Copyright © 2015 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and document use rules apply.
The bulk of the text of this specification is also available in the WHATWG HTML specification, under a license that permits reuse of the specification text.
This specification defines two mechanisms for communicating between browsing contexts in HTML documents.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This specification is the 7 April 2015 Proposed Recommendation of HTML5 Web Messaging. The W3C Web Applications Working Group is the W3C working group responsible for this specification's progress along the W3C Recommendation track. This document is intended to become a W3C Recommendation. A Candidate Recommendation (CR) of the specification was published on 01 May 2012.
The following changes were made to this specification after the Candidate Recommendation was published:
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MessageEvent
interfacesAll diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC2119. For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]
Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.
Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on user agents.
Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)
The only conformance class defined by this specification is user agents.
User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.
When support for a feature is disabled (e.g. as an emergency measure to mitigate a security problem, or to aid in development, or for performance reasons), user agents must act as if they had no support for the feature whatsoever, and as if the feature was not mentioned in this specification. For example, if a particular feature is accessed via an attribute in a Web IDL interface, the attribute itself would be omitted from the objects that implement that interface — leaving the attribute on the object but making it return null or throw an exception is insufficient.
This specification relies on several other underlying specifications.
Many fundamental concepts from HTML are used by this specification. [HTML]
The IDL blocks in this specification use the semantics of the WebIDL specification. [WEBIDL]
The construction "a Foo
object", where
Foo
is actually an interface, is sometimes
used instead of the more accurate "an object implementing the
interface Foo
".
The term DOM is used to refer to the API set made available to
scripts in Web applications, and does not necessarily imply the
existence of an actual Document
object or of any other
Node
objects as defined in the DOM specifications. [DOM]
An IDL attribute is said to be getting when its value is being retrieved (e.g. by author script), and is said to be setting when a new value is assigned to it.
MessageEvent
interfacesMessages in server-sent events, Web sockets, cross-document
messaging, channel messaging, and broadcast channels use the
MessageEvent
interface for their message
events:
[Constructor(DOMString type, optional MessageEventInit eventInitDict), Exposed=(Window,Worker)] interface MessageEvent : Event { readonly attribute any data; readonly attribute DOMString origin; readonly attribute DOMString lastEventId; readonly attribute (WindowProxy or MessagePort)? source; readonly attribute MessagePort[]? ports; void initMessageEvent(DOMString typeArg, boolean canBubbleArg, boolean cancelableArg, any dataArg, DOMString originArg, DOMString lastEventIdArg, (WindowProxy or MessagePort) sourceArg, sequence<MessagePort>? portsArg); }; dictionary MessageEventInit : EventInit { any data; DOMString origin; DOMString lastEventId; (WindowProxy or MessagePort)? source; sequence<MessagePort> ports; };
data
Returns the data of the message.
origin
Returns the origin of the message, for server-sent events and cross-document messaging.
lastEventId
Returns the last event ID string, for server-sent events.
source
Returns the WindowProxy
of the source window, for cross-document
messaging, and the MessagePort
being attached, in the connect
event fired at
SharedWorkerGlobalScope
objects.
ports
Returns the MessagePort
array sent with the message, for cross-document
messaging and channel messaging.
Open Bugs 27128
The data
attribute must return the value
it was initialised to. When the object is created, this attribute must be initialised to null. It
represents the message being sent.
The origin
attribute must return the
value it was initialised to. When the object is created, this attribute must be initialised to the
empty string. It represents, in server-sent events and cross-document
messaging, the origin of the document that sent the message (typically the
scheme, hostname, and port of the document, but not its path or fragment identifier).
The lastEventId
attribute must
return the value it was initialised to. When the object is created, this attribute must be
initialised to the empty string. It represents, in server-sent events, the last event ID string of the event source.
The source
attribute must return the
value it was initialised to. When the object is created, this attribute must be initialised to
null. It represents, in cross-document messaging, the WindowProxy
of the
browsing context of the Window
object from which the message came; and
in the connect
events used by shared workers, the newly connecting
MessagePort
.
The ports
attribute must return the
value it was initialised to. When the object is created, this attribute must be initialised to
null. It represents, in
cross-document messaging and channel messaging, the
MessagePort
array being sent, if any.
