HTML
Living Standard — Last Updated 11 August 2026
Living Standard — Last Updated 11 August 2026
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.JavaScript defines the concept of an agent. This section gives the mapping of that language-level concept on to the web platform.
Conceptually, the agent concept is an architecture-independent, idealized "thread" in which JavaScript code runs. Such code can involve multiple globals/realms that can synchronously access each other, and thus needs to run in a single execution thread.
Two Window objects having the same agent does not indicate
they can directly access all objects created in each other's realms. They would have to be
same origin-domain; see IsPlatformObjectSameOrigin.
The following types of agents exist on the web platform:
Contains various Window objects which can potentially reach each other, either
directly or by using document.domain.
If the encompassing agent cluster's is origin-keyed is true, then
all the Window objects will be same origin, can reach each other
directly, and document.domain will no-op.
Two Window objects that are same origin can be in
different similar-origin window agents, for
instance if they are each in their own browsing context group.
Contains a single DedicatedWorkerGlobalScope.
Contains a single SharedWorkerGlobalScope.
Contains a single ServiceWorkerGlobalScope.
Contains a single WorkletGlobalScope object.
Although a given worklet can have multiple realms, each such realm needs its own agent, as each realm can be executing code independently and at the same time as the others.
Only shared and dedicated worker agents allow the use of JavaScript Atomics APIs to
potentially block.
To create an agent, given a boolean canBlock:
Let signifier be a new unique internal value.
Let candidateExecution be a new candidate execution.
Let agent be a new agent whose [[CanBlock]] is canBlock, [[Signifier]] is signifier, [[CandidateExecution]] is candidateExecution, and [[IsLockFree1]], [[IsLockFree2]], and [[LittleEndian]] are set at the implementation's discretion.
Set agent's event loop to a new event loop.
Return agent.
The relevant agent for a platform object platformObject is platformObject's relevant realm's agent.
The agent equivalent of the current realm is the surrounding agent.
JavaScript also defines the concept of an agent cluster, which this standard maps to the web platform by placing agents appropriately when they are created using the obtain a similar-origin window agent or obtain a worker/worklet agent algorithms.
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.
Conceptually, the agent cluster concept is an architecture-independent, idealized "process boundary" that groups together multiple "threads" (agents). The agent clusters defined by the specification are generally more restrictive than the actual process boundaries implemented in user agents. By enforcing these idealized divisions at the specification level, we ensure that web developers see interoperable behavior with regard to shared memory, even in the face of varying and changing user agent process models.
An agent cluster has an associated cross-origin isolation mode, which is a
cross-origin isolation mode. It is initially "none".
An agent cluster has an associated is origin-keyed (a boolean), which is initially false.
The following defines the allocation of the agent clusters of similar-origin window agents.
An agent cluster key is a site or tuple origin. Without web developer action to achieve origin-keyed agent clusters, it will be a site.
An equivalent formulation is that an agent cluster key can be a scheme-and-host or an origin.
To obtain a similar-origin window agent, given an origin origin, a browsing context group group, and a boolean requestsOAC, run these steps:
Let site be the result of obtaining a site with origin.
Let key be site.
If group's cross-origin isolation
mode is not "none", then set
key to origin.
Otherwise, if group's historical agent cluster key map[origin] exists, then set key to group's historical agent cluster key map[origin].
Otherwise:
If requestsOAC is true, then set key to origin.
Set group's historical agent cluster key map[origin] to key.
If group's agent cluster map[key] does not exist:
Let agentCluster be a new agent cluster.
Set agentCluster's cross-origin isolation mode to group's cross-origin isolation mode.
If key is an origin:
Assert: key is origin.
Set agentCluster's is origin-keyed to true.
Add the result of creating an agent, given false, to agentCluster.
Set group's agent cluster map[key] to agentCluster.
Return the single similar-origin window agent contained in group's agent cluster map[key].
This means that there is only one similar-origin window agent per browsing context agent cluster. (However, dedicated worker and worklet agents might be in the same cluster.)
The following defines the allocation of the agent clusters of all other types of agents.
To obtain a worker/worklet agent, given an environment settings object or null outside settings, a boolean isTopLevel, and a boolean canBlock, run these steps:
Let agentCluster be null.
If isTopLevel is true:
Set agentCluster to a new agent cluster.
Set agentCluster's is origin-keyed to true.
These workers can be considered to be origin-keyed. However, this is not
exposed through any APIs (in the way that originAgentCluster exposes the origin-keyedness for
windows).
Otherwise:
Let agent be the result of creating an agent given canBlock.
Add agent to agentCluster.
Return agent.
To obtain a dedicated/shared worker agent, given an environment settings object outside settings and a boolean isShared, return the result of obtaining a worker/worklet agent given outside settings, isShared, and true.
To obtain a worklet agent, given an environment settings object outside settings, return the result of obtaining a worker/worklet agent given outside settings, false, and false.
To obtain a service worker agent, return the result of obtaining a worker/worklet agent given null, true, and false.
The JavaScript specification introduces the realm concept, representing a global environment in which script is run. Each realm comes with an implementation-defined global object; much of this specification is devoted to defining that global object and its properties.
For web specifications, it is often useful to associate values or algorithms with a
realm/global object pair. When the values are specific to a particular type of realm, they are
associated directly with the global object in question, e.g., in the definition of the
Window or WorkerGlobalScope interfaces. When the values have utility
across multiple realms, we use the environment settings object concept.
Finally, in some cases it is necessary to track associated values before a realm/global object/environment settings object even comes into existence (for example, during navigation). These values are tracked in the environment concept.
An environment is an object that identifies the settings of a current or potential execution environment (i.e., a navigation params's reserved environment or a request's reserved client). An environment has the following fields:
An opaque string that uniquely identifies this environment.
A URL that represents the location of the resource with which this environment is associated.
In the case of a Window environment settings object,
this URL might be distinct from its global
object's associated
Document's URL, due to
mechanisms such as history.pushState() which modify
the latter.
Null or a URL that represents the creation URL of the "top-level" environment. It is null for workers and worklets.
A for now implementation-defined value, null, or an origin. For a "top-level" potential execution environment it is null (i.e., when there is no response yet); otherwise it is the "top-level" environment's origin. For a dedicated worker or worklet it is the top-level origin of its creator. For a shared or service worker it is an implementation-defined value.
This is distinct from the top-level creation URL's origin when sandboxing, workers, and worklets are involved.
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.
Specifications may define environment discarding steps for environments. The steps take an environment as input.
The environment discarding steps are run for only a select few environments: the ones that will never become execution ready because, for example, they failed to load.
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 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 Source Text Module Record's Evaluate knows which realm to use.)
A module map that is used when importing JavaScript modules.
A URL used by APIs called by scripts that use this environment settings object to parse URLs.
An origin used in security checks.
A boolean used in security checks.
A policy container containing policies used for security checks.
A boolean representing whether scripts that use this environment settings object are allowed to use APIs that require cross-origin isolation.
An environment settings object's responsible event loop is its global object's relevant agent's event loop.
A global object is a JavaScript object that is the [[GlobalObject]] field of a realm.
In this specification, all realms are created with global
objects that are either Window, WorkerGlobalScope, or
WorkletGlobalScope objects.
A global object has an in error reporting mode boolean, which is initially false.
A global object has an outstanding rejected promises weak set, a
set of Promise objects, initially empty. This set
must not create strong references to any of its members, and implementations are free to limit
its size in an implementation-defined manner, e.g., by removing old entries from it
when new ones are added.
A global object has an about-to-be-notified rejected promises list, a
list of Promise objects, initially empty.
A global object has an import map, initially an empty import map.
For now, only Window global
objects have their import map modified
from the initial empty one. The import map is
only accessed for the resolution of a root module script.
A global object has a resolved module set, a set of specifier resolution records, initially empty.
The resolved module set ensures that module specifier resolution
returns the same result when called multiple times with the same (referrer, specifier) pair. It
does that by ensuring that import map rules that impact the specifier in its
referrer's scope cannot be defined after its initial resolution. For now, only Window
global objects have their module set data structures modified
from the initial empty one.
