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  6. Content Scripts
  7. Communicating With Other Scripts

Communicating With Other Scripts

In This Article
  1. main.js
  2. Content Scripts
  3. Page Scripts
    1. Using the DOM postMessage API
      1. Messaging From Content Script To Page Script
      2. Messaging From Page Script To Content Script
      3. Using postMessage() before Firefox 31
    2. Using Custom DOM Events
      1. Messaging From Content Script To Page Script
      2. Messaging From Page Script to Content Script

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.

This page is now obsolete, and its content has been incorporated into the main page on content scripts.

This section of the guide explains how content scripts can communicate with:

  • your main.js file, or any other modules in your add-on
  • other content scripts loaded by your add-on
  • page scripts (that is, scripts embedded in the web page or included using <script> tags)

main.js

Your content scripts can communicate with your add-on's "main.js" (or any other modules you're written for your add-on) by sending it messages, using either the port.emit() API or the postMessage() API. See the articles on using postMessage() and using port for details.

Content Scripts

Content scripts loaded into the same document at the same time using the same method can interact with each other directly as well as with the web content itself. However, content scripts which have been loaded into different documents cannot interact directly with each other.  Content scripts that have been loaded into the same document by different methods, or the same method called more than once, can pass messages directly to each other using the DOM postMessage() API or a CustomEvent. Any two content scripts can communicate by using the main add-on script to manually relay messages between the content scripts.

For example:

  • If an add-on creates a single panel object and loads several content scripts into the panel at the same time, then they can interact with each other.
  • If an add-on creates two panel objects and loads a script into each one, they can't interact with each other. But, they can each communicate with the main add-on script which could be written to relay messages between them.
  • If an add-on creates a single page-mod object and loads several content scripts into the page mod, then only content scripts associated with the same page can interact with each other: if two different matching pages are loaded, content scripts attached to page A cannot interact with those attached to page B.
  • If an add-on loads two or more content scripts into the same page, or panel, using two different method (e.g. a page-mod and a context-menu) they can directly communicate by dispatching and listening for CustomEvents on DOM objects they can access (e.g. the window object). This is true as long as both context scripts have direct access to the same DOM objects.

The web content has no access to objects created by the content script, unless the content script explicitly makes them available.

For more discussion and an example please see Content Scripts: Communicating with the add-on: Content script to content script.

Page Scripts

If a page includes its own scripts using <script> tags, either embedded in the page or linked to it using the src attribute, there are a couple of ways a content script can communicate with it:

  • using the DOM postMessage() API
  • using custom DOM events

Using the DOM postMessage API

Note that before Firefox 31 code in content scripts can't use window to access postMessage() and addEventListener() and instead must use document.defaultView. See the section below on using postMessage() before Firefox 31.

You can communicate between the content script and page scripts using window.postMessage().

Messaging From Content Script To Page Script

Suppose we have a page called "listen.html" hosted at "my-domain.org", and we want to send messages from the add-on to a script embedded in that page.

In the main add-on code, we have a page-mod that attaches the content script "talk.js" to the right page:

var data = require("sdk/self").data;
var pageMod = require("sdk/page-mod");
pageMod.PageMod({
  include: "http://my-domain.org/listen.html",
  contentScriptFile: data.url("talk.js")
});

The "talk.js" content script uses window.postMessage() to send the message to the page:

// content-script (talk.js)
window.postMessage("Message from content script", "http://my-domain.org/");

The second argument may be '*' which will allow communication with any domain.

Finally, "listen.html" uses window.addEventListener() to listen for messages from the content script:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <script>
      window.addEventListener('message', function(event) {
        window.alert(event.data);  // Message from content script
      }, false);
    </script>
  </body>
</html>

Messaging From Page Script To Content Script

Sending messages from the page script to the content script is just the same, but in reverse.

Here "main.js" creates a page-mod that attaches "listen.js" to the web page:

var data = require("sdk/self").data;
var pageMod = require("sdk/page-mod");
pageMod.PageMod({
  include: "http://my-domain.org/talk.html",
  contentScriptFile: data.url("listen.js")
});

The web page "talk.html" embeds a script that uses window.postMessage() to send the content script a message when the user clicks a button:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <script>
      function sendMessage() {
        window.postMessage("Message from page script", "http://my-domain.org/");
      }
    </script>
    <button onclick="sendMessage()">Send Message</button>
  </body>
</html>

Finally, the content script "listen.js" uses window.addEventListener() to listen for messages from the page script:

// listen.js
window.addEventListener('message', function(event) {
  console.log(event.data);    // Message from page script
  console.log(event.origin);
}, false);

Using postMessage() before Firefox 31

If your add-on is running in a version of Firefox before Firefox 31, then your content script can't access the postMessage() or addEventListener() APIs using window, but must access them using document.defaultView instead. So the content scripts in the above examples need to be rewritten like this:

// content-script.js
document.defaultView.postMessage("Message from content script", "http://my-domain.org/");
// content-script.js
document.defaultView.addEventListener('message', function(event) {
  console.log(event.data);    // Message from page script
  console.log(event.origin);
}, false);

Using Custom DOM Events

As an alternative to using postMessage() you can use custom DOM events to communicate between page scripts and content scripts.

