The Object.keys()
method returns an array of a given object's own enumerable property names, iterated in the same order that a normal loop would.
Syntax
Object.keys(obj)
Parameters
obj
- The object of which the enumerable's own properties are to be returned.
Return value
An array of strings that represent all the enumerable properties of the given object.
Description
Object.keys()
returns an array whose elements are strings corresponding to the enumerable properties found directly upon object
. The ordering of the properties is the same as that given by looping over the properties of the object manually.
Polyfill
To add compatible Object.keys
support in older environments that do not natively support it, copy the following snippet:
// From /docs/Web/JavaScript/Reference/Global_Objects/Object/keys if (!Object.keys) { Object.keys = (function() { 'use strict'; var hasOwnProperty = Object.prototype.hasOwnProperty, hasDontEnumBug = !({ toString: null }).propertyIsEnumerable('toString'), dontEnums = [ 'toString', 'toLocaleString', 'valueOf', 'hasOwnProperty', 'isPrototypeOf', 'propertyIsEnumerable', 'constructor' ], dontEnumsLength = dontEnums.length; return function(obj) { if (typeof obj !== 'function' && (typeof obj !== 'object' || obj === null)) { throw new TypeError('Object.keys called on non-object'); } var result = [], prop, i; for (prop in obj) { if (hasOwnProperty.call(obj, prop)) { result.push(prop); } } if (hasDontEnumBug) { for (i = 0; i < dontEnumsLength; i++) { if (hasOwnProperty.call(obj, dontEnums[i])) { result.push(dontEnums[i]); } } } return result; }; }()); }
Please note that the above code includes non-enumerable keys in IE7 (and maybe IE8), when passing in an object from a different window.
For a simple Browser Polyfill, see Javascript - Object.keys Browser Compatibility.
Examples
Using Object.keys
// simple array const arr = ['a', 'b', 'c']; console.log(Object.keys(arr)); // console: ['0', '1', '2'] // array-like object const obj = { 0: 'a', 1: 'b', 2: 'c' }; console.log(Object.keys(obj)); // console: ['0', '1', '2'] // array-like object with random key ordering const anObj = { 100: 'a', 2: 'b', 7: 'c' }; console.log(Object.keys(anObj)); // console: ['2', '7', '100'] // getFoo is a property which isn't enumerable const myObj = Object.create({}, { getFoo: { value: function () { return this.foo; } } }); myObj.foo = 1; console.log(Object.keys(myObj)); // console: ['foo']
If you want all properties—including non-enumerables—see Object.getOwnPropertyNames()
.
Non-object coercion
In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError
.
From ES2015 onwards, a non-object argument will be coerced to an object.
// In ES5 Object.keys('foo'); // TypeError: "foo" is not an object // In ES2015+ Object.keys('foo'); // ["0", "1", "2"]
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Object.keys' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
keys | Chrome Full support 5 | Edge Full support 12 | Firefox Full support 4 | IE Full support 9 | Opera Full support 12 | Safari Full support 5 | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 12 | Safari iOS Full support 5 | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
Legend
- Full support
- Full support