There is always a 1-to-1-to-1 mapping between realms, global objects, and environment settings objects:
A realm has a [[HostDefined]] field, which contains the realm's settings object.
A realm has a [[GlobalObject]] field, which contains the realm's global object.
Each global object in this specification is created during the creation of a corresponding 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.
To create a new realm in an agent agent, optionally with instructions to create a global object or a global this binding (or both), the following steps are taken:
Perform InitializeHostDefinedRealm() with the provided customizations for creating the global object and the global this binding.
Let realm execution context be the running JavaScript execution context.
This is the JavaScript execution context created in the previous step.
Remove realm execution context from the JavaScript execution context stack.
Let realm be realm execution context's Realm component.
If agent's agent cluster's cross-origin isolation mode is "none":
Let global be realm's global object.
Let status be ! global.[[Delete]]("SharedArrayBuffer").
Assert: status is true.
This is done for compatibility with web content and there is some hope that this
can be removed in the future. Web developers can still get at the constructor through
new WebAssembly.Memory({ shared:true, initial:0, maximum:0
}).buffer.constructor.
Return realm execution context.
When defining algorithm steps throughout this specification, it is often important to indicate what 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.
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 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.
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 realm, global object, and environment settings object.
When the print() method is called in response to pressing the
button in a.html:
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.
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 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 HostEnqueuePromiseJob 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:
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 starter example:
<!DOCTYPE html>
< iframe ></ iframe >
< script >
frames[ 0 ]. postMessage( "some data" , "*" );
</ 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]. This makes sense: the window
post message steps use the incumbent settings object to determine the source property of the resulting
MessageEvent, and in this case window is definitely the
source of the message.
Let us now explain how the steps given above give us our intuitively-desired result of window's relevant settings object.
When the window post message steps look up the incumbent settings
object, 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.
(Note how the environment settings object of frames[0] is
the relevant settings object of this at the time the postMessage() method is called, and thus is involved in
determining the target of the message. Whereas the incumbent is used to determine the
source.)
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 > This example is very similar to the previous one, but with an extra indirection through Function.prototype.bind as well as setTimeout(). But, the answer should be the same: invoking
algorithms asynchronously should not change the incumbent concept.
This time, the result involves more complicated mechanisms:
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 starter 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 Document, 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, also known as the "current Realm Record". [JAVASCRIPT]
Then, the current settings object is the environment settings object of the current realm.
Similarly, the current global object is the global object of the current realm.
The relevant realm for a platform object is the value of its [[Realm]] field.
Then, the relevant settings object for a platform object o is the environment settings object of the relevant realm for o.
Similarly, the relevant global object for a platform object o is the global object of the relevant realm for o.
Scripting is enabled for an environment settings object settings when all of the following conditions are true:
The user has not disabled scripting for settings at this time. (User agents may
provide users with the option to disable scripting globally, or in a finer-grained manner, e.g.,
on a per-origin basis, down to the level of individual environment settings objects.)Window object,
or settings's global object's
associated Document's active
sandboxing flag set does not have its sandboxed scripts browsing context flag
set.Scripting is disabled for an environment settings object when scripting is not enabled for it, i.e., when any of the above conditions are false.
Scripting is disabled for a platform object object if any of the following are true:
Scripting is disabled for object's relevant settings object.
The object implements Node, and object's node
document's browsing context is null.
The object implements Window and object's associated Document's browsing context is null.
Scripting is enabled for a platform object object, when object's scripting is not disabled.
An environment environment is a secure context if the following algorithm returns true:
If environment is an environment settings object:
Let global be environment's global object.
If global is a WorkerGlobalScope:
If global's owner set[0]'s relevant settings object is a secure context, then return true.
We only need to check the 0th item since they will necessarily all be consistent.
Return false.
If global is a WorkletGlobalScope, then return true.
Worklets can only be created in secure contexts.
If the result of Is url potentially trustworthy? given
environment's top-level creation URL is "Potentially
Trustworthy", then return true.
Return false.
An environment is a non-secure context if it is not a secure context.
A script is one of two possible structs (namely, a classic script or a module script). All scripts have:
An environment settings object, containing various settings that are shared with other scripts in the same context.
One of the following:
a script record, for classic scripts;
a Synthetic Module Record, for CSS module scripts, JSON module scripts, and text module scripts;
a WebAssembly Module Record, for WebAssembly module scripts; or
null, representing a parsing failure.
A JavaScript value, which has meaning only if the record is null, indicating that the corresponding script source text could not be parsed.
This value is used for internal tracking of immediate parse errors when creating scripts, and is not to be used directly. Instead, consult the error to rethrow when determining "what went wrong" for this script.
A JavaScript value representing an error that will prevent evaluation from succeeding. It will be re-thrown by any attempts to run the script.
This could be the script's parse error, but in the case of a module script it could instead be the parse error from one of its dependencies, or an error from resolve a module specifier.
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.
Null or a base URL used for resolving module specifiers. When non-null, 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.
Module scripts can be classified into four types:
A module script is a JavaScript module script if its record is a Source Text Module Record.
A module script is a CSS module script if its record is a Synthetic Module Record, and it was created via the create a CSS module script algorithm. CSS module scripts represent a parsed CSS style sheet.
A module script is a JSON module script if its record is a Synthetic Module Record, and it was created via the create a JSON module script algorithm. JSON module scripts represent a parsed JSON document.
A module script is a text module script if its record is a Synthetic Module Record, and it was created via the create a text module script algorithm. Text module scripts represent textual data encoded as UTF-8.
A module script is a WebAssembly module script if its record is a WebAssembly Module Record.
As CSS style sheets, JSON documents, and text do not import dependent modules, and do not throw exceptions on evaluation, the fetch options and base URL of CSS module scripts, JSON module scripts, and text module scripts are always null.
The active script is determined by the following algorithm:
Let record be GetActiveScriptOrModule().
If record is null, return null.
Return record.[[HostDefined]].
The active script concept is so far only used by the
import() feature, to determine the base
URL to use for resolving relative module specifiers.
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)
The referrer policy used for the initial fetch and for fetching any imported modules
This policy can mutate after a module script's response is received, to be the referrer policy parsed from the response, and used when fetching any module dependencies.
The boolean value of render-blocking used for the initial fetch and for fetching any imported modules. Unless otherwise stated, its value is false.
The priority used for the initial fetch
Recall that via the import() feature, classic scripts can import module scripts.
The default 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", credentials mode is "same-origin", referrer policy is the empty
string, and fetch priority
is "auto".
To set up the classic script request, given a request request and a script fetch options options, 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, its referrer policy to options's referrer policy, its render-blocking to options's render-blocking, and its priority to options's fetch priority.
To set up the module script request, given a request request and a script fetch options options, 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, its credentials mode to options's credentials mode, its referrer policy to options's referrer policy, its render-blocking to options's render-blocking, and its priority to options's fetch priority.
To get the descendant script fetch options given a script fetch options originalOptions, a URL url, and an environment settings object settingsObject:
Let newOptions be a copy of originalOptions.
Let integrity be the result of resolving a module integrity metadata with url and settingsObject.
Set newOptions's integrity metadata to integrity.
Set newOptions's fetch priority to "auto".
Return newOptions.
To resolve a module integrity metadata, given a URL url and an environment settings object settingsObject:
Let map be settingsObject's global object's import map.
If map's integrity[url] does not exist, then return the empty string.
Return map's integrity[url].
Several of the below algorithms can be customized with a perform the fetch
hook algorithm, which takes a request, a boolean isTopLevel, and a processCustomFetchResponse
algorithm. It runs processCustomFetchResponse with a response and either null (on failure) or a byte
sequence containing the response body. isTopLevel will be true for all classic
script fetches, and for the initial fetch when fetching an external module script graph or fetching a module worker script graph, but false for the fetches
resulting from import statements encountered throughout the graph or from
import() expressions.
By default, not supplying a perform the fetch hook 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 url, an environment settings object settingsObject, a script fetch options options, a CORS settings attribute state corsSetting, an encoding encoding, and an algorithm onComplete, run these steps. onComplete must be an algorithm accepting null (on failure) or a classic script (on success).
Let request be the result of creating a potential-CORS request given url, "script", and corsSetting.