Messaging From Content Script To Page Script

From Firefox 30 onwards, the execution environment for content scripts has changed, so content scripts can't directly share objects with page scripts. This affects the use of custom events to send messages from content scripts to page scripts.

Before Firefox 30

Here's an example showing how to use custom DOM events to send a message from a content script to a page script, before Firefox 30.

First, "main.js" will create a page-mod that will attach "content-script.js" to the target web page, and will then load the target web page:

var tabs = require("sdk/tabs");
var mod = require("sdk/page-mod");
var self = require("sdk/self");
var pageUrl = self.data.url("page.html")
var pageMod = mod.PageMod({
  include: pageUrl,
  contentScriptFile: self.data.url("content-script.js"),
  contentScriptWhen: "ready"
})
tabs.open(pageUrl);

The target web page "page.html" includes a button and a page script:

<html>
  <head>
    <meta charset="utf-8">
  </head>
  <body>
    <input id="message" type="button" value="Send a message"/>
    <script type="text/javascript" src="page-script.js"></script>
  </body>
</html>

The content script "content-script.js" adds an event listener to the button, that sends a custom event containing a message:

var messenger = document.getElementById("message");
messenger.addEventListener("click", sendCustomEvent, false);
function sendCustomEvent() {
  var greeting = {"greeting" : "hello world"};
  var event = document.createEvent('CustomEvent');
  event.initCustomEvent("addon-message", true, true, greeting);
  document.documentElement.dispatchEvent(event);
}

Finally, the page script "page-script.js" listens for the message and logs the greeting to the Web Console:

window.addEventListener("addon-message", function(event) {
  console.log(event.detail.greeting);
}, false);

After Firefox 30: clone the message object

This technique depends on being able to share the message payload between the content script scope and the page script scope. From Firefox 30 this sharing requires an extra step: the content script needs to explicitly clone the message payload into the page script's scope using the global cloneInto() function:

var messenger = document.getElementById("message");
messenger.addEventListener("click", sendCustomEvent, false);
function sendCustomEvent() {
  var greeting = {"greeting" : "hello world"};
  var cloned = cloneInto(greeting, document.defaultView);
  var event = document.createEvent('CustomEvent');
  event.initCustomEvent("addon-message", true, true, cloned);
  document.documentElement.dispatchEvent(event);
}

Messaging From Page Script to Content Script

Sending messages using custom DOM events from the page script to the content script is just the same, but in reverse. Also, there's no need to clone the message when using custom DOM events in this direction.

In this example, "main.js" creates a page-mod to target the page we are interested in:

var data = require("sdk/self").data;
var pageMod = require("sdk/page-mod");
pageMod.PageMod({
  include: "http://my-domain.org/talk.html",
  contentScriptFile: data.url("listen.js")
});

The web page "talk.html" creates and dispatches a custom DOM event, using initCustomEvent()'s detail parameter to supply the payload:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <script>
      function sendMessage() {
        var event = document.createEvent('CustomEvent');
        event.initCustomEvent("addon-message", true, true, { hello: 'world' });
        document.documentElement.dispatchEvent(event);
      }
    </script>
    <button onclick="sendMessage()">Send Message</button>
  </body>
</html>

Finally, the content script "listen.js" listens for the new event and retrieves the payload from its detail attribute:

window.addEventListener("addon-message", function(event) {
  console.log(JSON.stringify(event.detail));
}, false);

