Object.defineProperties()

Object.defineProperties() 函式可定義新的或是修改已存在的物件屬性，並回傳修改後的物件。

語法

Object.defineProperties(obj, props)

參數

obj

The object on which to define or modify properties.

props

An object whose own enumerable properties constitute descriptors for the properties to be defined or modified. Property descriptors present in objects come in two main flavors: data descriptors and accessor descriptors (see Object.defineProperty() for more details). Descriptors have the following keys:

configurable: true if and only if the type of this property descriptor may be changed and if the property may be deleted from the corresponding object.
預設為 false.
enumerable: 若該屬性設為 true，則該屬性可被物件所列舉。
預設為 false.
value: The value associated with the property. Can be any valid JavaScript value (number, object, function, etc).
預設為 undefined.
writable: 若該屬性為 true，則該屬性可透過賦予運算子 (en-US)所改變
預設為 false.
get: A function which serves as a getter for the property, or undefined if there is no getter. The function return will be used as the value of property.
預設為 undefined.
set: A function which serves as a setter for the property, or undefined if there is no setter. The function will receive as only argument the new value being assigned to the property.
預設為 undefined.

回傳值

The object that was passed to the function.

描述

Object.defineProperties, in essence, defines all properties corresponding to the enumerable own properties of props on the object obj object.

範例

var obj = {};
Object.defineProperties(obj, {
  'property1': {
    value: true,
    writable: true
  },
  'property2': {
    value: 'Hello',
    writable: false
  }
  // etc. etc.
});

Polyfill

Assuming a pristine execution environment with all names and properties referring to their initial values, Object.defineProperties is almost completely equivalent (note the comment in isCallable) to the following reimplementation in JavaScript:

function defineProperties(obj, properties) {
  function convertToDescriptor(desc) {
    function hasProperty(obj, prop) {
      return Object.prototype.hasOwnProperty.call(obj, prop);
    }

    function isCallable(v) {
      // NB: modify as necessary if other values than functions are callable.
      return typeof v === 'function';
    }

    if (typeof desc !== 'object' || desc === null)
      throw new TypeError('bad desc');

    var d = {};

    if (hasProperty(desc, 'enumerable'))
      d.enumerable = !!desc.enumerable;
    if (hasProperty(desc, 'configurable'))
      d.configurable = !!desc.configurable;
    if (hasProperty(desc, 'value'))
      d.value = desc.value;
    if (hasProperty(desc, 'writable'))
      d.writable = !!desc.writable;
    if (hasProperty(desc, 'get')) {
      var g = desc.get;

      if (!isCallable(g) && typeof g !== 'undefined')
        throw new TypeError('bad get');
      d.get = g;
    }
    if (hasProperty(desc, 'set')) {
      var s = desc.set;
      if (!isCallable(s) && typeof s !== 'undefined')
        throw new TypeError('bad set');
      d.set = s;
    }

    if (('get' in d || 'set' in d) && ('value' in d || 'writable' in d))
      throw new TypeError('identity-confused descriptor');

    return d;
  }

  if (typeof obj !== 'object' || obj === null)
    throw new TypeError('bad obj');

  properties = Object(properties);

  var keys = Object.keys(properties);
  var descs = [];

  for (var i = 0; i < keys.length; i++)
    descs.push([keys[i], convertToDescriptor(properties[keys[i]])]);

  for (var i = 0; i < descs.length; i++)
    Object.defineProperty(obj, descs[i][0], descs[i][1]);

  return obj;
}

規範

Specification
ECMAScript Language Specification # sec-object.defineproperties

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