Set request's client to settingsObject.
Set request's initiator
type to "script".
Set up the classic script request given request and options.
Fetch request with the following processResponseConsumeBody steps given response response and null, failure, or a byte sequence bodyBytes:
response can be either CORS-same-origin or CORS-cross-origin. This only affects how error reporting happens.
Set response to response's unsafe response.
If any of the following are true:
then run onComplete given null, and abort these steps.
For historical reasons, this algorithm does not include MIME type checking, unlike the other script-fetching algorithms in this section.
Let potentialMIMETypeForEncoding be the result of extracting a MIME type given response's header list.
Set encoding to the result of legacy extracting an encoding given potentialMIMETypeForEncoding and encoding.
This intentionally ignores the MIME type essence.
Let sourceText be the result of decoding bodyBytes to Unicode, using encoding as the fallback encoding.
The decode algorithm overrides encoding if the file contains a BOM.
Let mutedErrors be true if response was CORS-cross-origin, and false otherwise.
Let script be the result of creating a classic script given sourceText, settingsObject, response's URL, options, mutedErrors, and url.
To fetch a classic worker script given a URL url, an environment settings object fetchClient, a destination destination, an environment settings object settingsObject, an algorithm onComplete, and an optional perform the fetch hook performFetch, run these steps. onComplete must be an algorithm accepting null (on failure) or a classic script (on success).
Let request be a new request whose URL is url, client is fetchClient, destination is destination, initiator type is "other",
mode is "same-origin", credentials mode is "same-origin", parser
metadata is "not parser-inserted", and whose
use-URL-credentials flag is set.
If performFetch was given, run performFetch with request, true, and with processResponseConsumeBody as defined below.
Otherwise, fetch request with processResponseConsumeBody set to processResponseConsumeBody as defined below.
In both cases, let processResponseConsumeBody given response response and null, failure, or a byte sequence bodyBytes be the following algorithm:
Set response to response's unsafe response.
If any of the following are true:
then run onComplete given null, and abort these steps.
If all of the following are true:
response's URL's scheme is an HTTP(S) scheme; and
the result of extracting a MIME type from response's header list is not a JavaScript MIME type,
then run onComplete given null, and abort these steps.
Other fetch schemes are exempted from MIME type checking for historical web-compatibility reasons. We might be able to tighten this in the future; see issue #3255.
Let sourceText be the result of UTF-8 decoding bodyBytes.
Let script be the result of creating a classic script using sourceText, settingsObject, response's URL, and the default script fetch options.
Run onComplete given script.
To fetch a classic worker-imported script given a URL url, an environment settings object settingsObject, and an optional perform the fetch hook performFetch, run these steps. The algorithm will return a classic script on success, or throw an exception on failure.
Let response be null.
Let bodyBytes be null.
Let request be a new request whose URL is url, client is settingsObject, destination is "script",
initiator type is "other", parser metadata
is "not parser-inserted", and whose use-URL-credentials flag
is set.
If performFetch was given, run performFetch with request, isTopLevel, and with processResponseConsumeBody as defined below.
Otherwise, fetch request with processResponseConsumeBody set to processResponseConsumeBody as defined below.
In both cases, let processResponseConsumeBody given response res and null, failure, or a byte sequence bb be the following algorithm:
Set bodyBytes to bb.
Set response to res.
Pause until response is not null.
Unlike other algorithms in this section, the fetching process is synchronous here.
Set response to response's unsafe response.
If any of the following are true:
bodyBytes is null or failure;
the result of extracting a MIME type from response's header list is not a JavaScript MIME type,
then throw a "NetworkError" DOMException.
Let sourceText be the result of UTF-8 decoding bodyBytes.
Let mutedErrors be true if response was CORS-cross-origin, and false otherwise.
Let script be the result of creating a classic script given sourceText, settingsObject, response's URL, the default script fetch options, and mutedErrors.
Return script.
To fetch an external module script graph given a URL url, an environment settings object settingsObject, a script fetch options options, and an algorithm onComplete, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
Fetch a single module script given url, settingsObject,
"script", options, settingsObject, "client", true, and with the following steps given result:
If result is null, run onComplete given null, and abort these steps.
Fetch the descendants of
and link result given settingsObject, "script", and onComplete.
To fetch a modulepreload module script graph given a URL url, a destination destination, an environment settings object settingsObject, a script fetch options options, and an algorithm onComplete, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
Fetch a single module script given url, settingsObject,
destination, options, settingsObject, "client", true, and with the following steps given result:
Run onComplete given result.
Assert: settingsObject's global object implements
Window.
If result is not null, optionally fetch the descendants of and link result given settingsObject, destination, and an empty algorithm.
Generally, performing this step will be beneficial for performance, as it allows pre-loading the modules that will invariably be requested later, via algorithms such as fetch an external module script graph that fetch the entire graph. However, user agents might wish to skip them in bandwidth-constrained situations, or situations where the relevant fetches are already in flight.
To fetch an inline module script graph given a string sourceText, a URL baseURL, an environment settings object settingsObject, a script fetch options options, and an algorithm onComplete, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
Let script be the result of creating a JavaScript module script using sourceText, settingsObject, baseURL, and options.
Fetch the descendants of
and link script, given settingsObject, "script", and onComplete.
To fetch a module worker script graph given a URL url, an environment settings object fetchClient, a destination destination, a credentials mode credentialsMode, an environment settings object settingsObject, and an algorithm onComplete, fetch a worklet/module worker script graph given url, fetchClient, destination, credentialsMode, settingsObject, and onComplete.
To fetch a worklet script graph given a URL url, an environment settings object fetchClient, a destination destination, a credentials mode credentialsMode, an environment settings object settingsObject, a module responses map moduleResponsesMap, and an algorithm onComplete, fetch a worklet/module worker script graph given url, fetchClient, destination, credentialsMode, settingsObject, onComplete, and the following perform the fetch hook given request and processCustomFetchResponse:
Let requestURL be request's URL.
If moduleResponsesMap[requestURL] 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 moduleResponsesMap[requestURL] exists:
Let cached be moduleResponsesMap[requestURL].
Run processCustomFetchResponse with cached[0] and cached[1].
Return.
Set moduleResponsesMap[requestURL] to
"fetching".
Fetch request, with processResponseConsumeBody set to the following steps given response response and null, failure, or a byte sequence bodyBytes:
Set moduleResponsesMap[requestURL] to (response, bodyBytes).
Run processCustomFetchResponse with response and bodyBytes.
The following algorithms are meant for internal use by this specification only as part of fetching an external module script graph or other similar concepts above, and should not be used directly by other specifications.
This diagram illustrates how these algorithms relate to the ones above, as well as to each other:
To fetch a worklet/module worker script graph given a URL url, an environment settings object fetchClient, a destination destination, a credentials mode credentialsMode, an environment settings object settingsObject, an algorithm onComplete, and an optional perform the fetch hook performFetch, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
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", credentials mode is
credentialsMode, referrer
policy is the empty string, and fetch priority is "auto".
Fetch a single module script given url,
fetchClient, destination, options, settingsObject,
"client", true, and onSingleFetchComplete as defined below. If
performFetch was given, pass it along as well.
onSingleFetchComplete given result is the following algorithm:
If result is null, run onComplete given null, and abort these steps.
Fetch the descendants of and link result given fetchClient, destination, and onComplete. If performFetch was given, pass it along as well.
To fetch the descendants of and link a module script moduleScript, given an environment settings object fetchClient, a destination destination, an algorithm onComplete, and an optional perform the fetch hook performFetch, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
Let record be moduleScript's record.
If record is null:
Set moduleScript's error to rethrow to moduleScript's parse error.
Run onComplete given moduleScript.
Return.
Let state be Record { [[ErrorToRethrow]]: null, [[Destination]]: destination, [[PerformFetch]]: null, [[FetchClient]]: fetchClient }.
If performFetch was given, set state.[[PerformFetch]] to performFetch.
Let loadingPromise be record.LoadRequestedModules(state).
This step will recursively load all the module transitive dependencies.
Upon fulfillment of loadingPromise, run the following steps:
Perform record.Link().
This step will recursively call Link on all of the module's unlinked dependencies.
