setter

概述

set 语法可以将一个函数绑定在当前对象的指定属性上，当那个属性被赋值时，你所绑定的函数就会被调用。

语法

{set prop(val) { . . . }}
{set [expression](val) { . . . }}

参数

prop

将被指定函数绑定的属性名。

val

An alias for the variable that holds the value attempted to be assigned to prop.

expression

Starting with ECMAScript 6, you can also use expressions for a computed property name to bind to the given function.

描述

In JavaScript, a setter can be used to execute a function whenever a specified property is attempted to be changed. Setters are most often used in conjunction with getters to create a type of pseudo-property. It is not possible to simultaneously have a setter on a property that holds an actual value.

Note the following when working with the set syntax:

A setter can be removed using the delete operator.

示例

在对象初始化时定义 setter

This will define a pseudo-property current of object o that, when assigned a value, will update log with that value:

var o = {
  set current (str) {
    return this.log[this.log.length] = str;
  },
  log: []
}

请注意，current 属性是未定义的，访问它时将会返回 undefined。

用 delete 操作符移除一个 setter

我们可以使用 delete 操作符移除 setter。

delete o.current;

使用 defineProperty 为已存在的对象定义 setter

我们可以随时使用 Object.defineProperty() 给一个已经存在的对象添加一个 setter。

var o = { a:0 };

Object.defineProperty(o, "b", { set: function (x) { this.a = x / 2; } });

o.b = 10; // Runs the setter, which assigns 10 / 2 (5) to the 'a' property
console.log(o.a) // 5

使用 computed 属性名

var expr = "foo";

var obj = {
  baz: "bar",
  set [expr](v) { this.baz = v; }
};

console.log(obj.baz); // "bar"
obj.foo = "baz";      // run the setter
console.log(obj.baz); // "baz"

规范

浏览器兼容性

根据 Robert Nyman's page 显示，在不支持 setter 的浏览器上（尤其是 IE6-8），将会抛出一个语法错误。

SpiderMonkey 用户注意

• 从  JavaScript 1.8.1 开始 ，在设置对象属性和数组初始化时，setters 将不会被调用。

相关链接

• getter
• delete
• Object.defineProperty()
• __defineGetter__
• __defineSetter__
• Defining Getters and Setters in JavaScript Guide