Object.keys()

Object.keys() 方法會回傳一個由指定物件所有可列舉之屬性組成的陣列，該陣列中的的排列順序與使用 for...in 進行迭代的順序相同（兩者的差異在於 for-in 迴圈還會迭代出物件自其原型鏈所繼承來的可列舉屬性）。

語法

Object.keys(obj)

參數

obj

The object whose enumerable own properties are to be returned.

描述

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.

範例

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().

備註

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.

規範

瀏覽器相容性

參見

• Enumerability and ownership of properties
• Object.prototype.propertyIsEnumerable()
• Object.create()
• Object.getOwnPropertyNames()
• Object.values()
• Object.entries()