HTML

Living Standard — Last Updated 6 December 2019

1. 1. 9.4 Cross-document messaging
      1. 9.4.1 Introduction
      2. 9.4.2 Security
         1. 9.4.2.1 Authors
         2. 9.4.2.2 User agents
      3. 9.4.3 Posting messages
   2. 9.5 Channel messaging
      1. 9.5.1 Introduction
         1. 9.5.1.1 Examples
         2. 9.5.1.2 Ports as the basis of an object-capability model on the Web
         3. 9.5.1.3 Ports as the basis of abstracting out service implementations
      2. 9.5.2 Message channels
      3. 9.5.3 Message ports
      4. 9.5.4 Broadcasting to many ports
      5. 9.5.5 Ports and garbage collection
   3. 9.6 Broadcasting to other browsing contexts

9.4 Cross-document messaging

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.

This API has some privacy implications that might not be immediately obvious.

9.4.1 Introduction

この節は非規範的である。

var o = document.getElementsByTagName('iframe')[0];
o.contentWindow.postMessage('Hello world', 'https://b.example.org/');

window.addEventListener('message', receiver, false);
function receiver(e) {
  if (e.origin == 'https://example.com') {
    if (e.data == 'Hello world') {
      e.source.postMessage('Hello', e.origin);
    } else {
      alert(e.data);
    }
  }
}

9.4.2 Security

9.4.2.1 Authors

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 multiplied 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.

9.4.2.2 User agents

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.

9.4.3 Posting messages

window . postMessage(message [, options ] )

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 the transfer member of options are transferred, not just cloned, meaning that they are no longer usable on the sending side.

A target origin can be specified using the targetOrigin member of options. If not provided, it defaults to "/". This default restricts the message to same-origin targets only.

If the origin of the target window doesn't match the given target origin, the message is discarded, to avoid information leakage. To send the message to the target regardless of origin, set the target origin to "*".

Throws a "DataCloneError" DOMException if transfer array contains duplicate objects or if message could not be cloned.

window . postMessage(message, targetOrigin [, transfer ] )

This is an alternate version of postMessage() where the target origin is specified as a parameter. Calling window.postMessage(message, target, transfer) is equivalent to window.postMessage(message, {targetOrigin, transfer}).

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.

The window post message steps, given a targetWindow, message, and options, are as follows:

1. Let targetRealm be targetWindow's Realm.

2. Let incumbentSettings be the incumbent settings object.

3. Let targetOrigin be options["targetOrigin"].

4. If targetOrigin is a single U+002F SOLIDUS character (/), then set targetOrigin to incumbentSettings's origin.

5. Otherwise, if targetOrigin is not a single U+002A ASTERISK character (*), then:

   1. Let parsedURL be the result of running the URL parser on targetOrigin.

   2. If parsedURL is failure, then throw a "SyntaxError" DOMException.

   3. Set targetOrigin to parsedURL's origin.

6. Let transfer be options["transfer"].

7. Let serializeWithTransferResult be StructuredSerializeWithTransfer(message, transfer). 例外を再度投げる。

8. Queue a task on the posted message task source of targetWindow's relevant agent's event loop to run the following steps:

   1. If the targetOrigin argument is not a single literal U+002A ASTERISK character (*) and targetWindow's associated Document's origin is not same origin with targetOrigin, then return.

   2. Let origin be the serialization of incumbentSettings's origin.

   3. Let source be the WindowProxy object corresponding to incumbentSettings's global object (a Window object).

   4. Let deserializeRecord be StructuredDeserializeWithTransfer(serializeWithTransferResult, targetRealm).

      If this throws an exception, catch it, fire an event named messageerror at targetWindow, using MessageEvent, with the origin attribute initialized to origin and the source attribute initialized to source, and then return.

   5. Let messageClone be deserializeRecord.[[Deserialized]].

   6. Let newPorts be a new frozen array consisting of all MessagePort objects in deserializeRecord.[[TransferredValues]], if any, maintaining their relative order.

