file contains a BOM.
Let muted errors be true if response was CORS-cross-origin, and false otherwise.
Let script be the result of creating a classic script given source text, settings object, response's url, options, and muted errors.
To fetch a classic worker script given a url, a fetch client settings object, a destination, and a script settings object, run these steps. The algorithm will asynchronously complete with either null (on failure) or a new classic script (on success).
Let request be a new request whose url is url, client is fetch client settings object, destination is destination, mode is "same-origin", credentials mode is "same-origin", parser
metadata is "not parser-inserted", and whose
use-URL-credentials flag is set.
If the caller specified custom steps to perform the fetch, perform them on request, with the is top-level flag set. Return from this algorithm, and when the custom perform the fetch steps complete with response response, run the remaining steps.
Otherwise, fetch request. Return from this algorithm, and run the remaining steps as part of the fetch's process response for the response response.
Let response be response's unsafe response.
If response's type is "error", or response's status is not an ok status, asynchronously
complete this algorithm with null, and abort these steps.
Let source text be the result of UTF-8 decoding response's body.
Let script be the result of creating a classic script using source text, script settings object, response's url, and the default classic script fetch options.
To fetch a classic worker-imported script given a url and a settings object, run these steps. The algorithm will synchronously complete with a classic script on success, or throw an exception on failure.
Let request be a new request whose url is url, client is settings object, destination is "script", parser metadata is "not
parser-inserted", synchronous flag is set, and whose
use-URL-credentials flag is set.
If the caller specified custom steps to perform the fetch, perform them on request, with the is top-level flag set. Let response be the result.
Otherwise, fetch request, and let response be the result.
Unlike other algorithms in this section, the fetching process is synchronous here. Thus any perform the fetch steps will also finish their work synchronously.
Let response be response's unsafe response.
If response's type is "error", or response's status is not an ok status, throw a
"NetworkError" DOMException and abort these
steps.
Let source text be the result of UTF-8 decoding response's body.
Let muted errors be true if response was CORS-cross-origin, and false otherwise.
Let script be the result of creating a classic script given source text, settings object, response's url, the default classic script fetch options, and muted errors.
Return script.
To fetch a module script graph given a url, a settings object, a destination, and some options, run these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).
Let visited set be « url ».
Perform the internal module script graph fetching procedure given
url, settings object, destination, options,
settings object, visited set, "client", and with the
top-level module fetch flag set. If the caller of this algorithm specified custom perform the fetch steps, pass those along as
well.
When the internal module script graph fetching procedure asynchronously completes with result, asynchronously complete this algorithm with result.
To fetch a module worker script graph given a url, a fetch client settings object, a destination, a credentials mode, and a module map settings object, run these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).
Let visited set be « url ».
Let options be a script fetch options whose cryptographic nonce is the empty string, integrity metadata is the empty string,
parser metadata is "not-parser-inserted", and credentials mode is credentials
mode.
Perform the internal module script graph fetching procedure given
url, fetch client settings object, destination,
options, module map settings object, visited set, "client", and with the top-level module fetch flag set. If the caller
of this algorithm specified custom perform the
fetch steps, pass those along as well.
When the internal module script graph fetching procedure asynchronously completes with result, asynchronously complete this algorithm with result.
The following algorithms are meant for internal use by this specification only as part of fetching a module script graph or preparing a script, and should not be used directly by other specifications.
To perform the internal module script graph fetching procedure given a url, a fetch client settings object, a destination, a some options, a module map settings object, a visited set, a referrer, and a top-level module fetch flag, perform these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).
Assert: visited set contains url.
Fetch a single module script given url, fetch client settings object, destination, options, module map settings object, referrer, and the top-level module fetch flag. If the caller of this algorithm specified custom perform the fetch steps, pass those along while fetching a single module script.
Return from this algorithm, and run the following steps when fetching a single module script asynchronously completes with result:
If result is null, asynchronously complete this algorithm with null, and abort these steps.
If the top-level module fetch flag is set, fetch the descendants of and instantiate result given destination and visited set. Otherwise, fetch the descendants of result given the same arguments.
When the appropriate algorithm asynchronously completes with final result, asynchronously complete this algorithm with final result.
To fetch a single module script, given a url, a fetch client settings object, a destination, some options, a module map settings object, a referrer, and a top-level module fetch flag, run these steps. The algorithm will asynchronously complete with either null (on failure) or a module script (on success).
Let moduleMap be module map settings object's module map.
If moduleMap[url] is "fetching", wait
in parallel until that entry's value changes, then queue a task on
the networking task source to proceed with running the following steps.
If moduleMap[url] exists, asynchronously complete this algorithm with moduleMap[url], and abort these steps.
Set moduleMap[url] to "fetching".
Let request be a new request whose
url is url, destination is destination, mode is "cors", referrer is referrer, and client is fetch client settings
object.
Set up the module script request given request and options.
If the caller specified custom steps to perform the fetch, perform them on request, setting the is top-level flag if the top-level module fetch flag is set. Return from this algorithm, and when the custom perform the fetch steps complete with response response, run the remaining steps.
Otherwise, fetch request. Return from this algorithm, and run the remaining steps as part of the fetch's process response for the response response.
response is always CORS-same-origin.
If any of the following conditions are met, set moduleMap[url] to null, asynchronously complete this algorithm with null, and abort these steps:
response's type is "error"
The result of extracting a MIME type from response's header list (ignoring parameters) is not a JavaScript MIME type
For historical reasons, fetching a classic script does not include MIME type checking. In contrast, module scripts will fail to load if they are not of a correct MIME type.
Let source text be the result of UTF-8 decoding response's body.
Let module script be the result of creating a module script given source text, module map settings object, response's url, and options.
Set moduleMap[url] to module script, and asynchronously complete this algorithm with module script.
It is intentional that the module map is keyed by the request URL, whereas the base URL for the module script is set to the response URL. The former is used to deduplicate fetches, while the latter is used for URL resolution.
To fetch the descendants of a module script module script, given a destination and a visited set, run these steps. The algorithm will asynchronously complete with either null (on failure) or with module script (on success).
If module script's record is null, then asynchronously complete this algorithm with module script and abort these steps.
Let record be module script's record.
If record.[[RequestedModules]] is empty, asynchronously complete this algorithm with module script.
Let urls be a new empty list.
For each string requested of record.[[RequestedModules]],
Let url be the result of resolving a module specifier given module script and requested.
Assert: url is never failure, because resolving a module specifier must have been previously successful with these same two arguments.
If visited set does not contain url, then:
Let options be the descendant script fetch options for module script's fetch options.
For each url in urls, perform the internal module script graph fetching procedure given url, options, destination, module script's settings object, module script's settings object, visited set, module script's base URL, and with the top-level module fetch flag unset. If the caller of this algorithm specified custom perform the fetch steps, pass those along while performing the internal module script graph fetching procedure.
These invocations of the internal module script graph fetching procedure should be performed in parallel to each other.
If any of the invocations of the internal module script graph fetching procedure asynchronously complete with null, asynchronously complete this algorithm with null, aborting these steps.
Otherwise, wait until all of the internal module script graph fetching procedure invocations have asynchronously completed. Asynchronously complete this algorithm with module script.
To fetch the descendants of and instantiate a module script module script, given a destination and an optional visited set, run these steps. The algorithm will asynchronously complete with either null (on failure) or with module script (on success).
If visited set was not given, let it be an empty set.
Fetch the descendants of module script, given destination and visited set.
Return from this algorithm, and run the following steps when fetching the descendants of a module script asynchronously completes with result.
If result is null, then asynchronously complete this algorithm with result.
In this case, there was an error fetching one or more of the descendants. We will not attempt to instantiate.
Let parse error be the result of finding the first parse error given result.
If parse error is null, then:
Let record be result's record.
Perform record.Instantiate().
This step will recursively call Instantiate on all of the module's uninstantiated dependencies.
If this throws an exception, set result's error to rethrow to that exception.
Otherwise, set result's error to rethrow to parse error.
Asynchronously complete this algorithm with result.
To find the first parse error given a root moduleScript and an optional discoveredSet:
Let moduleMap be moduleScript's settings object's module map.
If discoveredSet was not given, let it be an empty set.
Append moduleScript to discoveredSet.
If moduleScript's record is null, then return moduleScript's parse error.
Let childSpecifiers be the value of moduleScript's record's [[RequestedModules]] internal slot.
Let childURLs be the list obtained by calling resolve a module specifier once for each item of childSpecifiers, given moduleScript and that item. (None of these will ever fail, as otherwise moduleScript would have been marked as itself having a parse error.)
Let childModules be the list obtained by getting each value in moduleMap whose key is given by an item of childURLs.
For each childModule of childModules:
Assert: childModule is a module script (i.e., it is not "fetching" or null); by now all module
scripts in the graph rooted at moduleScript will have successfully been
fetched.
Let childParseError be the result of finding the first parse error given childModule and discoveredSet.
If childParseError is not null, return childParseError.
Return null.
To create a classic script, given a JavaScript string source, an environment settings object settings, a URL baseURL, some script fetch options options, and an optional muted errors boolean:
If muted errors was not provided, let it be false.
If scripting is disabled for settings's responsible browsing context, then set source to the empty string.
Let script be a new classic script that this algorithm will subsequently initialize.
Set script's settings object to settings.
Set script's muted errors to muted errors.
Set script's parse error and error to rethrow to null.
Let result be ParseScript(source, settings's Realm, script).
Passing script as the last parameter here ensures result.[[HostDefined]] will be script.
If result is a list of errors, then:
Set script's parse error and its error to rethrow to result[0].
Return script.
Set script's record to result.
Set script's base URL to baseURL.
Set script's fetch options to options.
Return script.
To create a module script, given a JavaScript string source, an environment settings object settings, a URL baseURL, and some script fetch options options:
If scripting is disabled for settings's responsible browsing context, then set source to the empty string.
