MDN wants to learn about developers like you: https://qsurvey.mozilla.com/s3/d6d7ff2e2f9c

Add-ons

lang/type

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.

Starting from Firefox 53, no new legacy add-ons will be accepted on addons.mozilla.org (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

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 use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

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

Unstable

Functions for working with type detection.

Globals

Functions

isUndefined(value)

Returns true if value is undefined, false otherwise.

let { isUndefined } = require('sdk/lang/type');

var foo;
isUndefined(foo); // true
isUndefined(0); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is undefined.

isNull(value)

Returns true if value is null, false otherwise.

let { isNull } = require('sdk/lang/type');

isNull(null); // true
isNull(false); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is null.

isString(value)

Returns true if value is a String, false otherwise. Uses typeof operator to check type, and will only properly detect string primitives: for example, a string created with new String() will always return false.

let { isString } = require('sdk/lang/type');

isString('my string'); // true
isString(100); // false
isString('100'); // true
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a String.

isNumber(value)

Returns true if value is a Number, false otherwise. Uses typeof operator to check type, and will only properly detect number primitives: for example, a number created with new Number() will always return false.

let { isNumber } = require('sdk/lang/type');

isNumber(3.1415); // true
isNumber(100); // true
isNumber('100'); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a Number.

isRegExp(value)

Returns true if value is a RegExp, false otherwise.

let { isRegExp } = require('sdk/lang/type');

isRegExp(/[^\.]*\.js$/); // true
isRegExp(new RegExp('substring')); // true
isRegExp(1000); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a RegExp.

isDate(value)

Returns true if value is a Date, false otherwise.

let { isDate } = require('sdk/lang/type');

isDate(new Date()); // true
isDate('3/1/2013'); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a Date.

isFunction(value)

Returns true if value is a Function, false otherwise.

let { isFunction } = require('sdk/lang/type');

let fn = function () {};
isFunction(fn); // true;
isFunction(otherFn); // true;
isFunction(function () {}); // true;

function otherFn () {}
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a Function.

isObject(value)

Returns true if value is an Object and not null, false otherwise.

let { isObject } = require('sdk/lang/type');

isObject({}); // true
isObject(new Class()); // true
isObject(null); // false
isObject(5); // false

function Class () {}
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is an Object.

isArray(value)

Returns true if value is an Array, false otherwise. Uses native Array.isArray.

let { isArray } = require('sdk/lang/type');

isArray([]); // true
isArray({}); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is an Array.

isArguments(value)

Returns true if value is an array-like arguments object, false otherwise.

let { isArguments } = require('sdk/lang/type');

function run () {
  isArguments(arguments); // true
  isArguments([]); // false
  isArguments(Array.slice(arguments)); // false
}
run(1, 2, 3);
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is an arguments object.

isPrimitive(value)

Returns true if value is a primitive value: that is, any of null, undefined, number, boolean, or string. Returns false if value is not a primitive value.

let { isPrimitive } = require('sdk/lang/type');

isPrimitive(3); // true
isPrimitive('foo'); // true
isPrimitive({}); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a primitive.

isFlat(value)

Returns true if value is a direct descendant of Object.prototype or null. Similar to jQuery's isPlainObject.

let { isFlat } = require('sdk/lang/type');

isFlat({}); // true
isFlat(new Type()); // false

function Type () {}
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is a direct descendant of Object.prototype or null.

isEmpty(value)

Returns true if value is an Object with no properties and false otherwise.

let { isEmpty } = require('sdk/lang/type');

isEmpty({}); // true
isEmpty({ init: false }); // false
Parameters

value : object
The variable to check.

Returns

boolean : Boolean indicating if value is an Object with no properties.

isJSON(value)

Returns true if value is a string, number, boolean, null, array of JSON-serializable values, or an object whose property values are themselves JSON-serializable. Returns false otherwise.

let { isJSON } = require('sdk/lang/type');

isJSON({ value: 42 }); // true
isJSON({ fn: function () {} ); // false
Parameters

value : mixed
The variable to check.

Returns

boolean : Boolean indicating if value is an Array/flat Object containing only atomic values and other flat objects.

instanceOf(value, Type)

Returns true if value is an instance of a given Type. This is similar to the instanceof operator. The difference is that the Type constructor can be from a scope that has a different top level object: for example, it could be from a different iframe, module or sandbox.

let { instanceOf } = require('sdk/lang/type');

instanceOf(new Class(), Class); // true
function Class() {}
Parameters

value : object
The variable to check.

Type : object
The constructor to compare to value

Returns

boolean : Boolean indicating if value is an instance of Type.

source(value, indent, limit)

Returns the textual representation of value, containing property descriptors and types of properties contained within the object.

let { source } = require('sdk/lang/type');

var obj = {
  name: undefined,
  twitter: '@horse_js',
  tweets: [
    { id: 100, text: 'What happens to you if you break the monad laws?' },
    { id: 101, text: 'JAVASCRIPT DUBSTEP GENERATOR' }
  ]
};

console.log(source(obj));
// Prints the below
/*
{ // [object Object]
    // writable configurable enumerable
    name: undefined,
    // writable configurable enumerable
    twitter: "@horse_js",
    // writable configurable enumerable
    tweets: [
        { // [object Object]
            // writable configurable enumerable
            id: 100,
            // writable configurable enumerable
            text: "What happens to you if you break the monad laws?",
            "__proto__": { // [object Object]

            }
        },
        { // [object Object]
            // writable configurable enumerable
            id: 101,
            // writable configurable enumerable
            text: "JAVASCRIPT DUBSTEP GENERATOR",
            "__proto__": { // [object Object]

            }
        }
    ],
    "__proto__": { // [object Object]

    }
}
*/
Parameters

value : mixed
The source object to create a textual representation of.

indent : string
Optional. String to be used as indentation in output. 4 spaces by default.

limit : number
Optional. Number of properties to display per object.

Returns

string : The textual representation of value.

Document Tags and Contributors

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