Components.utils.import

Components.utils.import was introduced in Firefox 3 and is used for sharing code between different scopes easily. For example, you can import XPCOMUtils.jsm to avoid copy/pasting long XPCOM component registration boilerplate in your component files.

See Using JavaScript code modules for more details.

Note: Prior to Gecko 2.0, JavaScript code modules could only be loaded using file: or resource: URLs. Gecko 2.0 adds support for loading modules from chrome: URLs, even those inside JAR archives.

Syntax

Components.utils.import(url [, scope]);

// Or, if you use a tool such as jslint which reports compiler errors for the above,

Components.utils["import"](url [, scope]);

Parameters

url
A string of the URL of the script to be loaded. The URL must point to a file on the disk, possibly in a JAR.
scope
An optional object to import onto; if omitted, the global object is used.

Under Boot2Gecko, the scope is not optional. If your code is meant to work on all platforms, you should always provide a scope.

In case of doubt, this is generally a good scope.

import throws if it encounters an error (like a syntax error) in the file it reads.

Example

Components.utils.import("resource://gre/modules/XPCOMUtils.jsm", this);

Difference from mozIJSSubScriptLoader

The differences from mozIJSSubScriptLoader:

The behavior when importing/loading the same code from different locations:

  • the subscript loader evaluates the specified code each time it is invoked, with the caller's global object.
  • Components.utils.import evaluates the code of each module only once, in its own scope.

For example:

var scope1 = {}, scope2 = {};
Components.utils.import("resource://gre/modules/JSON.jsm", scope1);
Components.utils.import("resource://gre/modules/JSON.jsm", scope2);
assert(scope2.XPCOMUtils === scope1.XPCOMUtils);

...returns true, whereas:

var someURL = "resource://gre/modules/JSON.jsm"; 
var obj1 = {}, obj2 = {}; 
var loader = Components.classes["@mozilla.org/moz/jssubscript-loader;1"]
                       .getService(Components.interfaces.mozIJSSubScriptLoader); 
loader.loadSubScript(someURL, obj1); 
loader.loadSubScript(someURL, obj2);
assert(obj2 === obj1);

...returns false.

This means Components.utils.import is better suited for efficient sharing of code (and data?) between JS scripts running in different scope.

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Last updated by: Yoric,