Object.seal方法理解

在阅读react源码时注意到react在创建ref时使用了Object.seal来保证RefObject的一致性, 具体如下

/* ReactCreateRef.js */
// an immutable object with a single mutable value
export function createRef(): RefObject {
  const refObject = {
    current: null,
  };
  if (__DEV__) {
    Object.seal(refObject);
  }
  return refObject;
}

摘自MDN:

The Object.seal() method seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Values of present properties can still be changed as long as they are writable.

简单来说, Object.seal()方法被用来封闭一个对象，阻止添加新属性并将所有现有属性标记为不可配置。当前属性的值只要可写就可以改变, 并且返回一个被密封的对象的引用.

看看MDN上面的一些例子:

var obj = {
  prop: function() {},
  foo: 'bar'
};

// New properties may be added, existing properties
// may be changed or removed.
obj.foo = 'baz';
obj.lumpy = 'woof';
delete obj.prop;

var o = Object.seal(obj);

o === obj; // true
Object.isSealed(obj); // === true

// Changing property values on a sealed object
// still works.
obj.foo = 'quux';

// But you can't convert data properties to accessors,
// or vice versa.
Object.defineProperty(obj, 'foo', {
  get: function() { return 'g'; }
}); // throws a TypeError

// Now any changes, other than to property values,
// will fail.
obj.quaxxor = 'the friendly duck';
// silently doesn't add the property
delete obj.foo;
// silently doesn't delete the property

// ...and in strict mode such attempts
// will throw TypeErrors.
function fail() {
  'use strict';
  delete obj.foo; // throws a TypeError
  obj.sparky = 'arf'; // throws a TypeError
}
fail();

// Attempted additions through
// Object.defineProperty will also throw.
Object.defineProperty(obj, 'ohai', {
  value: 17
}); // throws a TypeError
Object.defineProperty(obj, 'foo', {
  value: 'eit'
}); // changes existing property value

从上面我们可以看出:

1. 被封闭对象仍旧全等该对象本身
2. 可以通过Object.isSealed来判断当前对象是否被封闭
3. 不能为被封闭对象添加任何未知属性, 也不能为其已知属性添加访问者
4. 可以修改已知的属性

再看下面例子

var a = {b: 1, c: 2, d: {e: 3}}
Object.seal(a)
a.d.f = 4;
a.d
{e: 3, f: 4}

可以看出: 与Object.freeze一样, Object.seal方法一样, Object.seal并不是递归的, 在封闭一个对象之后, 我们仍然可以访问/设置其下一级的属性.

参考: MDN