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.


Functions for working with objects.



merge(source, arguments)

Merges all of the properties of all arguments into the first argument. If two or more argument objects have properties with the same name, the property is overwritten with precedence from right to left, implying that properties of the object on the left are overwritten by a same named property of an object on the right. Properties are merged with descriptors onto the source object.

Any argument given with "falsy" values (null, undefined) in case of objects are skipped.

let { merge } = require("sdk/util/object");
var a = { jetpacks: "are yes", foo: 10 }
var b = merge(a, { foo: 5, bar: 6 }, { foo: 50, location: "SF" });

b === a    // true
b.jetpacks // "are yes"
b.foo      // 50
b.bar      // 6
b.location // "SF"

// Merge also translates property descriptors
var c = { "type": "addon" };
var d = {};
Object.defineProperty(d, "name", {
  value: "jetpacks",
  configurable: false
merge(c, d);
var props = Object.getOwnPropertyDescriptor(c, "name");
console.log(props.configurable); // true

source : object
The object that other properties are merged into.

arguments : object
n amount of additional objects that are merged into source object.


object : The source object.


Returns an object that inherits from the first argument and contains all of the properties from all following arguments, with precedence from right to left.

extend(source1, source2, source3) is the equivalent of merge(Object.create(source1), source2, source3).

let { extend } = require("sdk/util/object");
var a = { alpha: "a" };
var b = { beta: "b" };
var g = { gamma: "g", alpha: null };
var x = extend(a, b, g);

console.log(a); // { alpha: "a" }
console.log(b); // { beta: "b" }
console.log(g); // { gamma: "g", alpha: null }
console.log(x); // { alpha: null, beta: "b", gamma: "g" }

arguments : object
n arguments that get merged into a new object.


object : The new, merged object.

Document Tags and Contributors

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