   7. Fire an event named message at targetWindow, using MessageEvent, with the origin attribute initialized to origin, the source attribute initialized to source, the data attribute initialized to messageClone, and the ports attribute initialized to newPorts.

The postMessage(message, options) method, when invoked on a Window object, must run the following steps:

1. Let targetWindow be this Window object.

2. Run the window post message steps providing targetWindow, message, and options.

The postMessage(message, targetOrigin, transfer) method, when invoked on a Window object, must run the following steps:

1. Let targetWindow be this Window object.

2. Let options be «[ "targetOrigin" → targetOrigin, "transfer" → transfer ]».

3. Run the window post message steps providing targetWindow, message, and options.

9.5 Channel messaging

9.5.1 Introduction

この節は非規範的である。

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', 'https://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']);

9.5.1.1 Examples

この節は非規範的である。

<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>

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;
      // ...
    }
  };
};

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();
};

9.5.1.2 Ports as the basis of an object-capability model on the Web

この節は非規範的である。

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:

• Navigate the iframe to a new URL, such as the same URL but with a different fragment, causing the Window in the iframe to receive a hashchange event.
• Resize the iframe, causing the Window in the iframe to receive a resize event.
• Send a 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.

9.5.1.3 Ports as the basis of abstracting out service implementations

この節は非規範的である。

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 XMLHttpRequests-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.

9.5.2 Message channels

[Constructor, Exposed=(Window,Worker)]
interface MessageChannel {
  readonly attribute MessagePort port1;
  readonly attribute MessagePort port2;
};

channel = new MessageChannel()

Returns a new MessageChannel object with two new MessagePort objects.

channel . port1

Returns the first MessagePort object.

channel . port2

Returns the second MessagePort object.

When the MessageChannel() constructor is called, it must run the following algorithm:

1. Create a new MessagePort object whose owner is the incumbent settings object, and let port1 be that object.

2. Create a new MessagePort object whose owner is the incumbent settings object, and let port2 be that object.

3. Entangle the port1 and port2 objects.

4. Instantiate a new MessageChannel object, and let channel be that object.

5. Let the port1 attribute of the channel object be port1.

6. Let the port2 attribute of the channel object be port2.

7. Return channel.

The port1 and port2 attributes must return the values they were assigned when the MessageChannel object was created.

9.5.3 Message ports

Each channel has two message ports. Data sent through one port is received by the other port, and vice versa.

[Exposed=(Window,Worker,AudioWorklet), Transferable]
interface MessagePort : EventTarget {
  void postMessage(any message, sequence<object> transfer);
  void postMessage(any message, optional PostMessageOptions options = {});
  void start();
  void close();

  // event handlers
  attribute EventHandler onmessage;
  attribute EventHandler onmessageerror;
};

dictionary PostMessageOptions {
  sequence<object> transfer = [];
};

port . postMessage(message [, transfer] )

port . 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" DOMException if transfer contains duplicate objects or port, or if message could not be cloned.

port . start()

Begins dispatching messages received on the port.

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's responsible event loop is a window event loop, all tasks queued on its port message queue must be associated with the port's owner's responsible document.

If the port's owner's responsible document 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's responsible event loop is that 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 environment 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:

1. 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.

2. 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.

MessagePort objects are transferable objects. Their transfer steps, given value and dataHolder, are:

1. Set value's has been shipped flag to true.

2. Set dataHolder.[[PortMessageQueue]] to value's port message queue.

3. If value is entangled with another port remotePort, then:

   1. Set remotePort's has been shipped flag to true.

   2. Set dataHolder.[[RemotePort]] to remotePort.

4. Otherwise, set dataHolder.[[RemotePort]] to null.

Their transfer-receiving steps, given dataHolder and value, are:

1. Set value's has been shipped flag to true.

2. Set value's owner to value's relevant settings object.

3. Move all the tasks that are to fire message events in dataHolder.[[PortMessageQueue]] to the port message queue of value, if any, leaving value's port message queue in its initial disabled state, and, if value's owner's responsible event loop is a window event loop, associating the moved tasks with value's owner's responsible document.

