We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

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 instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

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


Building blocks for composing lists.



List(element1, element2, ...)

Constructor can takes any number of elements and creates an instance of List populated with the specified elements.


element1 : Object|String|Number

element2 : Object|String|Number

... : Object|String|Number


addListItem(list, item)

Function adds a item to a List.

removeListItem(list, item)

Function removes an item from a List


An ordered collection (also known as a sequence) disallowing duplicate elements. List is composed out of Iterable, therefore it provides custom enumeration behavior that is similar to array (enumerates only on the elements of the list).

List is a base trait and is meant to be part of a composition, since all of its API is private except for the length property.


var { List } = require("sdk/util/list");
var MyList = List.compose({
  add: function add(item1, item2, /*item3...*/) {
  remove: function remove(item1, item2, /*item3...*/) {
MyList('foo', 'bar', 'baz').length == 3;        // true
new MyList('new', 'keyword').length == 2;       // true
MyList.apply(null, [1, 2, 3]).length == 3;      // true
let list = MyList();
list.length == 0;                               // true
list.add(1, 2, 3) == 3;                         // true



Number of elements in this list.

Document Tags and Contributors

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