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Object.keys()

2016-06-13 15:51 453 查看

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 2015 (6th Edition, ECMA-262) The definition of 'Object.keys' in that specification.	Standard
ECMAScript 2017 Draft (ECMA-262) The definition of 'Object.keys' in that specification.	Draft

Browser compatibility

Desktop

Mobile

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	5	4.0 (2.0)	9	12	5