Let script be a new module script that this algorithm will subsequently initialize.
Set script's settings object to settings.
Set script's parse error and error to rethrow to null.
Let result be ParseModule(source, settings's Realm, script).
Passing script as the last parameter here ensures result.[[HostDefined]] will be script.
If result is a list of errors, then:
Set script's parse error to result[0].
Return script.
For each string requested of result.[[RequestedModules]]:
Let url be the result of resolving a module specifier given script and requested.
If url is failure, then:
Let error be a new TypeError exception.
Set script's parse error to error.
Return script.
This step is essentially validating all of the requested module specifiers. We treat a module with unresolvable module specifiers the same as one that cannot be parsed; in both cases, a syntactic issue makes it impossible to ever contemplate instantiating the module later.
Set script's record to result.
Set script's base URL to baseURL.
Set script's fetch options to options.
Return script.
To run a classic script given a classic script script and an optional rethrow errors boolean:
If rethrow errors is not given, let it be false.
Let settings be the settings object of script.
Check if we can run script with settings. If this returns "do not run" then return.
Prepare to run script given settings.
Let evaluationStatus be null.
If script's error to rethrow is not null, then set evaluationStatus to Completion { [[Type]]: throw, [[Value]]: script's error to rethrow, [[Target]]: empty }.
Otherwise, set evaluationStatus to ScriptEvaluation(script's record).
If ScriptEvaluation does not complete because the user agent has aborted the running script, leave evaluationStatus as null.
If evaluationStatus is an abrupt completion, then:
If rethrow errors is true and script's muted errors is false, then:
Clean up after running script with settings.
Rethrow evaluationStatus.[[Value]].
If rethrow errors is true and script's muted errors is true, then:
Clean up after running script with settings.
Throw a "NetworkError" DOMException.
Otherwise, rethrow errors is false. Perform the following steps:
Report the exception given by evaluationStatus.[[Value]] for script.
Clean up after running script with settings.
Return undefined.
Clean up after running script with settings.
If evaluationStatus is a normal completion, return evaluationStatus.[[Value]].
This value is only ever used by the javascript: URL steps.
If we've reached this point, evaluationStatus was left as null because the script was aborted prematurely during evaluation. Return undefined.
To run a module script given a module script script, with an optional rethrow errors boolean:
If rethrow errors is not given, let it be false.
Let settings be the settings object of script.
Check if we can run script with settings. If this returns "do not run" then return.
Prepare to run script given settings.
Let evaluationStatus be null.
If script's error to rethrow is not null, then set evaluationStatus to Completion { [[Type]]: throw, [[Value]]: script's error to rethrow, [[Target]]: empty }.
Otherwise:
Let record be script's record.
Set evaluationStatus to record.Evaluate().
This step will recursively evaluate all of the module's dependencies.
If Evaluate fails to complete as a result of the user agent
aborting the running script, then set
evaluationStatus to Completion { [[Type]]: throw, [[Value]]: a new
"QuotaExceededError" DOMException, [[Target]]: empty
}.
If evaluationStatus is an abrupt completion, then:
If rethrow errors is true, rethrow the exception given by evaluationStatus.[[Value]].
Otherwise, report the exception given by evaluationStatus.[[Value]] for script.
Clean up after running script with settings.
The steps to check if we can run script with an environment settings object settings are as follows. They return either "run" or "do not run".
If the global object specified by
settings is a Window object whose Document object is not
fully active, then return "do not run" and abort these steps.
If scripting is disabled for the responsible browsing context specified by settings, then return "do not run" and abort these steps.
Return "run".
The steps to prepare to run script with an environment settings object settings are as follows:
Push settings's realm execution context onto the JavaScript execution context stack; it is now the running JavaScript execution context.
The steps to clean up after running script with an environment settings object settings are as follows:
Assert: settings's realm execution context is the running JavaScript execution context.
Remove settings's realm execution context from the JavaScript execution context stack.
If the JavaScript execution context stack is now empty, perform a microtask checkpoint. (If this runs scripts, these algorithms will be invoked reentrantly.)
These algorithms are not invoked by one script directly calling another, but they can be invoked reentrantly in an indirect manner, e.g. if a script dispatches an event which has event listeners registered.
The running script is the script in the [[HostDefined]] field in the ScriptOrModule component of the running JavaScript execution context.
A global object is a JavaScript object that is the [[GlobalObject]] field of a JavaScript realm.
In this specification, all JavaScript
realms are initialized with global objects that are either Window or
WorkerGlobalScope objects.
There is always a 1-to-1-to-1 mapping between JavaScript realms, global objects, and environment settings objects:
A JavaScript realm has a [[HostDefined]] field, which contains the Realm's settings object.
A JavaScript realm has a [[GlobalObject]] field, which contains the Realm's global object.
Each global object in this specification is created during the initialization of a corresponding JavaScript realm, known as the global object's Realm.
Each global object in this specification is created alongside a corresponding environment settings object, known as its relevant settings object.
An environment settings object's realm execution context's Realm component is the environment settings object's Realm.
An environment settings object's Realm then has a [[GlobalObject]] field, which contains the environment settings object's global object.
When defining algorithm steps throughout this specification, it is often important to indicate what JavaScript realm is to be used—or, equivalently, what global object or environment settings object is to be used. In general, there are at least four possibilities:
Note how the entry, incumbent, and current concepts are usable without qualification, whereas the relevant concept must be applied to a particular platform object.
Consider the following pages, with a.html being loaded in a browser
window, b.html being loaded in an iframe as shown, and c.html and d.html omitted (they can simply be empty
documents):
<!-- a.html -->
<!DOCTYPE html>
<html lang="en">
<title>Entry page</title>
<iframe src="b.html"></iframe>
<button onclick="frames[0].hello()">Hello</button>
<!--b.html -->
<!DOCTYPE html>
<html lang="en">
<title>Incumbent page</title>
<iframe src="c.html" id="c"></iframe>
<iframe src="d.html" id="d"></iframe>
<script>
const c = document.querySelector("#c").contentWindow;
const d = document.querySelector("#d").contentWindow;
window.hello = () => {
c.print.call(d);
};
</script>
Each page has its own browsing context, and thus its own JavaScript realm, global object, and environment settings object.
When the print() method is called in response to pressing the
button in a.html, then:
The entry Realm is that of a.html.
The incumbent Realm is that of b.html.
The current Realm is that of c.html (since it is the print() method from
c.html whose code is running).
The relevant Realm of the object on which
the print() method is being called is that of d.html.
The incumbent and entry concepts should not be used by new specifications, as they are excessively complicated and unintuitive to work with. We are working to remove almost all existing uses from the platform: see issue #1430 for incumbent, and issue #1431 for entry.
In general, web platform specifications should use the relevant concept, applied to the object being operated
on (usually the this value of the current method). This mismatches the JavaScript
specification, where current is generally used as
the default (e.g. in determining the JavaScript realm whose Array constructor should be used to construct the result in Array.prototype.map). But this inconsistency is so embedded in the platform that
we have to accept it going forward.
Note that in constructors, where there is no this value yet, the current concept is the appropriate default.
One reason why the relevant concept is
generally a better default choice than the current concept is that it is more suitable for
creating an object that is to be persisted and returned multiple times. For example, the navigator.getBattery() method creates promises in the
relevant Realm for the Navigator object
on which it is invoked. This has the following impact: [BATTERY]
<!-- outer.html -->
<!DOCTYPE html>
<html lang="en">
<title>Relevant Realm demo: outer page</title>
<script>
function doTest() {
const promise = navigator.getBattery.call(frames[0].navigator);
console.log(promise instanceof Promise); // logs false
console.log(promise instanceof frames[0].Promise); // logs true
frames[0].hello();
}
</script>
<iframe src="inner.html" onload="doTest()"></iframe>
<!-- inner.html -->
<!DOCTYPE html>
<html lang="en">
<title>Relevant Realm demo: inner page</title>
<script>
function hello() {
const promise = navigator.getBattery();
console.log(promise instanceof Promise); // logs true
console.log(promise instanceof parent.Promise); // logs false
}
</script>
If the algorithm for the getBattery() method
had instead used the current Realm, all the results
would be reversed. That is, after the first call to getBattery() in outer.html, the
Navigator object in inner.html would be permanently storing
a Promise object created in outer.html's
JavaScript realm, and calls like that inside the hello()
function would thus return a promise from the "wrong" realm. Since this is undesirable, the
algorithm instead uses the relevant Realm, giving
the sensible results indicated in the comments above.
The rest of this section deals with formally defining the entry, incumbent, current, and relevant concepts.
The process of calling scripts will push or pop realm execution contexts onto the JavaScript execution context stack, interspersed with other execution contexts.
With this in hand, we define the entry execution context to be the most recently pushed item in the JavaScript execution context stack that is a realm execution context. The entry Realm is the entry execution context's Realm component.
Then, the entry settings object is the environment settings object of the entry Realm.
Similarly, the entry global object is the global object of the entry Realm.
All JavaScript execution contexts must contain, as part of their code evaluation state, a skip-when-determining-incumbent counter value, which is initially zero. In the process of preparing to run a callback and cleaning up after running a callback, this value will be incremented and decremented.
Every event loop has an associated backup incumbent settings object stack, initially empty. Roughly speaking, it is used to determine the incumbent settings object when no author code is on the stack, but author code is responsible for the current algorithm having been run in some way. The process of preparing to run a callback and cleaning up after running a callback manipulate this stack. [WEBIDL]
When Web IDL is used to invoke author code, or when EnqueueJob invokes a promise job, they use the following algorithms to track relevant data for determining the incumbent settings object:
To prepare to run a callback with an environment settings object settings:
Push settings onto the backup incumbent settings object stack.
Let context be the topmost script-having execution context.
If context is not null, increment context's skip-when-determining-incumbent counter.
To clean up after running a callback with an environment settings object settings:
Let context be the topmost script-having execution context.
