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Provides one of the building blocks for those modules that use content scripts to interact with web content, such as panel and page-mod.


The module exports a constructor for the Loader object, which inherits on(), once(), and removeListener() functions that enable its users to listen to events.

Loader adds code to initialize and validate a set of properties for managing content scripts:

  • contentURL
  • contentScript
  • contentScriptFile
  • contentScriptWhen
  • contentScriptOptions
  • allow

When certain of these properties are set, the Loader emits a propertyChange event, enabling its users to take the appropriate action.

The Loader is used by modules that use content scripts but don't themselves load content, such as page-mod.

Modules that load their own content, such as panel and page-worker, use the symbiont module instead. Symbiont inherits from Loader but contains its own frame into which it loads content supplied as the contentURL option.


The following code creates a wrapper on a hidden frame that reloads a web page in the frame every time the contentURL property is changed:

var hiddenFrames = require("sdk/frame/hidden-frame");
var { Loader } = require("sdk/content/content");
var PageLoader = Loader.compose({
  constructor: function PageLoader(options) {
    options = options || {};
    if (options.contentURL)
      this.contentURL = options.contentURL;
    this.on('propertyChange', this._onChange = this._onChange.bind(this));
    let self = this;
      onReady: function onReady() {
        let frame = self._frame = this.element;
        self._emit('propertyChange', { contentURL: self.contentURL });
  _onChange: function _onChange(e) {
    if ('contentURL' in e)
      this._frame.setAttribute('src', this._contentURL);




The local file URLs of content scripts to load. Content scripts specified by this property are loaded before those specified by the contentScript property.


The texts of content scripts to load. Content scripts specified by this property are loaded after those specified by the contentScriptFile property.


When to load the content scripts. This may take one of the following values:

  • "start": load content scripts immediately after the document element for the page is inserted into the DOM, but before the DOM content itself has been loaded
  • "ready": load content scripts once DOM content has been loaded, corresponding to the DOMContentLoaded event
  • "end": load content scripts once all the content (DOM, JS, CSS, images) for the page has been loaded, at the time the window.onload event fires


Read-only value exposed to content scripts under self.options property.

Any kind of jsonable value (object, array, string, etc.) can be used here. Optional.


The URL of the content loaded.


Permissions for the content, with the following keys:

Document Tags and Contributors

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