The initMessageEvent()
method must initialise the event in a manner analogous to the similarly-named initEvent()
method. [DOM]
Web browsers, for security and privacy reasons, prevent documents in different domains from affecting each other; that is, cross-site scripting is disallowed.
While this is an important security feature, it prevents pages from different domains from communicating even when those pages are not hostile. This section introduces a messaging system that allows documents to communicate with each other regardless of their source domain, in a way designed to not enable cross-site scripting attacks.
The task source for the tasks in cross-document messaging is the posted message task source.
This section is non-normative.
For example, if document A contains an iframe
element that contains document B,
and script in document A calls postMessage()
on the
Window
object of document B, then a message event will be fired on that object,
marked as originating from the Window
of document A. The script in document A might
look like:
var o = document.getElementsByTagName('iframe')[0]; o.contentWindow.postMessage('Hello world', 'http://b.example.org/');
To register an event handler for incoming events, the script would use addEventListener()
(or similar mechanisms). For example, the script in document B
might look like:
window.addEventListener('message', receiver, false); function receiver(e) { if (e.origin == 'http://example.com') { if (e.data == 'Hello world') { e.source.postMessage('Hello', e.origin); } else { alert(e.data); } } }
This script first checks the domain is the expected domain, and then looks at the message, which it either displays to the user, or responds to by sending a message back to the document which sent the message in the first place.
Use of this API requires extra care to protect users from hostile entities abusing a site for their own purposes.
Authors should check the origin
attribute to
ensure that messages are only accepted from domains that they expect to receive messages from.
Otherwise, bugs in the author's message handling code could be exploited by hostile sites.
Furthermore, even after checking the origin
attribute, authors should also check that the data in question is of the expected format.
Otherwise, if the source of the event has been attacked using a cross-site scripting flaw, further
unchecked processing of information sent using the postMessage()
method could result in the attack being
propagated into the receiver.
Authors should not use the wildcard keyword (*) in the targetOrigin argument in messages that contain any confidential information, as otherwise there is no way to guarantee that the message is only delivered to the recipient to which it was intended.
Authors who accept messages from any origin are encouraged to consider the risks of a denial-of-service attack. An attacker could send a high volume of messages; if the receiving page performs expensive computation or causes network traffic to be sent for each such message, the attacker's message could be multplied into a denial-of-service attack. Authors are encouraged to employ rate limiting (only accepting a certain number of messages per minute) to make such attacks impractical.
The integrity of this API is based on the inability for scripts of one origin to
post arbitrary events (using dispatchEvent()
or otherwise) to objects in
other origins (those that are not the same).
Implementors are urged to take extra care in the implementation of this feature. It allows authors to transmit information from one domain to another domain, which is normally disallowed for security reasons. It also requires that UAs be careful to allow access to certain properties but not others.
User agents are also encouraged to consider rate-limiting message traffic between different origins, to protect naïve sites from denial-of-service attacks.
postMessage
(message, targetOrigin [, transfer ] )Posts a message to the given window. Messages can be structured objects, e.g. nested objects
and arrays, can contain JavaScript values (strings, numbers, Date
objects, etc), and can
contain certain data objects such as File
Blob
, FileList
,
and ArrayBuffer
objects.
Objects listed in transfer are transferred, not just cloned, meaning that they are no longer usable on the sending side.
If the origin of the target window doesn't match the given origin, the message is discarded,
to avoid information leakage. To send the message to the target regardless of origin, set the
target origin to "*
". To restrict the message to same-origin targets only,
without needing to explicitly state the origin, set the target origin to "/
".
Throws a DataCloneError
exception if transfer array contains
duplicate objects or if message could not be cloned.
When posting a message to a Window
of a browsing context
that has just been navigated to a new Document
is likely to result in the message not
receiving its intended recipient: the scripts in the target browsing context have to
have had time to set up listeners for the messages. Thus, for instance, in situations where a
message is to be sent to the Window
of newly created child iframe
,
authors are advised to have the child Document
post a message to their parent
announcing their readiness to receive messages, and for the parent to wait for this message before
beginning posting messages.