This will be the same as the topmost script-having execution context inside the corresponding invocation of prepare to run a callback.
If context is not null, decrement context's skip-when-determining-incumbent counter.
Assert: the topmost entry of the backup incumbent settings object stack is settings.
Remove settings from the backup incumbent settings object stack.
Here, the topmost script-having execution context is the topmost entry of the JavaScript execution context stack that has a non-null ScriptOrModule component, or null if there is no such entry in the JavaScript execution context stack.
With all this in place, the incumbent settings object is determined as follows:
Let context be the topmost script-having execution context.
If context is null, or if context's skip-when-determining-incumbent counter is greater than zero, then:
Assert: the backup incumbent settings object stack is not empty.
This assert would fail if you try to obtain the incumbent settings object from inside an algorithm that was triggered neither by calling scripts nor by Web IDL invoking a callback. For example, it would trigger if you tried to obtain the incumbent settings object inside an algorithm that ran periodically as part of the event loop, with no involvement of author code. In such cases the incumbent concept cannot be used.
Return the topmost entry of the backup incumbent settings object stack.
Return context's Realm component's settings object.
Then, the incumbent Realm is the Realm of the incumbent settings object.
Similarly, the incumbent global object is the global object of the incumbent settings object.
The following series of examples is intended to make it clear how all of the different mechanisms contribute to the definition of the incumbent concept:
Consider the following very simple example:
<!DOCTYPE html> <iframe></iframe> <script> new frames[0].MessageChannel(); </script>
When the MessageChannel() constructor looks up the
incumbent settings object to use as the owner of the new MessagePort objects, the
topmost script-having execution context will be that corresponding to the
script element: it was pushed onto the JavaScript execution context
stack as part of ScriptEvaluation during the
run a classic script algorithm. Since there are no Web IDL callback invocations
involved, the context's skip-when-determining-incumbent counter is zero, so it is
used to determine the incumbent settings object; the result is the environment
settings object of window.
(In this example, the environment settings object of frames[0] is not involved at all. It is the current settings
object, but the MessageChannel() constructor
cares only about the incumbent, not current.)
Consider the following more complicated example:
<!DOCTYPE html> <iframe></iframe> <script> const bound = frames[0].postMessage.bind(frames[0], "some data", "*"); window.setTimeout(bound); </script>
There are two interesting environment settings
objects here: that of window, and that of frames[0]. Our concern is: what is the incumbent settings object at
the time that the algorithm for postMessage()
executes?
It should be that of window, to capture the intuitive notion that the
author script responsible for causing the algorithm to happen is executing in window, not frames[0]. Another way of capturing the
intuition here is that invoking algorithms asynchronously (in this case via setTimeout()) should not change the incumbent concept.
Let us now explain how the steps given above give us our intuitively-desired result of window's relevant settings object.
When bound is converted to a
Web IDL callback type, the incumbent settings object is that corresponding to window (in the same manner as in our simple example above). Web IDL stores this
as the resulting callback value's callback context.
When the task posted by setTimeout() executes, the algorithm for that task uses Web IDL to
invoke the stored callback value. Web IDL in
turn calls the above prepare to run a callback algorithm. This pushes the stored
callback context onto the backup incumbent settings object stack. At
this time (inside the timer task) there is no author code on the stack, so the topmost
script-having execution context is null, and nothing gets its
skip-when-determining-incumbent counter incremented.
Invoking the callback then calls bound, which in turn calls
the postMessage() method of frames[0]. When the postMessage()
algorithm looks up the incumbent settings object, there is still no author code on
the stack, since the bound function just directly calls the built-in method. So the
topmost script-having execution context will be null: the JavaScript execution
context stack only contains an execution context corresponding to postMessage(), with no ScriptEvaluation context or similar below it.
This is where we fall back to the backup incumbent settings object stack. As
noted above, it will contain as its topmost entry the relevant settings object of
window. So that is what is used as the incumbent settings
object while executing the postMessage()
algorithm.
Consider this final, even more convoluted example:
<!-- a.html -->
<!DOCTYPE html>
<button>click me</button>
<iframe></iframe>
<script>
const bound = frames[0].location.assign.bind(frames[0].location, "https://example.com/");
document.querySelector("button").addEventListener("click", bound);
</script>
<!-- b.html -->
<!DOCTYPE html>
<iframe src="a.html"></iframe>
<script>
const iframe = document.querySelector("iframe");
iframe.onload = function onLoad() {
iframe.contentWindow.document.querySelector("button").click();
};
</script>
Again there are two interesting environment
settings objects in play: that of a.html, and that of b.html. When the location.assign()
method triggers the Location-object navigate algorithm, what will be
the incumbent settings object? As before, it should intuitively be that of a.html: the click listener was originally
scheduled by a.html, so even if something involving b.html causes the listener to fire, the incumbent responsible is that of a.html.
The callback setup is similar to the previous example: when bound is
converted to a Web IDL callback type, the
incumbent settings object is that corresponding to a.html,
which is stored as the callback's callback context.
When the click() method is called inside b.html, it dispatches a click event on the button that is inside a.html. This time, when the prepare to run a callback algorithm
executes as part of event dispatch, there is author code on the stack; the topmost
script-having execution context is that of the onLoad function,
whose skip-when-determining-incumbent counter gets incremented. Additionally, a.html's environment settings object (stored as the
EventHandler's callback context) is pushed onto the
backup incumbent settings object stack.
Now, when the Location-object navigate algorithm looks up the
incumbent settings object, the topmost script-having execution
context is still that of the onLoad function (due to the fact we
are using a bound function as the callback). Its skip-when-determining-incumbent
counter value is one, however, so we fall back to the backup incumbent settings
object stack. This gives us the environment settings object of a.html, as expected.
Note that this means that even though it is the iframe inside a.html that navigates, it is a.html itself that is used
as the source browsing context, which determines among other things the request client. This is perhaps the only justifiable use
of the incumbent concept on the web platform; in all other cases the consequences of using it
are simply confusing and we hope to one day switch them to use current or relevant as appropriate.
The JavaScript specification defines the current Realm Record, sometimes abbreviated to the "current Realm". [JAVASCRIPT]
Then, the current settings object is the environment settings object of the current Realm Record.
Similarly, the current global object is the global object of the current Realm Record.
The relevant settings object for a platform object is defined as follows:
The relevant settings object for a non-global platform object o is the environment settings object whose global object is the global object of the global environment associated with o.
The "global environment associated with" concept is from the olden days, before the modern JavaScript specification and its concept of realms. We expect that as the Web IDL specification gets updated, every platform object will have a Realm associated with it, and this definition can be re-cast in those terms. [JAVASCRIPT] [WEBIDL]
Then, the relevant Realm for a platform object is the Realm of its relevant settings object.
Similarly, the relevant global object for a platform object is the global object of its relevant settings object.
Although the JavaScript specification does not account for this possibility, it's sometimes
necessary to abort a running script. This causes any ScriptEvaluation or Source Text Module Record
Evaluate invocations to cease immediately, emptying the
JavaScript execution context stack without triggering any of the normal mechanisms
like finally blocks. [JAVASCRIPT]
User agents may impose resource limitations on scripts, for example CPU quotas, memory limits,
total execution time limits, or bandwidth limitations. When a script exceeds a limit, the user
agent may either throw a "QuotaExceededError" DOMException,
abort the script without an exception, prompt the
user, or throttle script execution.
For example, the following script never terminates. A user agent could, after waiting for a few seconds, prompt the user to either terminate the script or let it continue.
<script>
while (true) { /* loop */ }
</script>
User agents are encouraged to allow users to disable scripting whenever the user is prompted
either by a script (e.g. using the window.alert() API) or because
of a script's actions (e.g. because it has exceeded a time limit).
If scripting is disabled while a script is executing, the script should be terminated immediately.
User agents may allow users to specifically disable scripts just for the purposes of closing a browsing context.
For example, the prompt mentioned in the example above could also offer the
user with a mechanism to just close the page entirely, without running any unload event handlers.
The JavaScript specification defines the JavaScript job and job queue abstractions in order to specify certain invariants about how promise operations execute with a clean JavaScript execution context stack and in a certain order. However, as of the time of this writing the definition of EnqueueJob in that specification is not sufficiently flexible to integrate with HTML as a host environment. [JAVASCRIPT]
This is not strictly true. It is in fact possible, by taking liberal advantage of the many "implementation defined" sections of the algorithm, to contort it to our purposes. However, the end result is a mass of messy indirection and workarounds that essentially bypasses the job queue infrastructure entirely, albeit in a way that is technically sanctioned within the bounds of implementation-defined behavior. We do not take this path, and instead introduce the following willful violation.
As such, user agents must instead use the following definition in place of that in the JavaScript specification. These ensure that the promise jobs enqueued by the JavaScript specification are properly integrated into the user agent's event loops.
The RunJobs abstract operation from the JavaScript specification must not be used by user agents.
When the JavaScript specification says to call the EnqueueJob abstract operation, the following algorithm must be used in place of JavaScript's EnqueueJob:
Assert: queueName is "PromiseJobs". ("ScriptJobs" must not be used by user agents.)
Let job settings be some appropriate environment settings object.
It is not yet clear how to specify the environment settings object that should be used here. In practice, this means that the entry concept is not correctly specified while executing a job. See discussion in issue #1189.
Let incumbent settings be the incumbent settings object.
Queue a microtask, on job settings's responsible event loop, to perform the following steps:
Check if we can run script with job settings. If this returns "do not run" then abort these steps.
Prepare to run script with job settings.
Prepare to run a callback with incumbent settings.
Let result be the result of performing the abstract operation specified by job, using the elements of arguments as its arguments.
Clean up after running a callback with incumbent settings.
Clean up after running script with job settings.
If result is an abrupt completion, report the exception given by result.[[Value]].