If this throws an exception, catch it, and set moduleScript's error to rethrow to that exception.
Run onComplete given moduleScript.
Upon rejection of loadingPromise, run the following steps:
If state.[[ErrorToRethrow]] is not null, set moduleScript's error to rethrow to state.[[ErrorToRethrow]] and run onComplete given moduleScript.
Otherwise, run onComplete given null.
state.[[ErrorToRethrow]] is null when loadingPromise is rejected due to a loading error.
To fetch a single module script, given a URL url, an environment settings object fetchClient, a destination destination, a script fetch options options, an environment settings object settingsObject, a referrer referrer, an optional ModuleRequest Record moduleRequest, a boolean isTopLevel, an algorithm onComplete, and an optional perform the fetch hook performFetch, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
Let moduleType be "javascript-or-wasm".
If moduleRequest was given, then set moduleType to the result of running the module type from module request steps given moduleRequest.
Assert: the result of running the module type allowed steps given moduleType and settingsObject is true. Otherwise, we would not have reached this point because a failure would have been raised when inspecting moduleRequest.[[Attributes]] in HostLoadImportedModule or fetch a single imported module script.
Let moduleMap be settingsObject's module map.
If moduleMap[(url, moduleType)] is a module script or null, run onComplete given moduleMap[(url, moduleType)], and return.
If moduleMap[(url, moduleType)] is a list, append onComplete to moduleMap[(url, moduleType)], and return.
Set moduleMap[(url, moduleType)] to « onComplete ».
Let request be a new request whose
URL is url, mode is "cors", referrer is referrer, and client is fetchClient.
Set request's destination to the result of running the fetch destination from module type steps given destination and moduleType.
If destination is "worker", "sharedworker", or "serviceworker", and
isTopLevel is true, then set request's mode to "same-origin".
Set request's initiator
type to "script".
Set up the module script request given request and options.
If performFetch was given, run performFetch with request, isTopLevel, and with processResponseConsumeBody as defined below.
Otherwise, fetch request with processResponseConsumeBody set to processResponseConsumeBody as defined below.
In both cases, let processResponseConsumeBody given response response and null, failure, or a byte sequence bodyBytes be the following algorithm:
response is always CORS-same-origin.
If any of the following are true:
then:
Let mimeType be the result of extracting a MIME type from response's header list.
Let moduleScript be null.
Let referrerPolicy be the result of parsing the `Referrer-Policy` header
given response. [REFERRERPOLICY]
If referrerPolicy is not the empty string, set options's referrer policy to referrerPolicy.
If mimeType's essence is
"application/wasm" and moduleType is "javascript-or-wasm", then set moduleScript to the result of
creating a WebAssembly module script given bodyBytes,
settingsObject, response's URL, and options.
Otherwise:
Let sourceText be the result of UTF-8 decoding bodyBytes.
If moduleType is "text", then set
moduleScript to the result of creating a text module script given
sourceText and settingsObject.
If mimeType is a JavaScript MIME type and moduleType
is "javascript-or-wasm", then set moduleScript to the result of
creating a JavaScript module script given sourceText,
settingsObject, response's URL, and options.
If the MIME type essence of mimeType is "text/css"
and moduleType is "css", then set moduleScript to
the result of creating a CSS module script given sourceText and
settingsObject.
If mimeType is a JSON MIME type and
moduleType is "json", then set moduleScript to
the result of creating a JSON module script given sourceText and
settingsObject.
Let callbacks be moduleMap[(url, moduleType)].
If moduleScript is null, then remove moduleMap[(url, moduleType)]; otherwise set moduleMap[(url, moduleType)] to moduleScript.
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.
For each callback of callbacks: run callback given moduleScript.
To fetch a single imported module script, given a URL url, an environment settings object fetchClient, a destination destination, a script fetch options options, environment settings object settingsObject, a referrer referrer, a ModuleRequest Record moduleRequest, an algorithm onComplete, and an optional perform the fetch hook performFetch, run these steps. onComplete must be an algorithm accepting null (on failure) or a module script (on success).
Assert: moduleRequest.[[Attributes]] does not contain any
Record entry such that entry.[[Key]] is not "type", because we only asked for "type" attributes in
HostGetSupportedImportAttributes.
Let moduleType be the result of running the module type from module request steps given moduleRequest.
If the result of running the module type allowed steps given moduleType and settingsObject is false, then run onComplete given null, and return.
Fetch a single module script given url, fetchClient, destination, options, settingsObject, referrer, moduleRequest, false, and onComplete. If performFetch was given, pass it along as well.
This standard refers to the following concepts defined there:
To create a classic script, given a string source, an environment settings object settings, a URL baseURL, a script fetch options options, an optional boolean mutedErrors (default false), an optional URL-or-null sourceURLForWindowScripts (default null), and an optional boolean bypassDisabledScripting (default false):
The bypassDisabledScripting parameter is intended to be used for running scripts even if scripting is disabled. This is required for some automation scenarios, e.g. for WebDriver BiDi command "script.evaluate".
The bypassDisabledScripting parameter is intended to be used for running scripts even if scripting is disabled. This is required for some automation scenarios, e.g. for WebDriver BiDi command "script.evaluate". When the scripting process (e.g., executing a script via a WebDriver BiDi command) is invoked with bypassDisabledScripting set to true, the event loop is allowed to run to resolve thenables (from promises) and weakmap finalizations created by that script, even if scripting is disabled. This is necessary to ensure the proper functioning of testing scripts, which often rely on asynchronous operations.
If mutedErrors is true, then set baseURL to
about:blank.
When mutedErrors is true, baseURL is the script's CORS-cross-origin response's url, which shouldn't be exposed to JavaScript. Therefore, baseURL is sanitized here.
If scripting is disabled for settings and bypassDisabledScripting is false, 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 base URL to baseURL.
Set script's fetch options to options.
Set script's muted errors to mutedErrors.
Set script's parse error and error to rethrow to null.
Record classic script creation time given script and sourceURLForWindowScripts.
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:
Set script's parse error and its error to rethrow to result[0].
Return script.
Set script's record to result.
Return script.
To create a JavaScript module script, given a string source, an environment settings object settings, a URL baseURL, and a script fetch options options:
If scripting is disabled for settings, 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 base URL to baseURL.
Set script's fetch options to options.
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:
Set script's parse error to result[0].
Return script.
Set script's record to result.
Return script.
To create a WebAssembly module script, given a byte sequence bodyBytes, an environment settings object settings, a URL baseURL, and a script fetch options options:
If scripting is disabled for settings, then set bodyBytes to the byte sequence 0x00 0x61 0x73 0x6D 0x01 0x00 0x00 0x00.
This byte sequence corresponds to an empty WebAssembly module with only the magic bytes and version number provided.
Let script be a new module script that this algorithm will subsequently initialize.
Set script's settings object to settings.
Set script's base URL to baseURL.
Set script's fetch options to options.
Set script's parse error and error to rethrow to null.
Let result be the result of parsing a WebAssembly module given bodyBytes, settings's realm, and script.
Passing script as the last parameter here ensures result.[[HostDefined]] will be script.
If the previous step threw an error error:
Set script's parse error to error.
Return script.
Set script's record to result.
Return script.
WebAssembly JavaScript Interface: ESM Integration specifies the hooks for the WebAssembly integration with ECMA-262 module loading. This includes support both for direct dependency imports, as well as for source phase imports, which support virtualization and multi-instantiation. [WASMESM]
To create a CSS module script, given a string source and an environment settings object settings:
Let script be a new module script that this algorithm will subsequently initialize.
Set script's settings object to settings.
Set script's base URL and fetch options to null.
Set script's parse error and error to rethrow to null.
Let sheet be the result of running the steps to create a constructed
CSSStyleSheet with an empty dictionary as the argument.
Run the steps to synchronously replace the rules of a CSSStyleSheet
on sheet given source.
If this throws an exception, catch it, and set script's parse error to that exception, and return script.
The steps to synchronously replace the rules of a
CSSStyleSheet will throw if source contains any @import rules. This is by-design for now because there is not yet an
agreement on how to handle these for CSS module scripts; therefore they are blocked altogether
until a consensus is reached.