Open Bugs 18242
When a script invokes the postMessage(message, targetOrigin, transfer)
method (with two or three arguments) on a
Window
object, the user agent must follow these steps:
If the value of the targetOrigin argument is neither a single U+002A
ASTERISK character (*), a single U+002F SOLIDUS character (/), nor an absolute URL,
then throw a SyntaxError
exception and abort the overall set of steps.
Let new ports be an empty array.
Let transfer map be an empty association list of
Transferable
objects to placeholder objects.
If the method was invoked with a third argument transfer, run these substeps:
If any object is listed in transfer more than once, or any of the
Transferable
objects listed in transfer are marked as neutered, then throw a
DataCloneError
exception and abort these steps.
For each object x in transfer in turn, add a
mapping from x to a new unique placeholder object created for x to transfer map, and if x is a
MessagePort
object, also append the placeholder object to the new
ports array.
Let message clone be the result of obtaining a structured clone of the message argument, with transfer map as the transfer map. If this throws an exception, then throw that exception and abort these steps.
If the method was invoked with a third argument transfer, run these substeps:
Let new owner be the environment settings object of the
Window
object on which the method was invoked.
For each object x in transfer in turn, obtain a new object y by transferring the object x to new owner, and replace the placeholder object that was created for the object x by the new object y wherever the placeholder exists (i.e. in message clone and in new ports).
Make new ports into a read only array.
Return from the postMessage()
method, but in parallel continue running these steps.
If the targetOrigin argument is a single literal U+002F SOLIDUS
character (/), and the Document
of the Window
object on which the
method was invoked does not have the same origin as the responsible
document specified by the entry settings object, then abort these steps
silently.
Otherwise, if the targetOrigin argument is an absolute URL,
and the Document
of the Window
object on which the method was invoked
does not have the same origin as targetOrigin, then abort
these steps silently.
Otherwise, the targetOrigin argument is a single literal U+002A ASTERISK character (*), and no origin check is made.
Create a trusted event that uses the
MessageEvent
interface, with the event type message
, which does not bubble, is not cancelable, and has no
default action. The data
attribute must be
initialised to the value of message clone, the origin
attribute must be initialised to the Unicode serialisation of the
origin specified by the incumbent settings object, the source
attribute must be initialised to the
WindowProxy
object corresponding to the global object (a
Window
object) specified by the incumbent settings object, and the
ports
attribute must be initialised to the new ports array.
Queue a task to dispatch the
event created in the previous step at the Window
object on which the method was
invoked. The task source for this task is the
posted message task source.
This section is non-normative.
To enable independent pieces of code (e.g. running in different browsing contexts) to communicate directly, authors can use channel messaging.
Communication channels in this mechanism are implemented as two-ways pipes, with a port at each end. Messages sent in one port are delivered at the other port, and vice-versa. Messages are delivered as DOM events, without interrupting or blocking running tasks.
To create a connection (two "entangled" ports), the MessageChannel()
constructor is called:
var channel = new MessageChannel();
One of the ports is kept as the local port, and the other port is sent to the remote code, e.g.
using postMessage()
:
otherWindow.postMessage('hello', 'http://example.com', [channel.port2]);
To send messages, the postMessage()
method on
the port is used:
channel.port1.postMessage('hello');
To receive messages, one listens to message
events:
channel.port1.onmessage = handleMessage; function handleMessage(event) { // message is in event.data // ... }
Data sent on a port can be structured data; for example here an array of strings is passed on a
MessagePort
:
port1.postMessage(['hello', 'world']);
This section is non-normative.
In this example, two JavaScript libraries are connected to each other using
MessagePort
s. This allows the libraries to later be hosted in different frames, or
in Worker
objects, without any change to the APIs.
<script src="contacts.js"></script> <!-- exposes a contacts object --> <script src="compose-mail.js"></script> <!-- exposes a composer object --> <script> var channel = new MessageChannel(); composer.addContactsProvider(channel.port1); contacts.registerConsumer(channel.port2); </script>
Here's what the "addContactsProvider()" function's implementation could look like:
function addContactsProvider(port) { port.onmessage = function (event) { switch (event.data.messageType) { 'search-result': handleSearchResult(event.data.results); break; 'search-done': handleSearchDone(); break; 'search-error': handleSearchError(event.data.message); break; // ... } }; };
Alternatively, it could be implemented as follows:
function addContactsProvider(port) { port.addEventListener('message', function (event) { if (event.data.messageType == 'search-result') handleSearchResult(event.data.results); }); port.addEventListener('message', function (event) { if (event.data.messageType == 'search-done') handleSearchDone(); }); port.addEventListener('message', function (event) { if (event.data.messageType == 'search-error') handleSearchError(event.data.message); }); // ... port.start(); };
The key difference is that when using addEventListener()
, the start()
method must also be invoked. When using onmessage
, the call to start()
is implied.