The JavaScript specification defines a syntax for modules, as well as some host-agnostic parts
of their processing model. This specification defines the rest of their processing model: how the
module system is bootstrapped, via the script element with type attribute set to "module", and how
modules are fetched, resolved, and executed. [JAVASCRIPT]
Although the JavaScript specification speaks in terms of "scripts" versus
"modules", in general this specification speaks in terms of classic
scripts versus module scripts, since both of them use
the script element.
A module map is a map of URL records to values that are either a module script, null (used to
represent failed fetches), or a placeholder value "fetching". Module maps are used to ensure that imported JavaScript modules are
only fetched, parsed, and evaluated once per Document or worker.
Since module maps are keyed by URL, the following code will create three separate entries in the module map, since it results in three different URLs:
import "https://example.com/module.js"; import "https://example.com/module.js#map-buster"; import "https://example.com/module.js?debug=true";
That is, URL queries and fragments can be varied to create distinct entries in the module map; they are not ignored. Thus, three separate fetches and three separate module evaluations will be performed.
In contrast, the following code would only create a single entry in the module map, since after applying the URL parser to these inputs, the resulting URL records are equal:
import "https://example.com/module2.js"; import "https:example.com/module2.js"; import "https://///example.com\\module2.js"; import "https://example.com/foo/../module2.js";
So in this second example, only one fetch and one module evaluation will occur.
Note that this behavior is the same as how shared workers are keyed by their parsed constructor url.
To resolve a module specifier given a script script and a JavaScript string specifier, perform the following steps. It will return either a URL record or failure.
Apply the URL parser to specifier. If the result is not failure, return the result.
If specifier does not start with the character U+002F SOLIDUS (/), the two-character sequence U+002E FULL STOP, U+002F SOLIDUS (./), or the three-character sequence U+002E FULL STOP, U+002E FULL STOP,
U+002F SOLIDUS (../), return failure and abort these steps.
This restriction is in place so that in the future we can allow custom module
loaders to give special meaning to "bare" import specifiers, like import "jquery" or import "web/crypto". For now any
such imports will fail, instead of being treated as relative URLs.
Return the result of applying the URL parser to specifier with script's base URL as the base URL.
The following are valid module specifiers according to the above algorithm:
https://example.com/apples.jshttp:example.com\pears.mjs (becomes http://example.com/pears.mjs as step 1 parses with no base URL)//example.com/bananas./strawberries.js.cgi../lychees/limes.jsxdata:text/javascript,export default 'grapes';blob:https://whatwg.org/d0360e2f-caee-469f-9a2f-87d5b0456f6fThe following are valid module specifiers according to the above algorithm, but will invariably cause failures when they are fetched:
javascript:export default 'artichokes';data:text/plain,export default 'kale';about:legumeswss://example.com/celeryThe following are not valid module specifiers according to the above algorithm:
https://eggplant:b/cpumpkins.js.tomato..zucchini.js.\yam.esJavaScript contains an implementation-defined HostResolveImportedModule abstract operation, very slightly updated by the import() proposal. User agents must use the following implementation: [JAVASCRIPT] [JSIMPORT]
Let referencing script be referencingScriptOrModule.[[HostDefined]].
Let moduleMap be referencing script's settings object's module map.
Let url be the result of resolving a module specifier given referencing script and specifier.
Assert: url is never failure, because resolving a module specifier must have been previously successful with these same two arguments.
Let resolved module script be moduleMap[url]. (This entry must exist for us to have gotten to this point.)
Assert: resolved module script is a module script (i.e., is not
null or "fetching").
Assert: resolved module script's record is not null.
Return resolved module script's record.
The import() proposal contains an implementation-defined HostImportModuleDynamically abstract operation. User agents must use the following implementation: [JSIMPORT]
Let referencing script be referencingScriptOrModule.[[HostDefined]].
Run the following steps in parallel:
Let url be the result of resolving a module specifier given referencing script and specifier.
If url is failure, then:
Let completion be Completion { [[Type]]: throw, [[Value]]: a new
TypeError, [[Target]]: empty }.
Perform FinishDynamicImport(referencingScriptOrModule, specifier, promiseCapability, completion).
Abort these steps.
Let options be the descendant script fetch options for referencing script's fetch options.
Fetch a module script graph given url, settings
object, "script", and options. Wait until the algorithm
asynchronously completes with result.
If result is null, then:
Let completion be Completion { [[Type]]: throw, [[Value]]: a new
TypeError, [[Target]]: empty }.
Perform FinishDynamicImport(referencingScriptOrModule, specifier, promiseCapability, completion).
Abort these steps.
Run the module script result, with the rethrow errors boolean set to true.
If running the module script throws an exception, then perform FinishDynamicImport(referencingScriptOrModule, specifier, promiseCapability, the thrown exception completion).
Otherwise, perform FinishDynamicImport(referencingScriptOrModule, specifier, promiseCapability, NormalCompletion(undefined)).
Return undefined.
JavaScript defines the concept of an agent. Until such a time that this standard has a better handle on lifetimes, we define five types of agents that user agents must allocate at the appropriate time.
In the future, when this specification has a better handle on lifetimes, we hope to define exactly when agents and agent clusters are created.
JavaScript is expected to define agents in more detail; in particular that they hold a set of realms: tc39/ecma262 issue #882.
An agent whose [[CanBlock]] is false and whose set of realms consists of all realms of Window objects whose
relevant settings object's responsible browsing context is in the same
unit of related similar-origin browsing contexts.
Two Window objects that are same origin can be in
different similar-origin window agents, for
instance if they are each in their own unit of related similar-origin browsing
contexts.
An agent whose [[CanBlock]] is true and whose set of realms consists of a single DedicatedWorkerGlobalScope object's Realm.
An agent whose [[CanBlock]] is true and whose set of realms consists a single SharedWorkerGlobalScope object's Realm.
An agent whose [[CanBlock]] is false and whose set of realms consists of a single ServiceWorkerGlobalScope object's Realm.
An agent whose [[CanBlock]] is false and whose set of realms consists of a single WorkletGlobalScope
object's Realm.
While conceptually it might be cleaner for worklets that end up with multiple realms to put all those in the same agent, it is not observable in practice.
Can share memory with defines an equivalence relation. An agent cluster consists of all agents in the same equivalence class with respect to the can share memory with equivalence relation.
A similar-origin window agent, dedicated worker agent, shared worker agent, or service worker agent, agent, can share memory with any dedicated worker agent whose single realm's global object's owner set contains an item whose relevant Realm belongs to agent.
We use item above as an owner set can contain Document
objects.
A worklet agent … currently worklets have no clearly defined owner, see: w3c/css-houdini-drafts issue #224.
In addition, any agent A can share memory with:
The agent cluster concept is crucial for defining the JavaScript memory model, and
in particular among which agents the backing data of
SharedArrayBuffer objects can be shared.
The following pairs of global objects are each within the same agent cluster, and
thus can use SharedArrayBuffer instances to share memory with each other:
Window object and a dedicated worker that it created.Window object A and the Window object of an
iframe element that A created that could be same
origin-domain with A.Window object and a same origin-domain Window
object that opened it.The following pairs of global objects are not within the same agent cluster, and thus cannot share memory:
When the user agent is required to report an error for a particular script script with a particular position line:col, using a particular target target, it must run these steps, after which the error is either handled or not handled:
If target is in error reporting mode, then abort these steps; the error is not handled.
Let target be in error reporting mode.
Let message be a user-agent-defined string describing the error in a
helpful manner.
Let errorValue be the value that represents the error: in the case of an
uncaught exception, that would be the value that was thrown; in the case of a JavaScript error
that would be an Error object. If there is no corresponding
value, then the null value must be used instead.
Let urlString be the result of applying the URL serializer to the URL record that corresponds to the resource from which script was obtained.
The resource containing the script will typically be the file from which the
Document was parsed, e.g. for inline script elements or event
handler content attributes; or the JavaScript file that the script was in, for external
scripts. Even for dynamically-generated scripts, user agents are strongly encouraged to attempt
to keep track of the original source of a script. For example, if an external script uses the
document.write() API to insert an inline
script element during parsing, the URL of the resource containing the script would
ideally be reported as being the external script, and the line number might ideally be reported
as the line with the document.write() call or where the
string passed to that call was first constructed. Naturally, implementing this can be somewhat
non-trivial.
User agents are similarly encouraged to keep careful track of the original line
numbers, even in the face of document.write() calls
mutating the document as it is parsed, or event handler content attributes spanning
multiple lines.
If script's muted errors is true, then set message to
"Script error.", urlString to the empty string, line
and col to 0, and errorValue to null.
Let notHandled be the result of firing an
event named error at target, using
ErrorEvent, with the cancelable attribute
initialized to true, the message attribute
initialized to message, the filename
attribute initialized to urlString, the lineno attribute initialized to line, the colno attribute initialized to col, and the error attribute initialized to
errorValue.
Let target no longer be in error reporting mode.
If notHandled is false, then the error is handled. Otherwise, the error is not handled.
Returning true in an event handler cancels the event per the event handler processing algorithm.
When the user agent is to report an exception E, the user agent must report the error for the relevant script, with the problematic position (line number and column number) in the resource containing the script, using the global object specified by the script's settings object as the target. If the error is still not handled after this, then the error may be reported to a developer console.
ErrorEvent interface[Constructor(DOMString type, optional ErrorEventInit eventInitDict), Exposed=(Window,Worker)]
interface ErrorEvent : Event {
readonly attribute DOMString message;
readonly attribute USVString filename;
readonly attribute unsigned long lineno;
readonly attribute unsigned long colno;
readonly attribute any error;
};
dictionary ErrorEventInit : EventInit {
DOMString message = "";
USVString filename = "";
unsigned long lineno = 0;
unsigned long colno = 0;
any error = null;
};
The message attribute must return the
value it was initialized to. It represents the error message.
The filename attribute must return the
value it was initialized to. It represents the URL of the script in which the error
originally occurred.