Document Tags and Contributors

 Contributors to this page: wbamberg, Makyen, Jaja, michaelkonecny, jsantell, evold
 Last updated by: wbamberg, Nov 30, 2016, 2:06:16 PM
See also
  1. WebExtensions
  2. Getting started
    1. What are WebExtensions?
    2. Your first WebExtension
    3. Your second WebExtension
    4. Anatomy of a WebExtension
    5. Example WebExtensions
  3. How to
    1. Intercept HTTP requests
    2. Modify a web page
    3. Add a button to the toolbar
    4. Implement a settings page
  4. Concepts
    1. Using the JavaScript APIs
    2. User interface components
    3. Content scripts
    4. Match patterns
    5. Internationalization
    6. Content Security Policy
    7. Native messaging
  5. Porting
    1. Porting a Google Chrome extension
    2. Porting a legacy Firefox add-on
    3. Embedded WebExtensions
    4. Comparison with the Add-on SDK
    5. Comparison with XUL/XPCOM extensions
    6. Chrome incompatibilities
  6. Firefox workflow
    1. Temporary Installation in Firefox
    2. Debugging
    3. Getting started with web-ext
    4. web-ext command reference
    5. WebExtensions and the Add-on ID
    6. Publishing your WebExtension
  7. JavaScript APIs
    1. Browser support for JavaScript APIs
    2. alarms
    3. bookmarks
    4. browserAction
    5. browsingData
    6. commands
    7. contextMenus
    8. contextualIdentities
    9. cookies
    10. downloads
    11. events
    12. extension
    13. extensionTypes
    14. history
    15. i18n
    16. identity
    17. idle
    18. management
    19. notifications
    20. omnibox
    21. pageAction
    22. runtime
    23. sessions
    24. sidebarAction
    25. storage
    26. tabs
    27. topSites
    28. webNavigation
    29. webRequest
    30. windows
  8. Manifest keys
    1. applications
    2. author
    3. background
    4. browser_action
    5. chrome_url_overrides
    6. commands
    7. content_scripts
    8. content_security_policy
    9. default_locale
    10. description
    11. developer
    12. homepage_url
    13. icons
    14. manifest_version
    15. name
    16. omnibox
    17. options_ui
    18. page_action
    19. permissions
    20. short_name
    21. sidebar_action
    22. version
    23. web_accessible_resources
  9. Add-on SDK
  10. Getting started
    1. Installation
    2. Getting started
    3. Troubleshooting
  11. High-Level APIs
    1. addon-page
    2. base64
    3. clipboard
    4. context-menu
    5. hotkeys
    6. indexed-db
    7. l10n
    8. notifications
    9. page-mod
    10. page-worker
    11. panel
    12. passwords
    13. private-browsing
    14. querystring
    15. request
    16. selection
    17. self
    18. simple-prefs
    19. simple-storage
    20. system
    21. tabs
    22. timers
    23. ui
    24. url
    25. webextension
    26. widget
    27. windows
  12. Low-Level APIs
    1. /loader
    2. chrome
    3. console/plain-text
    4. console/traceback
    5. content/content
    6. content/loader
    7. content/mod
    8. content/symbiont
    9. content/worker
    10. core/heritage
    11. core/namespace
    12. core/promise
    13. dev/panel
    14. event/core
    15. event/target
    16. frame/hidden-frame
    17. frame/utils
    18. fs/path
    19. io/byte-streams
    20. io/file
    21. io/text-streams
    22. lang/functional
    23. lang/type
    24. loader/cuddlefish
    25. loader/sandbox
    26. net/url
    27. net/xhr
    28. places/bookmarks
    29. places/favicon
    30. places/history
    31. platform/xpcom
    32. preferences/event-target
    33. preferences/service
    34. remote/child
    35. remote/parent
    36. stylesheet/style
    37. stylesheet/utils
    38. system/child_process
    39. system/environment
    40. system/events
    41. system/runtime
    42. system/unload
    43. system/xul-app
    44. tabs/utils
    45. test/assert
    46. test/harness
    47. test/httpd
    48. test/runner
    49. test/utils
    50. ui/button/action
    51. ui/button/toggle
    52. ui/frame
    53. ui/id
    54. ui/sidebar
    55. ui/toolbar
    56. util/array
    57. util/collection
    58. util/deprecate
    59. util/list
    60. util/match-pattern
    61. util/object
    62. util/uuid
    63. window/utils
  13. Firefox for Android
  14. Getting started
    1. Walkthrough
    2. Debugging
    3. Code snippets
  15. APIs
    1. Accounts.jsm
    2. BrowserApp
    3. HelperApps.jsm
    4. Home.jsm
    5. HomeProvider.jsm
    6. JavaAddonManager.jsm
    7. NativeWindow
    8. Notifications.jsm
    9. PageActions.jsm
    10. Prompt.jsm
    11. RuntimePermissions.jsm
    12. Snackbars.jsm
    13. Sound.jsm
    14. Tab
  16. Legacy
  17. Restartless extensions
    1. Overview
  18. Overlay extensions
    1. Overview
  19. Themes
  20. Lightweight themes
    1. Overview
  21. Complete themes
    1. Overview
  22. Publishing add-ons
  23. Guides
    1. Signing and distribution overview
    2. Submit an add-on
    3. Review policies
    4. Developer agreement
    5. Featured add-ons
    6. Contact addons.mozilla.org
  24. Community and support
  25. Channels
    1. Add-ons blog
    2. Add-on forums
    3. Stack Overflow
    4. Development newsgroup
    5. IRC Channel