The start()
method, whether called explicitly or
implicitly (by setting onmessage
), starts the
flow of messages: messages posted on message ports are initially paused, so that they don't get
dropped on the floor before the script has had a chance to set up its handlers.
This section is non-normative.
Ports can be viewed as a way to expose limited capabilities (in the object-capability model sense) to other actors in the system. This can either be a weak capability system, where the ports are merely used as a convenient model within a particular origin, or as a strong capability model, where they are provided by one origin provider as the only mechanism by which another origin consumer can effect change in or obtain information from provider.
For example, consider a situation in which a social Web site embeds in one iframe
the user's e-mail contacts provider (an address book site, from a second origin), and in a second
iframe
a game (from a third origin). The outer social site and the game in the second
iframe
cannot access anything inside the first iframe
; together they can
only:
iframe
to a new URL, such as the same
URL but with a different fragment identifier, causing the Window
in the
iframe
to receive a hashchange
event.iframe
, causing the Window
in the iframe
to
receive a resize
event.message
event to the Window
in the
iframe
using the window.postMessage()
API.The contacts provider can use these methods, most particularly the third one, to provide an API
that can be accessed by other origins to manipulate the user's address book. For example, it could
respond to a message "add-contact Guillaume Tell
<tell@pomme.example.net>
" by adding the given person and e-mail address to the user's
address book.
To avoid any site on the Web being able to manipulate the user's contacts, the contacts provider might only allow certain trusted sites, such as the social site, to do this.
Now suppose the game wanted to add a contact to the user's address book, and that the social site was willing to allow it to do so on its behalf, essentially "sharing" the trust that the contacts provider had with the social site. There are several ways it could do this; most simply, it could just proxy messages between the game site and the contacts site. However, this solution has a number of difficulties: it requires the social site to either completely trust the game site not to abuse the privilege, or it requires that the social site verify each request to make sure it's not a request that it doesn't want to allow (such as adding multiple contacts, reading the contacts, or deleting them); it also requires some additional complexity if there's ever the possibility of multiple games simultaneously trying to interact with the contacts provider.
Using message channels and MessagePort
objects, however, all of these problems can
go away. When the game tells the social site that it wants to add a contact, the social site can
ask the contacts provider not for it to add a contact, but for the capability to add a
single contact. The contacts provider then creates a pair of MessagePort
objects, and
sends one of them back to the social site, who forwards it on to the game. The game and the
contacts provider then have a direct connection, and the contacts provider knows to only honor a
single "add contact" request, nothing else. In other words, the game has been granted the
capability to add a single contact.
This section is non-normative.
Continuing the example from the previous section, consider the contacts provider in particular.
While an initial implementation might have simply used XMLHttpRequest
objects in the
service's iframe
, an evolution of the service might instead want to use a shared worker with a single WebSocket
connection.
If the initial design used MessagePort
objects to grant capabilities, or even just
to allow multiple simultaneous independent sessions, the service implementation can switch from
the XMLHttpRequest
s-in-each-iframe
model to the
shared-WebSocket
model without changing the API at all: the ports on the service
provider side can all be forwarded to the shared worker without it affecting the users of the API
in the slightest.
[Constructor, Exposed=(Window,Worker)] interface MessageChannel { readonly attribute MessagePort port1; readonly attribute MessagePort port2; };
MessageChannel
()Returns a new MessageChannel
object with two new MessagePort
objects.
port1
Returns the first MessagePort
object.
port2
Returns the second MessagePort
object.
When the MessageChannel()
constructor is
called, it must run the following algorithm:
Create a new MessagePort
object whose owner is the incumbent settings object, and let
port1 be that object.
Create a new MessagePort
object whose owner is the incumbent settings object, and let
port2 be that object.