The lineno attribute must return the
value it was initialized to. It represents the line number where the error occurred in the
script.
The colno attribute must return the value
it was initialized to. It represents the column number where the error occurred in the script.
The error attribute must return the value
it was initialized to. Where appropriate, it is set to the object representing the error (e.g.,
the exception object in the case of an uncaught DOM exception).
In addition to synchronous runtime script errors, scripts
may experience asynchronous promise rejections, tracked via the unhandledrejection and rejectionhandled events.
Support: unhandledrejectionChrome for Android 61+Chrome 36+iOS Safari NoneUC Browser for Android NoneFirefox NoneIE NoneSamsung Internet 5+Opera Mini NoneSafari 11+Edge NoneAndroid Browser 56+Opera 36+
Source: caniuse.com
When the user agent is to notify about rejected promises on a given environment settings object settings object, it must run these steps:
Let list be a copy of settings object's about-to-be-notified rejected promises list.
If list is empty, abort these steps.
Clear settings object's about-to-be-notified rejected promises list.
Queue a task to run the following substep:
For each promise p in list:
If p's [[PromiseIsHandled]] internal slot is true, continue to the next iteration of the loop.
Let notHandled be the result of firing an
event named unhandledrejection at
settings object's global
object, using PromiseRejectionEvent, with the cancelable attribute initialized to true, the promise attribute initialized to
p, and the reason attribute
initialized to the value of p's [[PromiseResult]] internal slot.
If notHandled is false, then the promise rejection is handled. Otherwise, the promise rejection is not handled.
If p's [[PromiseIsHandled]] internal slot is false, add p to settings object's outstanding rejected promises weak set.
This algorithm results in promise rejections being marked as handled or not handled. These concepts parallel handled and not handled script errors. If a rejection is still not handled after this, then the rejection may be reported to a developer console.
JavaScript contains an implementation-defined HostPromiseRejectionTracker(promise, operation) abstract operation. User agents must use the following implementation: [JAVASCRIPT]
Let script be the running script.
If script's muted errors is true, terminate these steps.
Let settings object be script's settings object.
If operation is "reject",
Add promise to settings object's about-to-be-notified rejected promises list.
If operation is "handle",
If settings object's about-to-be-notified rejected promises list contains promise, remove promise from that list and abort these steps.
If settings object's outstanding rejected promises weak set does not contain promise, abort these steps.
Remove promise from settings object's outstanding rejected promises weak set.
Queue a task to fire an event
named rejectionhandled at settings
object's global object, using
PromiseRejectionEvent, with the promise attribute initialized to
promise, and the reason
attribute initialized to the value of promise's [[PromiseResult]] internal
slot.
PromiseRejectionEvent interface[Constructor(DOMString type, PromiseRejectionEventInit eventInitDict), Exposed=(Window,Worker)]
interface PromiseRejectionEvent : Event {
readonly attribute Promise<any> promise;
readonly attribute any reason;
};
dictionary PromiseRejectionEventInit : EventInit {
required Promise<any> promise;
any reason;
};
The promise attribute must
return the value it was initialized to. It represents the promise which this notification is about.
The reason attribute must
return the value it was initialized to. It represents the rejection reason for the promise.
JavaScript contains an implementation-defined HostEnsureCanCompileStrings(callerRealm, calleeRealm) abstract operation. User agents must use the following implementation: [JAVASCRIPT]
Perform ? EnsureCSPDoesNotBlockStringCompilation(callerRealm, calleeRealm). [CSP]
To coordinate events, user interaction, scripts, rendering, networking, and so forth, user agents must use event loops as described in this section. There are two kinds of event loops: those for browsing contexts, and those for workers.
There must be at least one browsing context event loop per user agent, and at most one per unit of related similar-origin browsing contexts.
When there is more than one event loop for a unit of related browsing contexts, complications arise when a browsing context in that group is navigated such that it switches from one unit of related similar-origin browsing contexts to another. This specification does not currently describe how to handle these complications.
A browsing context event loop always has at least one browsing context. If such an event loop's browsing contexts all go away, then the event loop goes away as well. A browsing context always has an event loop coordinating its activities.
Worker event loops are simpler: each worker has one event loop, and the worker processing model manages the event loop's lifetime.
An event loop has one or more task queues. A task queue is an ordered list of tasks, which are algorithms that are responsible for such work as:
Dispatching an Event object at a particular
EventTarget object is often done by a dedicated task.
Not all events are dispatched using the task queue, many are dispatched during other tasks.
The HTML parser tokenizing one or more bytes, and then processing any resulting tokens, is typically a task.
Calling a callback is often done by a dedicated task.
When an algorithm fetches a resource, if the fetching occurs in a non-blocking fashion then the processing of the resource once some or all of the resource is available is performed by a task.
Some elements have tasks that trigger in response to DOM manipulation, e.g. when that element is inserted into the document.
Each task in a browsing context event
loop is associated with a Document; if the task was queued in the context of
an element, then it is the element's node document; if the task was queued in the context
of a browsing context, then it is the browsing context's active
document at the time the task was queued; if the task was queued by or for a script then the document is the responsible document
specified by the script's settings object.
A task is intended for a specific event loop:
the event loop that is handling tasks for the
task's associated Document or worker.
When a user agent is to queue a task, it must add the given task to one of the task queues of the relevant event loop.
Each task is defined as coming from a specific task source. All the tasks from one particular task source and
destined to a particular event loop (e.g. the callbacks generated by timers of a
Document, the events fired for mouse movements over that Document, the
tasks queued for the parser of that Document) must always be added to the same
task queue, but tasks from different task sources may be placed in different task
queues.
For example, a user agent could have one task queue for mouse and key events (the user interaction task source), and another for everything else. The user agent could then give keyboard and mouse events preference over other tasks three quarters of the time, keeping the interface responsive but not starving other task queues, and never processing events from any one task source out of order.
Each event loop has a currently running task. Initially, this is null. It is used to handle reentrancy. Each event loop also has a performing a microtask checkpoint flag, which must initially be false. It is used to prevent reentrant invocation of the perform a microtask checkpoint algorithm.
An event loop must continually run through the following steps for as long as it exists:
Let oldestTask be the oldest task
on one of the event loop's task queues, if any,
ignoring, in the case of a browsing context event loop, tasks whose
associated Documents are not fully active. The user agent may pick any
task queue. If there is no task to select, then jump to the microtasks step
below.
Set the event loop's currently running task to oldestTask.
Run oldestTask.
Set the event loop's currently running task back to null.
Remove oldestTask from its task queue.
Microtasks: Perform a microtask checkpoint.
Update the rendering: If this event loop is a browsing context event loop (as opposed to a worker event loop), then run the following substeps.
Let now be the value that would be returned by the Performance
object's now() method. [HRT]
Let docs be the list of Document objects associated with the
event loop in question, sorted arbitrarily except that the following conditions
must be met:
Any Document B that is nested through a Document A must be listed after
A in the list.
If there are two documents A and B whose browsing contexts are both nested browsing contexts and their browsing context containers are both elements in the same
Document C, then the order of A and B in the
list must match the relative tree order of their respective browsing context containers in
C.
In the steps below that iterate over docs, each Document must be
processed in the order it is found in the list.
If there are top-level browsing contexts
B that the user agent believes would not benefit from having their rendering
updated at this time, then remove from docs all Document objects whose
browsing context's top-level browsing
context is in B.
Whether a top-level browsing context would benefit from having its rendering
updated depends on various factors, such as the update frequency. For example, if the browser
is attempting to achieve a 60Hz refresh rate, then these steps are only necessary every 60th
of a second (about 16.7ms). If the browser finds that a top-level browsing
context is not able to sustain this rate, it might drop to a more sustainable 30Hz for
that set of Documents, rather than occasionally dropping frames. (This
specification does not mandate any particular model for when to update the rendering.)
Similarly, if a top-level browsing context is in the background, the user agent
might decide to drop that page to a much slower 4Hz, or even less.
Another example of why a browser might skip updating the rendering is to ensure certain tasks are executed immediately after each other, with only microtask checkpoints interleaved (and without, e.g., animation frame callbacks interleaved). For example, a user agent might wish to coalesce timer callbacks together, with no intermediate rendering updates.
If there are a nested browsing contexts
B that the user agent believes would not benefit from having their rendering
updated at this time, then remove from docs all Document objects whose
browsing context is in B.
As with top-level browsing contexts, a variety of factors can influence whether it is profitable for a browser to update the rendering of nested browsing contexts. For example, a user agent might wish to spend less resources rendering third-party content, especially if it is not currently visible to the user or if resources are constrained. In such cases, the browser could decide to update the rendering for such content infrequently or never.
For each fully active Document in docs, run
the resize steps for that Document, passing in now as the
timestamp. [CSSOMVIEW]
For each fully active Document in docs, run
the scroll steps for that Document, passing in now as the
timestamp. [CSSOMVIEW]
For each fully active Document in docs,
evaluate media queries and report changes for that Document, passing
in now as the timestamp. [CSSOMVIEW]
For each fully active Document in docs, run
CSS animations and send events for that Document, passing in now
as the timestamp. [CSSANIMATIONS]
For each fully active Document in docs, run
the fullscreen steps for that Document, passing in now as the
timestamp. [FULLSCREEN]
For each fully active Document in docs, run
the animation frame callbacks for that Document, passing in now
as the timestamp.
For each fully active Document in docs, run
the update intersection observations steps for that Document, passing in
now as the timestamp. [INTERSECTIONOBSERVER]
For each fully active Document in docs, update the
rendering or user interface of that Document and its browsing context to reflect the current state.
If this is a worker event loop (i.e. one running for a
WorkerGlobalScope), but there are no tasks in the
event loop's task queues and the
WorkerGlobalScope object's closing flag is true, then destroy the event
loop, aborting these steps, resuming the run a worker steps described in the
Web workers section below.