Set script's record to the result of CreateDefaultExportSyntheticModule(sheet).
Return script.
To create a JSON module script, given a string source and an environment settings object settings:
Let script be a new module script that this algorithm will subsequently initialize.
Set script's settings object to settings.
Set script's base URL and fetch options to null.
Set script's parse error and error to rethrow to null.
Let result be ParseJSONModule(source).
If this throws an exception, catch it, and set script's parse error to that exception, and return script.
Set script's record to result.
Return script.
To create a text module script, given a string text and an environment settings object settings:
Let script be a new module script that this algorithm will subsequently initialize.
Set script's settings object to settings.
Set script's base URL and fetch options to null.
Set script's parse error and error to rethrow to null.
Let result be CreateTextModule(text).
Set script's record to result.
Return script.
The module type from module request steps, given a ModuleRequest Record moduleRequest, are as follows:
Let moduleType be "javascript-or-wasm".
If moduleRequest.[[Attributes]] has a Record entry such
that entry.[[Key]] is "type":
If entry.[[Value]] is "javascript-or-wasm", then set
moduleType to null.
This specification uses the "javascript-or-wasm" module
type internally for JavaScript module scripts or
WebAssembly module scripts, so this step is
needed to prevent modules from being imported using a
"javascript-or-wasm" type attribute (a null moduleType will
cause the module type allowed check to fail).
Otherwise, set moduleType to entry.[[Value]].
Return moduleType.
The module type allowed steps, given a string moduleType and an environment settings object settings, are as follows:
If moduleType is not "javascript-or-wasm", "css", "json", or "text", then
return false.
If moduleType is "css" and the
CSSStyleSheet interface is not exposed in
settings's realm, then
return false.
Return true.
The fetch destination from module type steps, given a destination defaultDestination and a string moduleType, are as follows:
json", then return "json".css", then return "style".text", then return "text".To run a classic script given a classic script script and an optional boolean rethrow errors (default 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 NormalCompletion(empty).
Record classic script execution start time given script.
Prepare to run script given settings.
Let evaluationStatus be null.
If script's error to rethrow is not null, then set evaluationStatus to ThrowCompletion(script's error to rethrow).
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:
If rethrow errors is true and script's muted errors is false:
Clean up after running script with settings.
Rethrow evaluationStatus.[[Value]].
If rethrow errors is true and script's muted errors is true:
Clean up after running script with settings.
Throw a "NetworkError" DOMException.
Otherwise, rethrow errors is false. Perform the following steps:
Report an exception given by evaluationStatus.[[Value]] for script's settings object's global object.
Clean up after running script with settings.
Return evaluationStatus.
Clean up after running script with settings.
If evaluationStatus is a normal completion, then return evaluationStatus.
If we've reached this point, evaluationStatus was left as null because the
script was aborted prematurely during evaluation.
Return ThrowCompletion(a new QuotaExceededError).
To run a module script given a module script script and an optional boolean preventErrorReporting (default 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 a promise resolved with undefined.
Record module script execution start time given script.
Prepare to run script given settings.
Let evaluationPromise be null.
If script's error to rethrow is not null, then set evaluationPromise to a promise rejected with script's error to rethrow.
Otherwise:
Let record be script's record.
Set evaluationPromise 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
evaluationPromise to a promise rejected with a new
QuotaExceededError.
If preventErrorReporting is false, then upon rejection of evaluationPromise with reason, report an exception given by reason for script's settings object's global object.
Clean up after running script with settings.
Return evaluationPromise.
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".
If scripting is disabled for settings, then return "do not run".
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.
Add settings to the surrounding agent's event loop's currently running task's script evaluation environment settings object set.
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.
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, 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.
Support in all current engines.
self.reportError(e)Dispatches an error event at the global object for the
given value e, in the same fashion as an unhandled exception.
To extract error information from a JavaScript value exception:
Let attributes be an empty map keyed by IDL attributes.
Set attributes[error] to
exception.
Set attributes[message],
attributes[filename],
attributes[lineno], and
attributes[colno] to
implementation-defined values derived from exception.
Browsers implement behavior not specified here or in the JavaScript
specification to gather values which are helpful, including in unusual cases (e.g., eval). In the future, this might be specified in greater detail.
Return attributes.
To report an exception exception which is a JavaScript value, for a particular global object global and optional boolean omitError (default false):
Let notHandled be true.
Let errorInfo be the result of extracting error information from exception.
Let script be a script found in an implementation-defined way, or null. This should usually be the running script (most notably during run a classic script).
Implementations have not yet settled on interoperable behavior for which script is used to determine whether errors are muted in less common cases.
If script is a classic script and script's muted
errors is true, then set errorInfo[error] to null, errorInfo[message] to "Script error.",
errorInfo[filename] to the empty
string, errorInfo[lineno] to 0, and
errorInfo[colno] to 0.
If omitError is true, then set errorInfo[error] to null.
If global is not in error reporting mode:
If global implements EventTarget, then set notHandled to
the result of firing an event named error at global, using ErrorEvent, with
the cancelable attribute initialized to true,
and additional attributes initialized according to errorInfo.
Returning true in an event handler cancels the event per the event handler processing algorithm.
Set global's in error reporting mode to false.
If notHandled is true:
Set errorInfo[error] to
null.
If global implements DedicatedWorkerGlobalScope, queue a
global task on the DOM manipulation task source with the
global's associated Worker's relevant global object to
run these steps:
Let workerObject be the Worker object associated with
global.
Set notHandled to the result of firing
an event named error at workerObject,
using ErrorEvent, with the cancelable attribute initialized to true,
and additional attributes initialized according to errorInfo.
If notHandled is true, then report exception for workerObject's relevant global object with omitError set to true.
The actual exception value will not be available in the owner realm, but the user agent still carries through enough information to set the message, filename, and other attributes, as well as potentially report to a developer console.
Otherwise, the user agent may report exception to a developer console.
If the implicit port connecting a worker to its Worker object has been
disentangled (i.e. if the parent worker has been terminated), then the user agent must act as if
the Worker object had no error event handler and as
if that worker's onerror attribute was
null, but must otherwise act as described above.
Thus, error reports propagate up to the chain of dedicated workers up to the
original Document, even if some of the workers along this chain have been terminated
and garbage collected.
Previous revisions of this standard defined an algorithm to report the exception. As part of issue #958, this has been superseded by report an exception which behaves differently and takes different inputs. Issue #10516 tracks updating the specification ecosystem.
The reportError(e) method steps are to
report an exception e for this.
It is unclear whether muting is applicable here. In Chrome and Safari it is muted, but in Firefox it is not. See also issue #958.
Support in all current engines.
The ErrorEvent interface is defined as follows:
[Exposed=*]
interface ErrorEvent : Event {
constructor (DOMString type , optional ErrorEventInit eventInitDict = {});
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 ;
};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. It must initially be initialized to
undefined. Where appropriate, it is set to the object representing the error (e.g., the exception
object in the case of an uncaught exception).
Support in all current engines.
In addition to synchronous runtime script errors, scripts
may experience asynchronous promise rejections, tracked via the unhandledrejection and rejectionhandled events. Tracking these
rejections is done via the HostPromiseRejectionTracker abstract operation, but
reporting them is defined here.
To notify about rejected promises given a global object global:
Let list be a clone of global's about-to-be-notified rejected promises list.
If list is empty, then return.
Queue a global task on the DOM manipulation task source given global to run the following step:
For each promise p of list:
If p.[[PromiseIsHandled]] is true, then continue.
Let notCanceled be the result of firing
an event named unhandledrejection at
global, using PromiseRejectionEvent, with the cancelable attribute initialized to true, the promise attribute initialized to
p, and the reason
attribute initialized to p.[[PromiseResult]].
If notCanceled is true, then the user agent may report p.[[PromiseResult]] to a developer console.
If p.[[PromiseIsHandled]] is false, then append p to global's outstanding rejected promises weak set.
Support in all current engines.
The PromiseRejectionEvent interface is
defined as follows:
[Exposed=*]
interface PromiseRejectionEvent : Event {
constructor (DOMString type , PromiseRejectionEventInit eventInitDict );
readonly attribute object promise ;
readonly attribute any reason ;
};
dictionary PromiseRejectionEventInit : EventInit {
required object promise ;
any reason ;
};Support in all current engines.
