Object.defineProperty()

obj

要定義屬性的物件。

prop

要被定義或修改的屬性名字。

descriptor

要定義或修改物件敘述內容。

被定義完或修改完屬性的物件。

這個函式可以用來增加或修改物件中的屬性定義。在物件建立屬性後，這些屬性同時有被設定預設的設定，才能讓這些屬性被列舉、改變和刪除。而這個函式可以用來改變這些預設的設定。根據預設，被加到物件且使用Object.defineProperty()的值都是Immutable。

物件內的屬性描述器（Property descriptor）主要有兩種：資料描述器（data descriptor）與訪問描述器（accessor descriptor）。資料描述器（data descriptor）是可以選擇能否覆寫的屬性。訪問描述器（accessor descriptor） is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both.

data 和 accessor descriptors 皆為物件，兩者共享下面提及的 key：

configurable

true 則若且唯若此屬性則將可改變或刪除自相應物件。 預設為 false

enumerable

true 如果且唯若相應物件被列舉，將會列舉此屬性。 預設為 false

一個 data descriptor 還有以下可選的 key：

value

The value associated with the property. Can be any valid JavaScript value (number, object, function, etc). 預設 undefined (en-US).

writable

true 則該物件屬性可透過賦予運算子 (en-US)改變其值。 預設 false

一個 accessor descriptor 也擁有下述之 optional keys:

get

作為 getter 形式，為屬性存在的函式，如果沒有 getter 的話則回傳 undefined (en-US)。函式回傳將用於屬性值。 預設 undefined (en-US)

set

作為 setter 形式，為屬性存在的函式，如果沒有 setter 的話則回傳 undefined (en-US)。 The function will receive as only argument the new value being assigned to the property. 預設 undefined (en-US)

請注意，這些選項並不一定要是 descriptor 屬性，由原型鍊（prototype chain）繼承的屬性，也會被考慮到。要確保需要凍結（freeze）的 Object.prototype (en-US) upfront 預設能被保存，請明確指定所有選項，或把 Object.prototype.__proto__ (en-US) 屬性指向 null (en-US)。

// using __proto__
var obj = {};
Object.defineProperty(obj, "key", {
  __proto__: null, // no inherited properties
  value: "static", // not enumerable
  // not configurable
  // not writable
  // as defaults
});

// being explicit
Object.defineProperty(obj, "key", {
  enumerable: false,
  configurable: false,
  writable: false,
  value: "static",
});

// recycling same object
function withValue(value) {
  var d =
    withValue.d ||
    (withValue.d = {
      enumerable: false,
      writable: false,
      configurable: false,
      value: null,
    });
  d.value = value;
  return d;
}
// ... and ...
Object.defineProperty(obj, "key", withValue("static"));

// if freeze is available, prevents adding or
// removing the object prototype properties
// (value, get, set, enumerable, writable, configurable)
(Object.freeze || Object)(Object.prototype);

If you want to see how to use the Object.defineProperty method with a binary-flags-like syntax, see additional examples.

When the property specified doesn't exist in the object, Object.defineProperty() creates a new property as described. Fields may be omitted from the descriptor, and default values for those fields are imputed. All of the Boolean-valued fields default to false. The value, get, and set fields default to undefined (en-US). A property which is defined without get/set/value/writable is called 「generic」 and is 「typed」 as a data descriptor.

var o = {}; // Creates a new object

// Example of an object property added with defineProperty with a data property descriptor
Object.defineProperty(o, "a", {
  value: 37,
  writable: true,
  enumerable: true,
  configurable: true,
});
// 'a' property exists in the o object and its value is 37

// Example of an object property added with defineProperty with an accessor property descriptor
var bValue = 38;
Object.defineProperty(o, "b", {
  get: function () {
    return bValue;
  },
  set: function (newValue) {
    bValue = newValue;
  },
  enumerable: true,
  configurable: true,
});
o.b; // 38
// 'b' property exists in the o object and its value is 38
// The value of o.b is now always identical to bValue, unless o.b is redefined

// You cannot try to mix both:
Object.defineProperty(o, "conflict", {
  value: 0x9f91102,
  get: function () {
    return 0xdeadbeef;
  },
});
// throws a TypeError: value appears only in data descriptors, get appears only in accessor descriptors

如果該屬性已經存在, Object.defineProperty() 將會根據描述符內的值和物件當前的 configuration 來修改屬性。 如果舊的描述符之 configurable 的特徵為 false (屬性為 「non-configurable」), 那除了 writable 之外的特徵都將無法修改。 在這個情況，也不可能在 data 和 accessor 屬性類型中來回切換。

如果有一個屬性是 non-configurable, 那它的 writable 特徵只能被改變為 false.

若嘗試改變 non-configurable property attributes，將會丟出一個 TypeError (en-US)，除非當前之值與新值相同。

Writable attribute

當 writable 屬性特徵被設為 false, 此屬性為 「non-writable」. 它將無法被重新賦值。

var o = {}; // Creates a new object

Object.defineProperty(o, "a", {
  value: 37,
  writable: false,
});

console.log(o.a); // logs 37
o.a = 25; // No error thrown (it would throw in strict mode, even if the value had been the same)
console.log(o.a); // logs 37. The assignment didn't work.

As seen in the example, trying to write into the non-writable property doesn't change it but doesn't throw an error either.

