Object.keys()
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.
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"]
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 !== '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.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-object.keys |
Browser compatibility
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