The promise attribute must return the value it
was initialized to. It represents the promise which this notification is about.
Because of how Web IDL conversion rules for Promise<T> types always wrap the input into a new promise, the
promise attribute is of type object instead, which is more appropriate for representing an opaque
handle to the original promise object.
Support in all current engines.
The reason attribute must return the value it
was initialized to. It represents the rejection reason for the promise.
An import map parse result is a struct that is similar to a script, and also can be stored in a script element's
result, but is not counted as a script for other purposes. It has the following items:
To create an import map parse result given a string input and a URL baseURL:
Let result be an import map parse result whose import map is null and whose error to rethrow is null.
Parse an import map string given input and baseURL, catching any exceptions. If this threw an exception, then set result's error to rethrow to that exception. Otherwise, set result's import map to the return value.
Return result.
To register an import map given a Window global and an
import map parse result result:
If result's error to rethrow is not null, then report an exception given by result's error to rethrow for global and return.
Merge existing and new import maps, given global and result's import map.
A speculation rules parse result is a struct that is similar to a script, and also can be stored in a script element's
result, but is not counted as a script for other purposes. It has the following items:
To create a speculation rules parse result given a string
input and a Document document:
Let result be a speculation rules parse result whose speculation rule set is null and whose error to rethrow is null.
Parse a speculation rule set string given input, document, and document's document base URL, catching any exceptions. If this threw an exception, then set result's error to rethrow to that exception. Otherwise, set result's speculation rule set to the return value.
Return result.
To register speculation rules given a Window global, a
speculation rules parse result result, and an optional boolean
queueErrors (default false):
If result's error to rethrow is not null:
If queueErrors is true, then queue a global task on the DOM manipulation task source given global to perform the following step:
Report an exception given by result's error to rethrow for global.
Otherwise, report an exception given by result's error to rethrow for global.
Return.
Append result's speculation rule set to global's associated Document's speculation rule sets.
Consider speculative loads for global's associated Document.
To unregister speculation rules given a Window global and a
speculation rules parse result result:
If result's error to rethrow is not null, then return.
Remove result's speculation rule set from global's associated Document's speculation rule sets.
Consider speculative loads for global's associated Document.
To update speculation rules given a Window global, a
speculation rules parse result oldResult, and a speculation rules
parse result newResult:
Remove oldResult's speculation rule set from global's associated Document's speculation rule sets.
Register speculation rules given global, newResult, and true.
When updating speculation rules,
as opposed to registering them for the first time, we ensure that any error events are queued as tasks, instead of synchronously fired.
Although synchronously executing error event handlers is OK
when inserting script elements, it's best if other modifications do not cause such
synchronous script execution.
The resolve a module specifier algorithm is the primary entry point for converting module specifier strings into URLs. When no import maps are involved, it is relatively straightforward, and reduces to resolving a URL-like module specifier.
When there is a non-empty import map present, the behavior is more complex. It checks candidate entries from all applicable module specifier maps, from most-specific to least-specific scopes (falling back to the top-level unscoped imports), and from most-specific to least-specific prefixes. For each candidate, the resolve an imports match algorithm will give one of the following results:
Successful resolution of the specifier to a URL. Then the resolve a module specifier algorithm will return that URL.
Throwing an exception. Then the resolve a module specifier algorithm will rethrow that exception, without any further fallbacks.
Failing to resolve, without an error. In this case the outer resolve a module specifier algorithm will move on to the next candidate.
In the end, if no successful resolution is found via any of the candidate module specifier maps, resolve a module specifier will throw an exception. Thus the result is always either a URL or a thrown exception.
To resolve a module specifier given a script-or-null referringScript and a string specifier:
Let settingsObject and baseURL be null.
If referringScript is not null:
Set settingsObject to referringScript's settings object.
Set baseURL to referringScript's base URL.
Otherwise:
Assert: there is a current settings object.
Set settingsObject to the current settings object.
Set baseURL to settingsObject's API base URL.
Let importMap be an empty import map.
If settingsObject's global
object implements Window, then set importMap to
settingsObject's global object's
import map.
Let serializedBaseURL be baseURL, serialized.
Let asURL be the result of resolving a URL-like module specifier given specifier and baseURL.
Let normalizedSpecifier be the serialization of asURL, if asURL is non-null; otherwise, specifier.
Let result be a URL-or-null, initially null.
For each scopePrefix → scopeImports of importMap's scopes:
If scopePrefix is serializedBaseURL, or if scopePrefix ends with U+002F (/) and scopePrefix is a code unit prefix of serializedBaseURL:
Let scopeImportsMatch be the result of resolving an imports match given normalizedSpecifier, asURL, and scopeImports.
If scopeImportsMatch is not null, then set result to scopeImportsMatch, and break.
If result is null, set result to the result of resolving an imports match given normalizedSpecifier, asURL, and importMap's imports.
If result is null, set it to asURL.
By this point, if result was null, specifier wasn't remapped to anything by importMap, but it might have been able to be turned into a URL.
If result is not null:
Add module to resolved module set given settingsObject, serializedBaseURL, normalizedSpecifier, and asURL.
Return result.
Throw a TypeError indicating that specifier was a bare specifier,
but was not remapped to anything by importMap.
To resolve an imports match, given a string normalizedSpecifier, a URL-or-null asURL, and a module specifier map specifierMap:
For each specifierKey → resolutionResult of specifierMap:
If specifierKey is normalizedSpecifier:
If resolutionResult is null, then throw a TypeError indicating
that resolution of specifierKey was blocked by a null entry.
This will terminate the entire resolve a module specifier algorithm, without any further fallbacks.
Return resolutionResult.
If all of the following are true:
specifierKey ends with U+002F (/);
specifierKey is a code unit prefix of normalizedSpecifier; and
either asURL is null, or asURL is special,
then:
If resolutionResult is null, then throw a TypeError indicating
that the resolution of specifierKey was blocked by a null entry.
This will terminate the entire resolve a module specifier algorithm, without any further fallbacks.
Let afterPrefix be the portion of normalizedSpecifier after the initial specifierKey prefix.
Assert: resolutionResult, serialized, ends with U+002F (/), as enforced during parsing.
Let url be the result of URL parsing afterPrefix with resolutionResult.
If url is failure, then throw a TypeError indicating that
resolution of normalizedSpecifier was blocked since the afterPrefix
portion could not be URL-parsed relative to the resolutionResult mapped to by
the specifierKey prefix.
This will terminate the entire resolve a module specifier algorithm, without any further fallbacks.
If the serialization of
resolutionResult is not a code unit prefix of the serialization of url, then throw a
TypeError indicating that the resolution of normalizedSpecifier was
blocked due to it backtracking above its prefix specifierKey.
This will terminate the entire resolve a module specifier algorithm, without any further fallbacks.
Return url.
Return null.
The resolve a module specifier algorithm will fall back to a
less-specific scope, or to "imports", if possible.
To resolve a URL-like module specifier, given a string specifier and a URL baseURL:
If specifier starts with "/", "./", or "../":
Let url be the result of URL parsing specifier with baseURL.
If url is failure, then return null.
One way this could happen is if specifier is "../foo" and baseURL is a data: URL.
Return url.
This includes cases where specifier starts with "//", i.e., scheme-relative URLs. Thus, url might end up with a
different host than baseURL.
Let url be the result of URL parsing specifier (with no base URL).
If url is failure, then return null.
Return url.
An import map allows control over module specifier resolution. Import maps
are delivered via inline script elements with their type attribute set to "importmap", and
with their child text content containing a JSON representation of the import
map.
A Document can have multiple import maps processed, which can happen either
before or after any modules have been imported, e.g., via import() expressions or
script elements with their type attribute set
to "module". The merge existing and new import maps algorithm
ensures that new import maps cannot define the module resolution for modules that were already
defined by past import maps, or for ones that were already resolved.
The simplest use of import maps is to globally remap a bare module specifier:
{
"imports" : {
"moment" : "/node_modules/moment/src/moment.js"
}
} This enables statements like import moment from "moment";
to work, fetching and evaluating the JavaScript module at the /node_modules/moment/src/moment.js URL.
