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    Object.keys()

    Summary

    The Object.keys() method returns an array of a given object's own enumerable properties, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

    Syntax

    Object.keys(obj)

    Parameters

    obj
    The object whose enumerable own properties are to be returned.

    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.

    Examples

    var arr = ['a', 'b', 'c'];
    console.log(Object.keys(arr)); // console: ['0', '1', '2']
    
    // array like object
    var obj = { 0: 'a', 1: 'b', 2: 'c' };
    console.log(Object.keys(obj)); // console: ['0', '1', '2']
    
    // array like object with random key ordering
    var an_obj = { 100: 'a', 2: 'b', 7: 'c' };
    console.log(Object.keys(an_obj)); // console: ['2', '7', '100']
    
    // getFoo is property which isn't enumerable
    var my_obj = Object.create({}, { getFoo: { value: function() { return this.foo; } } });
    my_obj.foo = 1;
    
    console.log(Object.keys(my_obj)); // console: ['foo']
    

    If you want all properties, even not enumerables, see Object.getOwnPropertyNames().

    Notes

    In ES5, if the argument to this method is not an object (a primitive), then it will cause a TypeError. In ES6, a non-object argument will be coerced to an object.

    > Object.keys("foo")
    TypeError: "foo" is not an object // ES5 code
    
    > Object.keys("foo")
    ["0", "1", "2"]                   // ES6 code
    

    Polyfill

    To add compatible Object.keys support in older environments that do not natively support it, copy the following snippet:

    // From https://developer.mozilla.org/en-US/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 !== 'object' && (typeof obj !== 'function' || 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.

    Specifications

    Specification Status Comment
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Object.keys' in that specification.
    Standard Initial definition. Implemented in JavaScript 1.8.5.
    ECMAScript 6 (ECMA-262)
    The definition of 'Object.keys' in that specification.
    Release Candidate  

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support 5 4.0 (2.0) 9 12 5
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support ? ? ? ? ? ?

    Based on Kangax's compat table.

    See also

    Document Tags and Contributors

    Last updated by: jscape,