可列舉 attribute

The enumerable property attribute defines whether the property shows up in a for...in loop and Object.keys() or not.

var o = {};
Object.defineProperty(o, "a", { value: 1, enumerable: true });
Object.defineProperty(o, "b", { value: 2, enumerable: false });
Object.defineProperty(o, "c", { value: 3 }); // enumerable defaults to false
o.d = 4; // enumerable defaults to true when creating a property by setting it

for (var i in o) {
  console.log(i);
}
// logs 'a' and 'd' (in undefined order)

Object.keys(o); // ['a', 'd']

o.propertyIsEnumerable("a"); // true
o.propertyIsEnumerable("b"); // false
o.propertyIsEnumerable("c"); // false

可設定 attribute

The configurable attribute controls at the same time whether the property can be deleted from the object and whether its attributes (other than writable) can be changed.

var o = {};
Object.defineProperty(o, "a", {
  get: function () {
    return 1;
  },
  configurable: false,
});

Object.defineProperty(o, "a", { configurable: true }); // throws a TypeError
Object.defineProperty(o, "a", { enumerable: true }); // throws a TypeError
Object.defineProperty(o, "a", { set: function () {} }); // throws a TypeError (set was undefined previously)
Object.defineProperty(o, "a", {
  get: function () {
    return 1;
  },
}); // throws a TypeError (even though the new get does exactly the same thing)
Object.defineProperty(o, "a", { value: 12 }); // throws a TypeError

console.log(o.a); // logs 1
delete o.a; // Nothing happens
console.log(o.a); // logs 1

If the configurable attribute of o.a had been true, none of the errors would be thrown and the property would be deleted at the end.

It's important to consider the way default values of attributes are applied. There is often a difference between simply using dot notation to assign a value and using Object.defineProperty(), as shown in the example below.

var o = {};

o.a = 1;
// is equivalent to:
Object.defineProperty(o, "a", {
  value: 1,
  writable: true,
  configurable: true,
  enumerable: true,
});

// On the other hand,
Object.defineProperty(o, "a", { value: 1 });
// is equivalent to:
Object.defineProperty(o, "a", {
  value: 1,
  writable: false,
  configurable: false,
  enumerable: false,
});

Example below shows how to implement a self-archiving object. When temperature property is set, the archive array gets a log entry.

function Archiver() {
  var temperature = null;
  var archive = [];

  Object.defineProperty(this, "temperature", {
    get: function () {
      console.log("get!");
      return temperature;
    },
    set: function (value) {
      temperature = value;
      archive.push({ val: temperature });
    },
  });

  this.getArchive = function () {
    return archive;
  };
}

var arc = new Archiver();
arc.temperature; // 'get!'
arc.temperature = 11;
arc.temperature = 13;
arc.getArchive(); // [{ val: 11 }, { val: 13 }]

or

var pattern = {
  get: function () {
    return "I always return this string, whatever you have assigned";
  },
  set: function () {
    this.myname = "this is my name string";
  },
};

function TestDefineSetAndGet() {
  Object.defineProperty(this, "myproperty", pattern);
}

var instance = new TestDefineSetAndGet();
instance.myproperty = "test";
console.log(instance.myproperty); // I always return this string, whatever you have assigned

console.log(instance.myname); // this is my name string

It is possible to redefine the length property of arrays, subject to the usual redefinition restrictions. (The length property is initially non-configurable, non-enumerable, and writable. Thus on an unaltered array, it's possible to change the length property's value or to make it non-writable. It is not allowed to change its enumerability or configurability, or if it is non-writable to change its value or writability.) However, not all browsers permit this redefinition.

Firefox 4 through 22 will throw a TypeError (en-US) on any attempt whatsoever (whether permitted or not) to redefine the length property of an array.

Versions of Chrome which implement Object.defineProperty() in some circumstances ignore a length value different from the array's current length property. In some circumstances changing writability seems to silently not work (and not throw an exception). Also, relatedly, some array-mutating methods like Array.prototype.push don't respect a non-writable length.

Versions of Safari which implement Object.defineProperty() ignore a length value different from the array's current length property, and attempts to change writability execute without error but do not actually change the property's writability.

Only Internet Explorer 9 and later, and Firefox 23 and later, appear to fully and correctly implement redefinition of the length property of arrays. For now, don't rely on redefining the length property of an array to either work, or to work in a particular manner. And even when you can rely on it, there's really no good reason to do so.

Internet Explorer 8 implemented a Object.defineProperty() method that could only be used on DOM objects. A few things need to be noted:

• Trying to use Object.defineProperty() on native objects throws an error.
• Property attributes must be set to some values. The configurable, enumerable and writable attributes should all be set to true for data descriptor and true for configurable, false for enumerable for accessor descriptor.(?) Any attempt to provide other value(?) will result in an error being thrown.
• Reconfiguring a property requires first deleting the property. If the property isn't deleted, it stays as it was before the reconfiguration attempt.

• Enumerability and ownership of properties (en-US)
• Object.defineProperties() (en-US)
• Object.propertyIsEnumerable() (en-US)
• Object.getOwnPropertyDescriptor() (en-US)
• Object.prototype.watch()
• Object.prototype.unwatch()
• get (en-US)
• set (en-US)
• Object.create()
• Additional Object.defineProperty examples
• Reflect.defineProperty() (en-US)