4. If dataHolder.[[RemotePort]] is not null, then entangle dataHolder.[[RemotePort]] and value. (This will disentangle dataHolder.[[RemotePort]] from the original port that was transferred.)

The message port post message steps, given a targetPort, message and options are as follows:

1. Let transfer be options["transfer"].

2. If transfer contains this MessagePort, then throw a "DataCloneError" DOMException.

3. Let doomed be false.

4. If targetPort is not null and transfer contains targetPort, then set doomed to true and optionally report to a developer console that the target port was posted to itself, causing the communication channel to be lost.

5. Let serializeWithTransferResult be StructuredSerializeWithTransfer(message, transfer). 例外を再度投げる。

6. If targetPort is null, or if doomed is true, then return.

7. Add a task that runs the following steps to the port message queue of targetPort:

   1. Let finalTargetPort be the MessagePort in whose port message queue the task now finds itself.

      This can be different from targetPort, if targetPort itself was transferred and thus all its tasks moved along with it.

   2. Let targetRealm be finalTargetPort's relevant Realm.

   3. Let deserializeRecord be StructuredDeserializeWithTransfer(serializeWithTransferResult, targetRealm).

      If this throws an exception, catch it, fire an event named messageerror at finalTargetPort, using MessageEvent, and then return.

   4. Let messageClone be deserializeRecord.[[Deserialized]].

   5. Let newPorts be a new frozen array consisting of all MessagePort objects in deserializeRecord.[[TransferredValues]], if any, maintaining their relative order.

   6. Fire an event named message at finalTargetPort, using MessageEvent, with the data attribute initialized to messageClone and the ports attribute initialized to newPorts.

The postMessage(message, options) method, when invoked on a MessagePort object must run the following steps:

1. Let targetPort be the port with which this MessagePort is entangled, if any; otherwise let it be null.

2. Run the message port post message steps providing targetPort, message and options.

The postMessage(message, transfer) method, when invoked on a MessagePort object must run the following steps:

1. Let targetPort be the port with which this MessagePort is entangled, if any; otherwise let it be null.

2. Let options be «[ "transfer" → transfer ]».

3. Run the message port post message steps providing targetPort, message and options.

The start() method, when invoked, must enable this MessagePort object's port message queue, if it is not already enabled.

The close() method, when invoked, must run these steps:

1. Set this MessagePort object's [[Detached]] internal slot value to true.

2. If this MessagePort object is entangled, disentangle it.

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:

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.

9.5.4 Broadcasting to many ports

この節は非規範的である。

Broadcasting to many ports is in principle relatively simple: keep an array of MessagePort objects to send messages to, and iterate through the array to send a message. However, this has one rather unfortunate effect: it prevents the ports from being garbage collected, even if the other side has gone away. To avoid this problem, implement a simple protocol whereby the other side acknowledges it still exists. If it doesn't do so after a certain amount of time, assume it's gone, close the MessagePort object, and let it be garbage collected.

9.5.5 Ports and garbage collection

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.

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 MessagePorts where garbage collection can involve cross-process coordination.

9.6 Broadcasting to other browsing contexts

Pages on a single origin opened by the same user in the same user agent but in different unrelated browsing contexts sometimes need to send notifications to each other, for example "hey, the user logged in over here, check your credentials again".

For elaborate cases, e.g. to manage locking of shared state, to manage synchronization of resources between a server and multiple local clients, to share a WebSocket connection with a remote host, and so forth, shared workers are the most appropriate solution.

For simple cases, though, where a shared worker would be an unreasonable overhead, authors can use the simple channel-based broadcast mechanism described in this section.