An import map can remap a class of module specifiers into a class of URLs by using trailing slashes, like so:
{
"imports" : {
"moment/" : "/node_modules/moment/src/"
}
} This enables statements like import localeData from
"moment/locale/zh-cn.js"; to work, fetching and evaluating the JavaScript module at the
/node_modules/moment/src/locale/zh-cn.js URL. Such trailing-slash
mappings are often combined with bare-specifier mappings, e.g.
{
"imports" : {
"moment" : "/node_modules/moment/src/moment.js" ,
"moment/" : "/node_modules/moment/src/"
}
} so that both the "main module" specified by "moment" and the
"submodules" specified by paths such as "moment/locale/zh-cn.js" are
available.
Bare specifiers are not the only type of module specifiers which import maps can remap.
"URL-like" specifiers, i.e., those that are either parseable as absolute URLs or start with
"/", "./", or "../", can be
remapped as well:
{
"imports" : {
"https://cdn.example.com/vue/dist/vue.runtime.esm.js" : "/node_modules/vue/dist/vue.runtime.esm.js" ,
"/js/app.mjs" : "/js/app-8e0d62a03.mjs" ,
"../helpers/" : "https://cdn.example/helpers/"
}
} Note how the URL to be remapped, as well as the URL being mapped to, can be specified either
as absolute URLs, or as relative URLs starting with "/", "./", or "../". (They cannot be specified as relative
URLs without those starting sigils, as those help distinguish from bare module specifiers.) Also
note how the trailing slash mapping works in
this context as well.
Such remappings operate on the post-canonicalization URL, and do not require a match between
the literal strings supplied in the import map key and the imported module specifier. So for
example, if this import map was included on https://example.com/app.html,
then not only would import "/js/app.mjs" be remapped, but so
would import "./js/app.mjs" and import "./foo/../js/app.mjs".
All previous examples have globally remapped module specifiers, by using the top-level "imports" key in the import map. The top-level "scopes"
key can be used to provide localized remappings, which only apply when the referring module
matches a specific URL prefix. For example:
{
"scopes" : {
"/a/" : {
"moment" : "/node_modules/moment/src/moment.js"
},
"/b/" : {
"moment" : "https://cdn.example.com/moment/src/moment.js"
}
}
} With this import map, the statement import "moment" will have
different meanings depending on which referrer script contains the statement:
Inside scripts located under /a/, this will import /node_modules/moment/src/moment.js.
Inside scripts located under /b/, this will import https://cdn.example.com/moment/src/moment.js.
Inside scripts located under /c/, this will fail to resolve and
thus throw an exception.
A typical usage of scopes is to allow multiple versions of the "same" module to exist in a web application, with some parts of the module graph importing one version, and other parts importing another version.
Scopes can overlap each other, and overlap the global "imports"
specifier map. At resolution time, scopes are consulted in order of most- to least-specific,
where specificity is measured by sorting the scopes using the code unit less than
operation. So, for example, "/scope2/scope3/" is treated as more specific
than "/scope2/", which is treated as more specific than the top-level
(unscoped) mappings.
The following import map illustrates this:
{
"imports" : {
"a" : "/a-1.mjs" ,
"b" : "/b-1.mjs" ,
"c" : "/c-1.mjs"
},
"scopes" : {
"/scope2/" : {
"a" : "/a-2.mjs"
},
"/scope2/scope3/" : {
"b" : "/b-3.mjs"
}
}
} This results in the following resolutions (using relative URLs for brevity):
| Specifier | ||||
|---|---|---|---|---|
"a"
| "b"
| "c"
| ||
| Referrer | /scope1/r.mjs
| /a-1.mjs
| /b-1.mjs
| /c-1.mjs
|
/scope2/r.mjs
| /a-2.mjs
| /b-1.mjs
| /c-1.mjs
| |
/scope2/scope3/r.mjs
| /a-2.mjs
| /b-3.mjs
| /c-1.mjs
| |
Import maps can also be used to provide modules with integrity metadata to be used in Subresource Integrity checks. [SRI]
The following import map illustrates this:
{
"imports" : {
"a" : "/a-1.mjs" ,
"b" : "/b-1.mjs" ,
"c" : "/c-1.mjs"
},
"integrity" : {
"/a-1.mjs" : "sha384-Li9vy3DqF8tnTXuiaAJuML3ky+er10rcgNR/VqsVpcw+ThHmYcwiB1pbOxEbzJr7" ,
"/d-1.mjs" : "sha384-MBO5IDfYaE6c6Aao94oZrIOiC6CGiSN2n4QUbHNPhzk5Xhm0djZLQqTpL0HzTUxk"
}
} The above example provides integrity metadata to be enforced on the modules /a-1.mjs and /d-1.mjs, even if the latter is not defined
as an import in the map.
The child text content of a script element representing an
import map must match the following import map authoring
requirements:
It must be valid JSON. [JSON]
The JSON must represent a JSON object, with at most the three keys "imports", "scopes", and "integrity".
The values corresponding to the "imports", "scopes", and "integrity" keys, if present,
must themselves be JSON objects.
The value corresponding to the "imports" key, if present, must be
a valid module specifier map.
The value corresponding to the "scopes" key, if present, must be a
JSON object, whose keys are valid URL strings and whose
values are valid module specifier
maps.
The value corresponding to the "integrity" key, if present, must
be a JSON object, whose keys are valid URL strings and
whose values fit the requirements of the integrity attribute.
A valid module specifier map is a JSON object that meets the following requirements:
All of its keys must be nonempty.
All of its values must be strings.
Each value must be either a valid absolute URL or a
valid URL string that starts with "/", "./", or "../".
If a given key ends with "/", then the corresponding
value must also.
Formally, an import map is a struct with three items:
imports, a module specifier map;
scopes, an ordered map of URLs to module specifier maps; and
integrity, a module integrity map.
A module specifier map is an ordered map whose keys are strings and whose values are either URLs or nulls.
A module integrity map is an ordered map whose keys are URLs and whose values are strings that will be used as integrity metadata.
An empty import map is an import map with its imports and scopes both being empty maps.
A specifier resolution record is a struct. It has the following items:
Implementations can replace specifier as a URL with a boolean that indicates that the specifier is either bare or URL-like that is special.
To add module to resolved module set given an environment settings object settingsObject, a string serializedBaseURL, a string normalizedSpecifier, and a URL-or-null asURL:
Let global be settingsObject's global object.
If global does not implement Window, then return.
Let record be a new specifier resolution record, with serialized base URL set to serializedBaseURL, specifier set to normalizedSpecifier, and specifier as a URL set to asURL.
Append record to global's resolved module set.
To parse an import map string, given a string input and a URL baseURL:
Let parsed be the result of parsing a JSON string to an Infra value given input.
If parsed is not an ordered map, then throw a
TypeError indicating that the top-level value needs to be a JSON object.
Let sortedAndNormalizedImports be an empty ordered map.
If parsed["imports"] exists:
If parsed["imports"] is not an ordered
map, then throw a TypeError indicating that the value for the "imports" top-level key needs to be a JSON object.
Set sortedAndNormalizedImports to the result of sorting and
normalizing a module specifier map given parsed["imports"] and baseURL.
Let sortedAndNormalizedScopes be an empty ordered map.
If parsed["scopes"] exists:
If parsed["scopes"] is not an ordered
map, then throw a TypeError indicating that the value for the "scopes" top-level key needs to be a JSON object.
Set sortedAndNormalizedScopes to the result of sorting and normalizing
scopes given parsed["scopes"] and
baseURL.
Let normalizedIntegrity be an empty ordered map.
If parsed["integrity"] exists:
If parsed["integrity"] is not an ordered
map, then throw a TypeError indicating that the value for the "integrity" top-level key needs to be a JSON object.
Set normalizedIntegrity to the result of normalizing a module
integrity map given parsed["integrity"] and
baseURL.
If parsed's keys contains any items besides "imports", "scopes", or "integrity", then the user agent should
report a warning to the console indicating that an invalid top-level key was
present in the import map.