Each event loop has a microtask queue. A microtask is a task that is originally to be queued on the microtask queue rather than a task queue. There are two kinds of microtasks: solitary callback microtasks, and compound microtasks.
This specification only has solitary callback microtasks. Specifications that use compound microtasks have to take extra care to wrap callbacks to handle spinning the event loop.
When an algorithm requires a microtask to be queued, it must be appended to the relevant event loop's microtask queue; the task source of such a microtask is the microtask task source.
It is possible for a microtask to be moved to a regular task queue, if, during its initial execution, it spins the event loop. In that case, the microtask task source is the task source used. Normally, the task source of a microtask is irrelevant.
When a user agent is to perform a microtask checkpoint, if the performing a microtask checkpoint flag is false, then the user agent must run the following steps:
Set the performing a microtask checkpoint flag to true.
While the event loop's microtask queue is not empty:
Let oldestMicrotask be the oldest microtask on the event loop's microtask queue.
Set the event loop's currently running task to oldestMicrotask.
Run oldestMicrotask.
This might involve invoking scripted callbacks, which eventually calls the clean up after running script steps, which call this perform a microtask checkpoint algorithm again, which is why we use the performing a microtask checkpoint flag to avoid reentrancy.
Set the event loop's currently running task back to null.
Remove oldestMicrotask from the microtask queue.
For each environment settings object whose responsible event loop is this event loop, notify about rejected promises on that environment settings object.
Set the performing a microtask checkpoint flag to false.
If, while a compound microtask is running, the user agent is required to execute a compound microtask subtask to run a series of steps, the user agent must run the following steps:
Let parent be the event loop's currently running task (the currently running compound microtask).
Let subtask be a new task that consists of running the given series of steps. The task source of such a microtask is the microtask task source. This is a compound microtask subtask.
Set the event loop's currently running task to subtask.
Run subtask.
Set the event loop's currently running task back to parent.
When an algorithm running in parallel is to await a stable state, the user agent must queue a microtask that runs the following steps, and must then stop executing (execution of the algorithm resumes when the microtask is run, as described in the following steps):
Run the algorithm's synchronous section.
Resumes execution of the algorithm in parallel, if appropriate, as described in the algorithm's steps.
Steps in synchronous sections are marked with ⌛.
When an algorithm says to spin the event loop until a condition goal is met, the user agent must run the following steps:
Let task be the event loop's currently running task.
This might be a microtask, in which case it is a solitary callback microtask. It could also be a compound microtask subtask, or a regular task that is not a microtask. It will not be a compound microtask.
Let task source be task's task source.
Let old stack be a copy of the JavaScript execution context stack.
Empty the JavaScript execution context stack.
Stop task, allowing whatever algorithm that invoked it to resume, but continue these steps in parallel.
This causes one of the following algorithms to continue: the event loop's main set of steps, the perform a microtask checkpoint algorithm, or the execute a compound microtask subtask algorithm.
Wait until the condition goal is met.
Queue a task to continue running these steps, using the task source task source. Wait until this new task runs before continuing these steps.
Replace the JavaScript execution context stack with the old stack.
Return to the caller.
Some of the algorithms in this specification, for historical reasons, require the user agent to pause while running a task until a condition goal is met. This means running the following steps:
If necessary, update the rendering or user interface of any Document or
browsing context to reflect the current state.
Wait until the condition goal is met. While a user agent has a paused task, the corresponding event loop must not run further tasks, and any script in the currently running task must block. User agents should remain responsive to user input while paused, however, albeit in a reduced capacity since the event loop will not be doing anything.
Pausing is highly detrimental to the user experience, especially in scenarios where a single event loop is shared among multiple documents. User agents are encouraged to experiment with alternatives to pausing, such as spinning the event loop or even simply proceeding without any kind of suspended execution at all, insofar as it is possible to do so while preserving compatibility with existing content. This specification will happily change if a less-drastic alternative is discovered to be web-compatible.
In the interim, implementers should be aware that the variety of alternatives that user agents might experiment with can change subtle aspects of event loop behavior, including task and microtask timing. Implementations should continue experimenting even if doing so causes them to violate the exact semantics implied by the pause operation.
The following task sources are used by a number of mostly unrelated features in this and other specifications.
This task source is used for features that react to DOM manipulations, such as things that happen in a non-blocking fashion when an element is inserted into the document.
This task source is used for features that react to user interaction, for example keyboard or mouse input.
Events sent in response to user input (e.g. click events) must be fired using tasks queued with the user
interaction task source. [UIEVENTS]
This task source is used for features that trigger in response to network activity.
This task source is used to queue calls to history.back() and similar APIs.
Writing specifications that correctly interact with the event loop can be tricky. This is compounded by how this specification uses concurrency-model-independent terminology, so we say things like "event loop" and "in parallel" instead of using more familiar model-specific terms like "main thread" or "on a background thread".
By default, specification text generally runs on the event loop. This falls out from the formal event loop processing model, in that you can eventually trace most algorithms back to a task queued there.
The algorithm steps for any JavaScript method will be invoked by author code
calling that method. And author code can only be run via queued tasks, usually originating
somewhere in the script processing model.
From this starting point, the overriding guideline is that any work a specification needs to perform that would otherwise block the event loop must instead be performed in parallel with it. This includes (but is not limited to):
performing heavy computation;
displaying a user-facing prompt;
performing operations which could require involving outside systems (i.e. "going out of process").
The next complication is that, in algorithm sections that are in parallel, you must not create or manipulate objects associated to a specific JavaScript realm, global, or environment settings object. (Stated in more familiar terms, you must not directly access main-thread artifacts from a background thread.) Doing so would create data races observable to JavaScript code, since after all, your algorithm steps are running in parallel to the JavaScript code.
You can, however, manipulate specification-level data structures and values from the WHATWG Infra Standard, as those are realm-agnostic. They are never directly exposed to JavaScript without a specific conversion taking place (often via Web IDL). [INFRA] [WEBIDL]
To affect the world of observable JavaScript objects, then, you must queue a task to perform any such manipulations. This ensures your steps are properly interleaved with respect to other things happening on the event loop. Furthermore, you must choose a task source when queueing a task; this governs the relative order of your steps versus others. If you are unsure which task source to use, pick one of the generic task sources that sounds most applicable.
Most invocations of queue a task implicitly use "the relevant event loop", i.e., the one that is obvious from context. That is because it is very rare for algorithms to be invoked in contexts involving multiple event loops. (Unlike contexts involving multiple global objects, which happen all the time!) So unless you are writing a specification which, e.g., deals with manipulating workers, you can omit this argument when queueing a task.
Putting this all together, we can provide a template for a typical algorithm that needs to do work asynchronously:
Do any synchronous setup work, while still on the event loop. This may include converting realm-specific JavaScript values into realm-agnostic specification-level values.
Perform a set of potentially-expensive steps in parallel, operating entirely on realm-agnostic values, and producing a realm-agnostic result.
Queue a task, on a specified task source, to convert the realm-agnostic result back into observable effects on the observable world of JavaScript objects on the event loop.
The following is an algorithm that "encrypts" a passed-in list of scalar value strings input, after parsing them as URLs:
Let urls be an empty list.
For each string of input:
Let parsed be the result of parsing string relative to the current settings object.
If parsed is failure, return a promise rejected with a
"SyntaxError" DOMException.
Let serialized be the result of applying the URL serializer to parsed.
Append serialized to urls.
Let realm be the current Realm Record.
Let p be a new promise.
Run the following steps in parallel:
Let encryptedURLs be an empty list.
For each url of urls:
Wait 100 milliseconds, so that people think we're doing heavy-duty encryption.
Let encrypted be a new JavaScript string derived from url, whose nth code unit is equal to url's nth code unit plus 13.
Append encrypted to encryptedURLs.
Queue a task, on the networking task source, to perform the following steps:
Let array be the result of converting encryptedURLs to a JavaScript array, in realm.
Resolve p with array.
Return p.
Here are several things to notice about this algorithm:
It does its URL parsing up front, on the event loop, before going to the in parallel steps. This is necessary, since parsing depends on the current settings object, which would no longer be current after going in parallel.
Alternately, it could have saved a reference to the current settings object's API base URL and used it during the in parallel steps; that would have been equivalent. However, we recommend instead doing as much work as possible up front, as this example does. Attempting to save the correct values can be error prone; for example, if we'd saved just the current settings object, instead of its API base URL, there would have been a potential race.
It implicitly passes a list of JavaScript strings from the initial steps to the in parallel steps. This is OK, as both lists and JavaScript strings are realm-agnostic.
It performs "expensive computation" (waiting for 100 milliseconds per input URL) during the in parallel steps, thus not blocking the main event loop.
Promises, as observable JavaScript objects, are never created and manipulated during the in parallel steps. p is created before entering those steps, and then is manipulated during a task that is queued specifically for that purpose.
The creation of a JavaScript array object also happens during the queued task, and is careful to specify which realm it creates the array in since that is no longer obvious from context.
(On these last two points, see also w3ctag/promises-guide#52, heycam/webidl#135, and heycam/webidl#371, where we are still mulling over the subtleties of the above promise-resolution pattern.)
Another thing to note is that, in the event this algorithm was called from a Web IDL-specified
operation taking a sequence<USVString>, there was an automatic conversion from realm-specific JavaScript objects provided by the author as
input, into the realm-agnostic sequence<USVString> Web IDL type, which we then treat as a
list of scalar value strings. So depending
on how your specification is structured, there may be other implicit steps happening on the main
event loop that play a part in this whole process of getting you ready to go
in parallel.
Many objects can have event handlers specified. These act as non-capture event listeners for the object on which they are specified. [DOM]
An event handler has a name, which always starts with
"on" and is followed by the name of the event for which it is intended.
An event handler has a value, which is either null, or is
a callback object, or is an internal raw uncompiled handler.