[Exposed=(Window,Worker)]
interface BroadcastChannel : EventTarget {
  constructor(DOMString name);

  readonly attribute DOMString name;
  void postMessage(any message);
  void close();
  attribute EventHandler onmessage;
  attribute EventHandler onmessageerror;
};

broadcastChannel = new BroadcastChannel(name)

Returns a new BroadcastChannel object via which messages for the given channel name can be sent and received.

broadcastChannel . name

Returns the channel name (as passed to the constructor).

broadcastChannel . postMessage(message)

Sends the given message to other BroadcastChannel objects set up for this channel. Messages can be structured objects, e.g. nested objects and arrays.

broadcastChannel . close()

Closes the BroadcastChannel object, opening it up to garbage collection.

A BroadcastChannel object has a channel name, a BroadcastChannel settings object, and a closed flag.

The BroadcastChannel() constructor, when invoked, must create and return a BroadcastChannel object whose channel name is the constructor's first argument, whose BroadcastChannel settings object is the incumbent settings object, and whose closed flag is false.

The name attribute must return the channel name.

The postMessage(message) method, when invoked on a BroadcastChannel object, must run the following steps:

1. Let source be this BroadcastChannel.

2. Let sourceSettings be source's BroadcastChannel settings object.

3. If source's closed flag is true, then throw an "InvalidStateError" DOMException.

4. Let sourceChannel be source's channel name.

5. Let targetRealm be a user-agent defined Realm.

6. Let serialized be StructuredSerialize(message). 例外を再度投げる。

7. Let destinations be a list of BroadcastChannel objects that match the following criteria:

   • Their BroadcastChannel settings object specifies either:

     • a global object that is a Window object whose associated Document is fully active, or

     • a global object that is a WorkerGlobalScope object whose closing flag is false and whose worker is not a suspendable worker.

   • Their BroadcastChannel settings object's origin is same origin with sourceSettings's origin.

   • Their channel name is a case-sensitive match for sourceChannel.

   • Their closed flag is false.

8. Remove source from destinations.

9. Sort destinations such that all BroadcastChannel objects whose BroadcastChannel settings objects specify the same responsible event loop are sorted in creation order, oldest first. (This does not define a complete ordering. Within this constraint, user agents may sort the list in any user-agent defined manner.)

10. For each BroadcastChannel object destination in destinations, queue a task on the DOM manipulation task source of destination's relevant agent's event loop that runs the following steps. If that event loop is a window event loop, then the task's document must be set to destination's BroadcastChannel settings object's responsible document.

    1. Let targetRealm be destination's relevant Realm.

    2. Let data be StructuredDeserialize(serialized, targetRealm).

       If this throws an exception, catch it, fire an event named messageerror at destination, using MessageEvent, with the origin attribute initialized to the serialization of sourceSettings's origin, and then return.

    3. Fire an event named message at destination, using MessageEvent, with the data attribute initialized to data and the origin attribute initialized to the serialization of sourceSettings's origin.

While a BroadcastChannel object whose closed flag is false has an event listener registered for message events, there must be a strong reference from global object specified by the BroadcastChannel object's BroadcastChannel settings object to the BroadcastChannel object itself.

The close() method must set the closed flag of the BroadcastChannel object on which it was invoked to true.

Authors are strongly encouraged to explicitly close BroadcastChannel objects when they are no longer needed, so that they can be garbage collected. Creating many BroadcastChannel objects and discarding them while leaving them with an event listener and without closing them can lead to an apparent memory leak, since the objects will continue to live for as long as they have an event listener (or until their page or worker is closed).

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 BroadcastChannel interface:

var authChannel = new BroadcastChannel('auth');
authChannel.onmessage = function (event) {
  if (event.data == 'logout')
    showLogout();
}

function logoutRequested() {
  // called when the user asks us to log them out
  doLogout();
  showLogout();
  authChannel.postMessage('logout');
}

function doLogout() {
  // actually log the user out (e.g. clearing cookies)
  // ...
}

function showLogout() {
  // update the UI to indicate we're logged out
  // ...
}