Entangle the port1 and port2 objects.
Instantiate a new MessageChannel
object, and let channel
be that object.
Let the port1
attribute of the channel object be port1.
Let the port2
attribute of the channel object be port2.
Return channel.
The port1
and port2
attributes must return the values they were
assigned when the MessageChannel
object was created.
Each channel has two message ports. Data sent through one port is received by the other port, and vice versa.
[Exposed=(Window,Worker)] interface MessagePort : EventTarget { void postMessage(any message, optional sequence<Transferable> transfer); void start(); void close(); // event handlers attribute EventHandler onmessage; }; // MessagePort implements Transferable;
postMessage
(message [, transfer] )Posts a message through the channel. Objects listed in transfer are transferred, not just cloned, meaning that they are no longer usable on the sending side.
Throws a DataCloneError
exception if transfer array contains
duplicate objects or the source or target ports, or if message could not be
cloned.
start
()Begins dispatching messages received on the port.
close
()Disconnects the port, so that it is no longer active.
Each MessagePort
object can be entangled with another (a symmetric relationship).
Each MessagePort
object also has a task source called the port
message queue, initially empty. A port message queue can be enabled or
disabled, and is initially disabled. Once enabled, a port can never be disabled again (though
messages in the queue can get moved to another queue or removed altogether, which has much the
same effect). A MessagePort
also has a has been shipped flag, which must
initially be false, and an owner, which is a settings
object set when the object is created, as described below.
When a port's port message queue is enabled, the event loop must use it as one of its task sources. When a port's owner specifies a responsible event loop that is a browsing context event loop, all tasks queued on its port message queue must be associated with the responsible document specified by the port's owner.
If the port's owner specifies a responsible document that is fully active, but the event listeners all have scripts whose settings objects specify responsible documents that are not fully active, then the messages will be lost.
Each event loop has a task source called the unshipped port
message queue. This is a virtual task source: it must act as if it contained
the tasks of each port message queue of each
MessagePort
whose has been shipped flag is false, whose port
message queue is enabled, and whose owner
specifies that event loop as the responsible event loop, in the order in
which they were added to their respective task source. When a task would be removed from the unshipped port message
queue, it must instead be removed from its port message queue.
When a MessagePort
's has been shipped flag is false, its port
message queue must be ignored for the purposes of the event loop. (The
unshipped port message queue is used instead.)
The has been shipped flag is set to true when a port, its twin, or
the object it was cloned from, is or has been transferred. When a MessagePort
's has been shipped flag
is true, its port message queue acts as a first-class task source,
unaffected to any unshipped port message queue.
When the user agent is to create a new MessagePort
object with a
particular settings object as its owner, it must instantiate a
new MessagePort
object, and let its owner be
owner.
When the user agent is to entangle two MessagePort
objects, it must run
the following steps:
If one of the ports is already entangled, then disentangle it and the port that it was entangled with.
If those two previously entangled ports were the two ports of a
MessageChannel
object, then that MessageChannel
object no longer
represents an actual channel: the two ports in that object are no longer entangled.
Associate the two ports to be entangled, so that they form the two parts of a new channel.
(There is no MessageChannel
object that represents this channel.)
Two ports A and B that have gone through this step are now said to be entangled; one is entangled to the other, and vice versa.
While this specification describes this process as instantaneous, implementations are more likely to implement it via message passing. As with all algorithms, the key is "merely" that the end result be indistinguishable, in a black-box sense, from the specification.
When the user agent is to clone a port original port, with the
clone being owned by owner, it must run the following steps, which return a
new MessagePort
object. These steps must be run atomically.
Set original port's has been shipped flag to true.
Create a new MessagePort
object whose owner is owner, and let new port be that object.
Set new port's has been shipped flag to true.
Move all the tasks that are to fire message
events in the port message queue of original port to the port message queue of new
port, if any, leaving the new port's port message queue
in its initial disabled state, and, if the new port's owner specifies a responsible event loop that is
a browsing context event loop, associating the moved tasks with the responsible document specified by new port's owner.
If the original port is entangled with another port, then run these substeps:
Let the remote port be the port with which the original port is entangled.
Set remote port's has been shipped flag to true.