The EventHandler callback function type describes how this is exposed to scripts.
Initially, an event handler's value must be set
to null.
Event handlers are exposed in one of two ways.
The first way, common to all event handlers, is as an event handler IDL attribute.
The second way is as an event handler content
attribute. Event handlers on HTML elements and some of the event handlers on
Window objects are exposed in this way.
An event handler IDL attribute is an IDL attribute for a specific event handler. The name of the IDL attribute is the same as the name of the event handler.
Event handler IDL attributes, on setting, must set the corresponding event handler to their new value, and on getting, must return the result of getting the current value of the event handler in question.
If an event handler IDL attribute exposes an event handler of an object that doesn't exist, it must always return null on getting and must do nothing on setting.
This can happen in particular for event
handler IDL attribute on body elements that do not have corresponding
Window objects.
Certain event handler IDL attributes have additional requirements, in particular
the onmessage attribute of
MessagePort objects.
An event handler content attribute is a content attribute for a specific event handler. The name of the content attribute is the same as the name of the event handler.
Event handler content attributes, when specified, must contain valid JavaScript code which, when parsed, would match the FunctionBody production after automatic semicolon insertion.
When an event handler content attribute is set, execute the following steps:
If the Should element's inline behavior be blocked by Content Security
Policy? algorithm returns "Blocked" when executed upon the
attribute's element, "script attribute", and the attribute's
value, then abort these steps. [CSP]
Set the corresponding event handler to an internal raw uncompiled handler consisting of the attribute's new value and the script location where the attribute was set to this value
When an event handler content attribute is removed, the user agent must set the corresponding event handler to null.
When an event handler H of an element or object
T implementing the EventTarget interface is first set to a non-null value,
the user agent must append an event listener to the
list of event listeners associated with T
with type set to the event handler event type corresponding to
H and callback set to the result of creating a Web IDL EventListener instance representing a reference to a function of
one argument that executes the steps of the event handler processing algorithm, given
H and its argument. The EventListener's
callback context can be arbitrary; it does not impact the steps of the event
handler processing algorithm. [DOM]
The callback is emphatically not the event handler itself. Every event handler ends up registering the same callback, the algorithm defined below, which takes care of invoking the right callback, and processing the callback's return value.
This only happens the first time the event
handler's value is set. Since listeners are called in the order they were registered, the
order of event listeners for a particular event type will always be first the event listeners
registered with addEventListener() before
the first time the event handler was set to a non-null value,
then the callback to which it is currently set, if any, and finally the event listeners registered
with addEventListener() after the
first time the event handler was set to a non-null value.
This example demonstrates the order in which event listeners are invoked. If the button in this example is clicked by the user, the page will show four alerts, with the text "ONE", "TWO", "THREE", and "FOUR" respectively.
<button id="test">Start Demo</button>
<script>
var button = document.getElementById('test');
button.addEventListener('click', function () { alert('ONE') }, false);
button.setAttribute('onclick', "alert('NOT CALLED')"); // event handler listener is registered here
button.addEventListener('click', function () { alert('THREE') }, false);
button.onclick = function () { alert('TWO'); };
button.addEventListener('click', function () { alert('FOUR') }, false);
</script>
The interfaces implemented by the event object do not influence whether an event handler is triggered or not.
The event handler processing algorithm for an event
handler H and an Event object E is as
follows:
Let callback be the result of getting the current value of the event handler H.
If callback is null, then abort these steps.
Let special error event handling be true if E is an
ErrorEvent object, E's type is error, and E's currentTarget implements the
WindowOrWorkerGlobalScope mixin. Otherwise, let special error event
handling be false.
Process the Event object E as follows:
Invoke callback with five
arguments, the first one having the value of E's message attribute, the second having the value of
E's filename attribute, the third
having the value of E's lineno
attribute, the fourth having the value of E's colno attribute, the fifth having the value of
E's error attribute, and with the callback this value set to E's currentTarget. Let return value be the
callback's return value. [WEBIDL]
Invoke callback
with one argument, the value of which is the Event object E,
with the callback this value set to E's currentTarget. Let return value be the callback's return value. [WEBIDL]
If an exception gets thrown by the callback, end these steps and allow the exception to propagate. (It will propagate to the DOM event dispatch logic, which will then report the exception.)
Process return value as follows:
BeforeUnloadEvent object and E's type is beforeunloadIn this case, the event handler
IDL attribute's type will be OnBeforeUnloadEventHandler, so return
value will have been coerced into either null or a DOMString.
If return value is not null, then:
Set E's canceled flag.
If E's returnValue
attribute's value is the empty string, then set E's returnValue attribute's value to
return value.
If return value is true, then set E's canceled flag.
If return value is false, then set E's canceled flag.
If we've gotten to this "Otherwise" clause because E's type is beforeunload but E is not a
BeforeUnloadEvent object, then return value will never be false, since
in such cases return value will have been coerced into either null or a DOMString.
The EventHandler callback function type represents a callback used for event
handlers. It is represented in Web IDL as follows:
[TreatNonObjectAsNull] callback EventHandlerNonNull = any (Event event); typedef EventHandlerNonNull? EventHandler;
In JavaScript, any Function object implements
this interface.
For example, the following document fragment:
<body onload="alert(this)" onclick="alert(this)">
...leads to an alert saying "[object Window]" when the document is
loaded, and an alert saying "[object HTMLBodyElement]" whenever the
user clicks something in the page.
The return value of the function affects whether the event is canceled or not: as described above, if the return value is false, the event is canceled.
There are two exceptions in the platform, for historical reasons:
The onerror handlers on global objects, where
returning true cancels the event
The onbeforeunload handler, where
returning any non-null and non-undefined value will cancel the event.
For historical reasons, the onerror handler has different
arguments:
[TreatNonObjectAsNull] callback OnErrorEventHandlerNonNull = any ((Event or DOMString) event, optional DOMString source, optional unsigned long lineno, optional unsigned long colno, optional any error); typedef OnErrorEventHandlerNonNull? OnErrorEventHandler;
window.onerror = (message, source, lineno, colno, error) => { … };
Similarly, the onbeforeunload handler has a
different return value:
[TreatNonObjectAsNull] callback OnBeforeUnloadEventHandlerNonNull = DOMString? (Event event); typedef OnBeforeUnloadEventHandlerNonNull? OnBeforeUnloadEventHandler;
An internal raw uncompiled handler is a tuple with the following information:
When the user agent is to get the current value of the event handler H, it must run these steps:
If H's value is an internal raw uncompiled handler, run these substeps:
If H is an element's event handler, then let element be the element, and document be the element's node document.
Otherwise, H is a Window object's event handler:
let element be null, and let document be H's associated Document.
If scripting is disabled for document, then return null.
Let body be the uncompiled script body in the internal raw uncompiled handler.
Let location be the location where the script body originated, as given by the internal raw uncompiled handler.
If element is not null and element has a form owner, let form owner be that form owner. Otherwise, let form owner be null.
Let settings object be the relevant settings object of document.
If body is not parsable as FunctionBody or if parsing detects an early error, then follow these substeps:
Set H's value to null.
Report the error for the appropriate script and with the appropriate position (line number and column number) given by location, using settings object's global object. If the error is still not handled after this, then the error may be reported to a developer console.
Return null.
If body begins with a Directive Prologue that contains a Use Strict Directive then let strict be true, otherwise let strict be false.
Push settings object's realm execution context onto the JavaScript execution context stack; it is now the running JavaScript execution context.
This is necessary so the subsequent invocation of FunctionCreate takes place in the correct JavaScript Realm.
Let function be the result of calling FunctionCreate, with arguments:
onerror event handler of a Window objectevent, source, lineno, colno, and
error.event.If H is an element's event handler, then let Scope be NewObjectEnvironment(document, the global environment).
Otherwise, H is a Window object's event handler: let Scope be the global environment.
If form owner is not null, let Scope be NewObjectEnvironment(form owner, Scope).
If element is not null, let Scope be NewObjectEnvironment(element, Scope).
Remove settings object's realm execution context from the JavaScript execution context stack.
Set H's value to the result of creating a Web IDL callback function whose object reference is function and whose callback context is settings object.
Return H's value.
