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Communicating using "postMessage"

Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.

From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).

From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

As an alternative to port, content modules support the built-in message event. In most cases port is preferable to message events. However, the context-menu module does not support port, so to send messages from a content script to the add-on via a context menu object, you must use message events.

Handling Message Events in the Content Script

To send a message from a content script, you use the postMessage function of the global self object:

self.postMessage(contentScriptMessage);

This takes a single parameter, the message payload, which may be any JSON-serializable value.

To receive a message from the add-on script, use self's on function:

self.on("message", function(addonMessage) {
  // Handle the message
});

Like all event-registration functions, this takes two parameters: the name of the event, and the handler function. The handler function is passed the message payload.

Handling Message Events in the Add-on Script

To send a message to a content script, use the worker's postMessage function. Again, panel and page integrate worker directly:

// Post a message to the panel's content scripts
panel.postMessage(addonMessage);

However, for page-mod objects you need to listen to the onAttach event and use the worker supplied to that:

var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.postMessage(addonMessage);
  }
});

To receive messages from a content script, use the worker's on function. To simplify this most content modules provide an onMessage property as an argument to the constructor:

panel = require("sdk/panel").Panel({
  onMessage: function(contentScriptMessage) {
    // Handle message from the content script
  }
});

Message Events Versus User-Defined Events

You can use message events as an alternative to user-defined events:

var pageModScript = "window.addEventListener('mouseover', function(event) {" +
                    "  self.postMessage(event.target.toString());" +
                    "}, false);";

var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.on('message', function(message) {
      console.log('mouseover: ' + message);
    });
  }
});

The reason to prefer user-defined events is that as soon as you need to send more than one type of message, then both sending and receiving messages gets more complex.

Suppose the content script wants to send mouseout events as well as mouseover. Now we have to embed the event type in the message payload, and implement a switch function in the receiver to dispatch the message:

var pageModScript = "window.addEventListener('mouseover', function(event) {" +
                    "  self.postMessage({" +
                    "    kind: 'mouseover'," +
                    "    element: event.target.toString()" +
                    "  });" +
                    "}, false);" +
                    "window.addEventListener('mouseout', function(event) {" +
                    "  self.postMessage({" +
                    "    kind: 'mouseout'," +
                    "    element: event.target.toString()" +
                    "  });" +
                    " }, false);"

var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.on('message', function(message) {
    switch(message.kind) {
      case 'mouseover':
        console.log('mouseover: ' + message.element);
        break;
      case 'mouseout':
        console.log('mouseout: ' + message.element);
        break;
      }
    });
  }
});

Implementing the same add-on with user-defined events is shorter and more readable:

var pageModScript = "window.addEventListener('mouseover', function(event) {" +
                    "  self.port.emit('mouseover', event.target.toString());" +
                    "}, false);" +
                    "window.addEventListener('mouseout', function(event) {" +
                    "  self.port.emit('mouseout', event.target.toString());" +
                    "}, false);";

var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.port.on('mouseover', function(message) {
      console.log('mouseover :' + message);
    });
    worker.port.on('mouseout', function(message) {
      console.log('mouseout :' + message);
    });
  }
});

Document Tags and Contributors

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 Contributors to this page: wbamberg, evold
 Last updated by: wbamberg,