Entangle the remote port and new port objects. The original port object will be disentangled by this process.
Return new port. It is the clone.
To transfer a
MessagePort
object old to a new owner owner,
a user agent must clone the old object with
the clone being owned by owner, thus obtaining new, must
neuter the old port, and
must finally return new.
The postMessage()
method, when
called on a port source port, must cause the user agent to run the following
steps:
Let target port be the port with which source port is entangled, if any.
Let doomed be false. It is set to true if a condition is detected that will make this message cause the port to be unusable; specifically, if the message contains target port as one of the objects being transferred. (This condition cannot necessarily be detected when the method is called.)
Let new ports be an empty array.
Let transfer map be an empty association list of
Transferable
objects to placeholder objects.
If the method was invoked with a second argument transfer, run these substeps:
If any object is listed in transfer more than once, or any of the
Transferable
objects listed in transfer are marked as neutered, then throw a
DataCloneError
exception and abort these steps.
If any of the objects in transfer are the source
port, then throw a DataCloneError
exception and abort these steps.
If any of the objects in transfer are the target port, if any, then let doomed be true, and optionally report to a developer console that the target port was posted to itself, causing the communication channel to be lost.
For each object x in transfer in turn, add a
mapping from x to a new unique placeholder object created for x to transfer map, and if x is a
MessagePort
object, also append the placeholder object to the new
ports array.
Let message clone be the result of obtaining a structured clone of the message argument, with transfer map as the transfer map. If this throws an exception, then throw that exception and abort these steps.
If the method was invoked with a second argument transfer, run these substeps:
Let new owner be the owner of
target port, if there is a target port and doomed is false, or else some arbitrary owner. (This new
owner is used when transferring objects below. If there is no target
port, or if the target port is one of the objects being transferred, the Transferable
objects given in the second argument, if any, are still transferred, but since they are then discarded, it doesn't matter where they
are transferred to.)
For each object x in transfer in turn, obtain a new object y by transferring the object x to new owner, and replace the placeholder object that was created for the object x by the new object y wherever the placeholder exists (i.e. in message clone and in new ports).
Make new ports into a read only array.
If there is no target port (i.e. if source port is not entangled), or if doomed is true, then abort these steps.
Create an event e that uses the MessageEvent
interface, with the name message
, which does not bubble, is not cancelable, and has no
default action.
Let the data
attribute of e be
initialised to the value of message clone.
Let the ports
attribute of e be
initialised to the new ports array.
Add a task that runs the following steps to the port message queue of target port:
Let target be the MessagePort
in whose port message
queue the event e now finds itself.
Dispatch e at target.
The start()
method must enable its port's
port message queue, if it is not already enabled.
The close()
method, when called on a port
local port that is entangled with another port, must cause the user agent to
disentangle the two ports. If the method is called on a port that is not entangled, then the
method must do nothing.
The following are the event handlers (and their corresponding event handler event types) that must be supported, as event
handler IDL attributes, by all objects implementing the MessagePort
interface:
Event handler | Event handler event type |
---|---|
onmessage | message
|
The first time a MessagePort
object's onmessage
IDL attribute is set, the port's port
message queue must be enabled, as if the start()
method had been called.
When a MessagePort
object o is entangled, user agents must
either act as if o's entangled MessagePort
object has a strong
reference to o, or as if the global object specified by o's owner has a strong reference to o.
Thus, a message port can be received, given an event listener, and then forgotten, and so long as that event listener could receive a message, the channel will be maintained.
Of course, if this was to occur on both sides of the channel, then both ports could be garbage collected, since they would not be reachable from live code, despite having a strong reference to each other.
Furthermore, a MessagePort
object must not be garbage collected while there exists
an event referenced by a task in a task queue that is to be dispatched on that MessagePort
object, or while the MessagePort
object's port message queue is enabled
and not empty.
Authors are strongly encouraged to explicitly close MessagePort
objects to disentangle them, so that their resources can be recollected. Creating many
MessagePort
objects and discarding them without closing them can lead to high
transient memory usage since garbage collection is not necessarily performed promptly, especially
for MessagePort
s where garbage collection can involve cross-process coordination.
All references are normative unless marked "Non-normative".
For a full list of acknowledgements, please see the HTML specification. [HTML]