Document objects, and Window objectsThe following are the event handlers (and their corresponding event handler event types) that must be
supported by all HTML elements, as both event handler content attributes
and event handler IDL attributes; and that must be
supported by all Document and Window objects, as event handler IDL
attributes:
| Event handler | Event handler event type |
|---|---|
onabort | abort
|
onauxclick | auxclick
|
oncancel | cancel
|
oncanplay | canplay
|
oncanplaythrough | canplaythrough
|
onchange | change
|
onclick | click
|
onclose | close
|
oncontextmenu | contextmenu
|
oncuechange | cuechange
|
ondblclick | dblclick
|
ondrag | drag
|
ondragend | dragend
|
ondragenter | dragenter
|
ondragexit | dragexit
|
ondragleave | dragleave
|
ondragover | dragover
|
ondragstart | dragstart
|
ondrop | drop
|
ondurationchange | durationchange
|
onemptied | emptied
|
onended | ended
|
oninput | input
|
oninvalid | invalid
|
onkeydown | keydown
|
onkeypress | keypress
|
onkeyup | keyup
|
onloadeddata | loadeddata
|
onloadedmetadata | loadedmetadata
|
onloadend | loadend
|
onloadstart | loadstart
|
onmousedown | mousedown
|
onmouseenter | mouseenter
|
onmouseleave | mouseleave
|
onmousemove | mousemove
|
onmouseout | mouseout
|
onmouseover | mouseover
|
onmouseup | mouseup
|
onwheel | wheel
|
onpause | pause
|
onplay | play
|
onplaying | playing
|
onprogress | progress
|
onratechange | ratechange
|
onreset | reset
|
onsecuritypolicyviolation | securitypolicyviolation
|
onseeked | seeked
|
onseeking | seeking
|
onselect | select
|
onstalled | stalled
|
onsubmit | submit
|
onsuspend | suspend
|
ontimeupdate | timeupdate
|
ontoggle | toggle
|
onvolumechange | volumechange
|
onwaiting | waiting
|
The following are the event handlers (and their corresponding event handler event types) that must be
supported by all HTML elements other than body and frameset
elements, as both event handler content attributes and event handler IDL
attributes; that must be supported by all Document
objects, as event handler IDL attributes; and that must be
supported by all Window objects, as event handler IDL attributes on the
Window objects themselves, and with corresponding event handler content
attributes and event handler IDL attributes exposed on all body
and frameset elements that are owned by that Window object's associated Document:
| Event handler | Event handler event type |
|---|---|
onblur | blur
|
onerror | error
|
onfocus | focus
|
onload | load
|
onresize | resize
|
onscroll | scroll
|
The following are the event handlers (and their corresponding event handler event types) that must be
supported by Window objects, as event handler IDL attributes on the
Window objects themselves, and with corresponding event handler content
attributes and event handler IDL attributes exposed on all body
and frameset elements that are owned by that Window object's associated Document:
| Event handler | Event handler event type |
|---|---|
onafterprint | afterprint
|
onbeforeprint | beforeprint
|
onbeforeunload | beforeunload
|
onhashchange | hashchange
|
onlanguagechange | languagechange
|
onmessage | message
|
onmessageerror | messageerror
|
onoffline | offline
|
ononline | online
|
onpagehide | pagehide
|
onpageshow | pageshow
|
onpopstate | popstate
|
onrejectionhandled | rejectionhandled
|
onstorage | storage
|
onunhandledrejection | unhandledrejection
|
onunload | unload
|
The following are the event handlers (and their corresponding event handler event types) that must be
supported by all HTML elements, as both event handler content attributes
and event handler IDL attributes; and that must be
supported by all Document objects, as event handler IDL attributes:
| Event handler | Event handler event type |
|---|---|
oncut | cut
|
oncopy | copy
|
onpaste | paste
|
The following are the event handlers (and their corresponding event handler event types) that must be
supported on Document objects as event handler IDL attributes:
| Event handler | Event handler event type |
|---|---|
onreadystatechange | readystatechange
|
[Exposed=Window,
NoInterfaceObject]
interface GlobalEventHandlers {
attribute EventHandler onabort;
attribute EventHandler onauxclick;
attribute EventHandler onblur;
attribute EventHandler oncancel;
attribute EventHandler oncanplay;
attribute EventHandler oncanplaythrough;
attribute EventHandler onchange;
attribute EventHandler onclick;
attribute EventHandler onclose;
attribute EventHandler oncontextmenu;
attribute EventHandler oncuechange;
attribute EventHandler ondblclick;
attribute EventHandler ondrag;
attribute EventHandler ondragend;
attribute EventHandler ondragenter;
attribute EventHandler ondragexit;
attribute EventHandler ondragleave;
attribute EventHandler ondragover;
attribute EventHandler ondragstart;
attribute EventHandler ondrop;
attribute EventHandler ondurationchange;
attribute EventHandler onemptied;
attribute EventHandler onended;
attribute OnErrorEventHandler onerror;
attribute EventHandler onfocus;
attribute EventHandler oninput;
attribute EventHandler oninvalid;
attribute EventHandler onkeydown;
attribute EventHandler onkeypress;
attribute EventHandler onkeyup;
attribute EventHandler onload;
attribute EventHandler onloadeddata;
attribute EventHandler onloadedmetadata;
attribute EventHandler onloadend;
attribute EventHandler onloadstart;
attribute EventHandler onmousedown;
[LenientThis] attribute EventHandler onmouseenter;
[LenientThis] attribute EventHandler onmouseleave;
attribute EventHandler onmousemove;
attribute EventHandler onmouseout;
attribute EventHandler onmouseover;
attribute EventHandler onmouseup;
attribute EventHandler onwheel;
attribute EventHandler onpause;
attribute EventHandler onplay;
attribute EventHandler onplaying;
attribute EventHandler onprogress;
attribute EventHandler onratechange;
attribute EventHandler onreset;
attribute EventHandler onresize;
attribute EventHandler onscroll;
attribute EventHandler onsecuritypolicyviolation;
attribute EventHandler onseeked;
attribute EventHandler onseeking;
attribute EventHandler onselect;
attribute EventHandler onstalled;
attribute EventHandler onsubmit;
attribute EventHandler onsuspend;
attribute EventHandler ontimeupdate;
attribute EventHandler ontoggle;
attribute EventHandler onvolumechange;
attribute EventHandler onwaiting;
};
[Exposed=Window,
NoInterfaceObject]
interface WindowEventHandlers {
attribute EventHandler onafterprint;
attribute EventHandler onbeforeprint;
attribute OnBeforeUnloadEventHandler onbeforeunload;
attribute EventHandler onhashchange;
attribute EventHandler onlanguagechange;
attribute EventHandler onmessage;
attribute EventHandler onmessageerror;
attribute EventHandler onoffline;
attribute EventHandler ononline;
attribute EventHandler onpagehide;
attribute EventHandler onpageshow;
attribute EventHandler onpopstate;
attribute EventHandler onrejectionhandled;
attribute EventHandler onstorage;
attribute EventHandler onunhandledrejection;
attribute EventHandler onunload;
};
[Exposed=Window,
NoInterfaceObject]
interface DocumentAndElementEventHandlers {
attribute EventHandler oncopy;
attribute EventHandler oncut;
attribute EventHandler onpaste;
};
Certain operations and methods are defined as firing events on elements. For example, the click() method on the HTMLElement interface is defined as
firing a click event on the element. [UIEVENTS]
Firing a synthetic mouse event named e at target, with an optional not trusted flag, means running these steps:
Let event be the result of creating an event using
MouseEvent.
Initialize event's type attribute to
e.
Initialize event's bubbles and cancelable attributes to true.
Set event's composed flag.
If the not trusted flag is set, initialize event's isTrusted attribute to false.
Initialize event's ctrlKey, shiftKey, altKey, and metaKey
attributes according to the current state of the key input device, if any (false for any keys
that are not available).
Initialize event's view attribute to
target's node document's Window object, if any, and null
otherwise.
event's getModifierState() method is to return values
appropriately describing the current state of the key input device.
Return the result of dispatching event at target.
Firing a click event
at target means firing a synthetic mouse
event named click at target.
WindowOrWorkerGlobalScope mixinThe WindowOrWorkerGlobalScope mixin is for use of APIs that are to be exposed on
Window and WorkerGlobalScope objects.
Other standards are encouraged to further extend it using partial
interface WindowOrWorkerGlobalScope { … }; along with an appropriate
reference.
typedef (DOMString or Function) TimerHandler;
[NoInterfaceObject, Exposed=(Window,Worker)]
interface WindowOrWorkerGlobalScope {
[Replaceable] readonly attribute USVString origin;
// base64 utility methods
DOMString btoa(DOMString data);
ByteString atob(DOMString data);
// timers
long setTimeout(TimerHandler handler, optional long timeout = 0, any... arguments);
void clearTimeout(optional long handle = 0);
long setInterval(TimerHandler handler, optional long timeout = 0, any... arguments);
void clearInterval(optional long handle = 0);
// ImageBitmap
Promise<ImageBitmap> createImageBitmap(ImageBitmapSource image, optional ImageBitmapOptions options);
Promise<ImageBitmap> createImageBitmap(ImageBitmapSource image, long sx, long sy, long sw, long sh, optional ImageBitmapOptions options);
};
Window implements WindowOrWorkerGlobalScope;
WorkerGlobalScope implements WindowOrWorkerGlobalScope;
originReturns the global object's origin, serialized as string.
Developers are strongly encouraged to use self.origin over location.origin. The former returns the origin of the environment,
the latter of the URL of the environment. Imagine the following script executing in a document on
https://stargate.example/:
var frame = document.createElement("iframe")
frame.onload = function() {
var frameWin = frame.contentWindow
console.log(frameWin.location.origin) // "null"
console.log(frameWin.origin) // "https://stargate.example"
}
document.body.appendChild(frame)
self.origin is a more reliable security indicator.
The origin attribute's getter must return this
object's relevant settings object's origin, serialized.
Support: atob-btoaChrome for Android 61+Chrome 4+iOS Safari 3.2+UC Browser for Android 11.4+Firefox 2+IE 10+Samsung Internet 4+Opera Mini all+Safari 3.1+Edge 12+Android Browser 2.1+Opera 10.6+
Source: caniuse.com
The atob() and btoa() methods
allow developers to transform content to and from the base64 encoding.
In these APIs, for mnemonic purposes, the "b" can be considered to stand for "binary", and the "a" for "ASCII". In practice, though, for primarily historical reasons, both the input and output of these functions are Unicode strings.
btoa( data )Takes the input data, in the form of a Unicode string containing only characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, and converts it to its base64 representation, which it returns.
Throws an "InvalidCharacterError" DOMException
exception if the input string contains any out-of-range characters.
atob( data )Takes the input data, in the form of a Unicode string containing base64-encoded binary data, decodes it, and returns a string consisting of characters in the range U+0000 to U+00FF, each representing a binary byte with values 0x00 to 0xFF respectively, corresponding to that binary data.
Throws an "InvalidCharacterError" DOMException if the
input string is not valid base64 data.
The btoa(data)
method must throw an "InvalidCharacterError" DOMException
if data contains any character whose code point is greater than U+00FF. Otherwise, the
user agent must convert data to a byte sequence whose nth byte is the
eight-bit representation of the nth code point of data, and then must apply
forgiving-base64 encode to that byte sequence and return the result.
The atob(data)
method, when invoked, must run the following steps:
Let decodedData be the result of running forgiving-base64 decode on data.
If decodedData is failure, then throw an
"InvalidCharacterError" DOMException.
Return decodedData.