This can help detect typos. It is not an error, because that would prevent any future extensions from being added backward-compatibly.
Return an import map whose imports are sortedAndNormalizedImports, whose scopes are sortedAndNormalizedScopes, and whose integrity are normalizedIntegrity.
The import map that results from this parsing algorithm is highly normalized.
For example, given a base URL of https://example.com/base/page.html, the
input
{
"imports" : {
"/app/helper" : "node_modules/helper/index.mjs" ,
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} will generate an import map with imports of
«[
"https://example.com/app/helper" → https://example.com/base/node_modules/helper/index.mjs
"lodash" → https://example.com/node_modules/lodash-es/lodash.js
]»and (despite nothing being present in the input string) an empty ordered map for its scopes.
To merge module specifier maps, given a module specifier map newMap and a module specifier map oldMap:
Let mergedMap be a deep copy of oldMap.
For each specifier → url of newMap:
If specifier exists in oldMap:
The user agent may report a warning to the console indicating the ignored rule. They may choose to avoid reporting if the rule is identical to an existing one.
Set mergedMap[specifier] to url.
Return mergedMap.
To merge existing and new import maps, given a global object global and an import map newImportMap:
Let newImportMapScopes be a deep copy of newImportMap's scopes.
We're mutating these copies and removing items from them when they are used to ignore scope-specific rules. This is true for newImportMapScopes, as well as to newImportMapImports below.
Let oldImportMap be global's import map.
Let newImportMapImports be a deep copy of newImportMap's imports.
For each scopePrefix → scopeImports of newImportMapScopes:
For each record of global's resolved module set:
If scopePrefix is record's serialized base URL, or if scopePrefix ends with U+002F (/) and scopePrefix is a code unit prefix of record's serialized base URL:
For each specifierKey → resolutionResult of scopeImports:
If specifierKey is record's specifier, or if all of the following conditions are true:
specifierKey ends with U+002F (/);
specifierKey is a code unit prefix of record's specifier;
either record's specifier as a URL is null or is special,
then:
The user agent may report a warning to the console indicating the ignored rule. They may choose to avoid reporting if the rule is identical to an existing one.
Remove scopeImports[specifierKey].
Implementers are encouraged to implement a more efficient matching algorithm when working with the resolved module set. As guidance, the number of resolved/mapped modules in a large application can be on the order of thousands.
If scopePrefix exists in oldImportMap's scopes, then set oldImportMap's scopes[scopePrefix] to the result of merging module specifier maps, given scopeImports and oldImportMap's scopes[scopePrefix].
Otherwise, set oldImportMap's scopes[scopePrefix] to scopeImports.
For each url → integrity of newImportMap's integrity:
If url exists in oldImportMap's integrity:
The user agent may report a warning to the console indicating the ignored rule. They may choose to avoid reporting if the rule is identical to an existing one.
Set oldImportMap's integrity[url] to integrity.
For each record of global's resolved module set:
For each specifier → url of newImportMapImports:
If specifier starts with record's specifier:
The user agent may report a warning to the console indicating the ignored rule. They may choose to avoid reporting if the rule is identical to an existing one.
Remove newImportMapImports[specifier].
Set oldImportMap's imports to the result of merge module specifier maps, given newImportMapImports and oldImportMap's imports.
The above algorithm merges a new import map into the given environment settings object's global object's import map. Let's examine a few examples:
When the new import map impacts an already resolved module, that rule gets dropped from the import map.
So, if the resolved module set already contains the "/app/helper", the following new import map:
{
"imports" : {
"/app/helper" : "./helper/index.mjs" ,
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} Would be equivalent to the following one:
{
"imports" : {
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} The same is true for rules that impact already resolved modules defined in specific scopes.
If we already resolved "/app/helper" from "/app/main.mjs" the following new import map:
{
"scopes" : {
"/app/" : {
"/app/helper" : "./helper/index.mjs"
}
},
"imports" : {
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} Would similarly be equivalent to:
{
"imports" : {
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} We could also have cases where a single already-resolved module specifier has multiple rules for its resolution, depending on the referring script. In such cases, only the relevant rules would not be added to the map.
For example, if we already resolved "/app/helper" from "/app/vendor/main.mjs", the following new import map:
{
"scopes" : {
"/app/" : {
"/app/helper" : "./helper/index.mjs"
},
"/app/vendor/" : {
"/app/" : "./vendor_helper/"
},
"/vendor/" : {
"/app/helper" : "./helper/vendor_index.mjs"
}
},
"imports" : {
"lodash" : "/node_modules/lodash-es/lodash.js"
"/app/" : "./general_app_path/"
"/app/helper" : "./other_path/helper/index.mjs"
}
} Would be equivalent to:
{
"scopes" : {
"/vendor/" : {
"/app/helper" : "./helper/vendor_index.mjs"
}
},
"imports" : {
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} This is achieved by the fact that the merge algorithm tracks already resolved modules and removes rules affecting them from new import maps before they are merged into the existing one.
When the new import map has conflicting rules to the existing import map, with no impacted already resolved modules, the existing import map rules persist.
For example, the following existing and new import maps:
{
"imports" : {
"/app/helper" : "./helper/index.mjs" ,
"lodash" : "/node_modules/lodash-es/lodash.js"
}
} {
"imports" : {
"/app/helper" : "./main/helper/index.mjs"
}
} Would be equivalent to the following single import map:
{
"imports" : {
"/app/helper" : "./helper/index.mjs" ,
"lodash" : "/node_modules/lodash-es/lodash.js" ,
}
} To sort and normalize a module specifier map, given an ordered map originalMap and a URL baseURL:
Let normalized be an empty ordered map.
For each specifierKey → value of originalMap:
Let normalizedSpecifierKey be the result of normalizing a specifier key given specifierKey and baseURL.
If normalizedSpecifierKey is null, then continue.
If value is not a string:
The user agent may report a warning to the console indicating that addresses need to be strings.
Set normalized[normalizedSpecifierKey] to null.
Let addressURL be the result of resolving a URL-like module specifier given value and baseURL.
If addressURL is null:
The user agent may report a warning to the console indicating that the address was invalid.
Set normalized[normalizedSpecifierKey] to null.
If specifierKey ends with U+002F (/), and the serialization of addressURL does not end with U+002F (/):
The user agent may report a warning to the console indicating that an invalid address was given for the specifier key specifierKey; since specifierKey ends with a slash, the address needs to as well.
Set normalized[normalizedSpecifierKey] to null.
Set normalized[normalizedSpecifierKey] to addressURL.
Return the result of sorting in descending order normalized, with an entry a being less than an entry b if a's key is code unit less than b's key.
To sort and normalize scopes, given an ordered map originalMap and a URL baseURL:
Let normalized be an empty ordered map.
For each scopePrefix → potentialSpecifierMap of originalMap:
If potentialSpecifierMap is not an ordered map, then throw a
TypeError indicating that the value of the scope with prefix
scopePrefix needs to be a JSON object.
Let scopePrefixURL be the result of URL parsing scopePrefix with baseURL.
If scopePrefixURL is failure:
The user agent may report a warning to the console that the scope prefix URL was not parseable.
Let normalizedScopePrefix be the serialization of scopePrefixURL.
Set normalized[normalizedScopePrefix] to the result of sorting and normalizing a module specifier map given potentialSpecifierMap and baseURL.
Return the result of sorting in descending order normalized, with an entry a being less than an entry b if a's key is code unit less than b's key.
In the above two algorithms, sorting keys and scopes in descending order has the
effect of putting "foo/bar/" before "foo/". This in
turn gives "foo/bar/" a higher priority than "foo/"
during module specifier resolution.
To normalize a module integrity map, given an ordered map originalMap:
Let normalized be an empty ordered map.
For each key → value of originalMap:
Let resolvedURL be the result of resolving a URL-like module specifier given key and baseURL.
Unlike "imports", keys of the integrity map are treated
as URLs, not module specifiers. However, we use the resolve a URL-like module specifier algorithm to prohibit "bare" relative
URLs like foo, which could be mistaken for module specifiers.
If resolvedURL is null:
The user agent may report a warning to the console indicating that the key failed to resolve.
If value is not a string:
The user agent may report a warning to the console indicating that