元编程

从ECMAScript 2015 开始,JavaScript 获得了 Reflect 对象的支持,允许你拦截并定义基本语言操作的自定义行为(例如,属性查找,赋值,枚举,函数调用等)。借助这两个对象,你可以在 JavaScript 元级别进行编程。

代理

在 ECMAScript 6 中引入的 Proxy 对象可以拦截某些操作并实现自定义行为。例如获取一个对象上的属性:

let handler = {
  get: function(target, name){
    return name in target ? target[name] : 42;
}};

let p = new Proxy({}, handler);
p.a = 1;

console.log(p.a, p.b); // 1, 42

Proxy 对象定义了一个目标(这里是一个空对象)和一个实现了 get 陷阱的 handle 对象。这里,代理的对象在获取未定义的属性时不会返回 undefined,而是返回 42。

更多例子参见 Proxy 页面 。

术语

在讨论代理的功能时会用到以下术语。

handler
包含陷阱的占位符对象。
traps
提供属性访问的方法。这类似于操作系统中陷阱的概念。
target
代理虚拟化的对象。它通常用作代理的存储后端。根据目标验证关于对象不可扩展性或不可配置属性的不变量(保持不变的语义)。
invariants
实现自定义操作时保持不变的语义称为不变量。如果你违反处理程序的不变量,则会抛出一个 TypeError

句柄和陷阱

以下表格中总结了 Proxy 对象可用的陷阱。详细的解释和例子请看参考页

Handler / trap Interceptions Invariants
handler.getPrototypeOf() instanceof
  • getPrototypeOf方法一定返回一个对象或null.
  • 如果 target 不可扩展,Object.getPrototypeOf(proxy) 必须返回和 Object.getPrototypeOf(target)一样的值。
handler.setPrototypeOf() Reflect.setPrototypeOf() 如果 target 不可扩展,prototype 参数必须与Object.getPrototypeOf(target)的值相同。
handler.isExtensible() Reflect.isExtensible() Object.isExtensible(proxy) 必须返回和Object.isExtensible(target)一样的值。
handler.preventExtensions() Reflect.preventExtensions()  如果Object.isExtensible(proxy) 值为 false,Object.preventExtensions(proxy) 只返回true。
handler.getOwnPropertyDescriptor() Reflect.getOwnPropertyDescriptor()
  • getOwnPropertyDescripton 只能返回对象或者undefined.
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
  • A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.
  • A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.
  • The result of Object.getOwnPropertyDescriptor(target) can be applied to the target object using Object.defineProperty and will not throw an exception.
handler.defineProperty() Reflect.defineProperty()
  • A property cannot be added, if the target object is not extensible.
  • A property cannot be added as or modified to be non-configurable, if it does not exists as a non-configurable own property of the target object.
  • A property may not be non-configurable, if a corresponding configurable property of the target object exists.
  • If a property has a corresponding target object property then Object.defineProperty(target, prop, descriptor) will not throw an exception.
  • In strict mode, a false return value from the defineProperty handler will throw a TypeError exception.
handler.has() Property query: foo in proxy
Inherited property query: foo in Object.create(proxy)
Reflect.has()
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
handler.get() Property access: proxy[foo]and proxy.bar
Inherited property access: Object.create(proxy)[foo]
Reflect.get()
  • The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable data property.
  • The value reported for a property must be undefined if the corresponding target object property is non-configurable accessor property that has undefined as its [[Get]] attribute.
handler.set() Property assignment: proxy[foo] = bar and proxy.foo = bar
Inherited property assignment: Object.create(proxy)[foo] = bar
Reflect.set()
  • Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable data property.
  • Cannot set the value of a property if the corresponding target object property is a non-configurable accessor property that has undefined as its [[Set]] attribute.
  • In strict mode, a false return value from the set handler will throw a TypeError exception.
handler.deleteProperty() Property deletion: delete proxy[foo] and delete proxy.foo
Reflect.deleteProperty()
A property cannot be deleted, if it exists as a non-configurable own property of the target object.
handler.enumerate() Property enumeration / for...in: for (var name in proxy) {...}
Reflect.enumerate()
The enumerate method must return an object.
handler.ownKeys() Reflect.ownKeys()
  • The result of ownKeys is a List.
  • The Type of each result List element is either Symbol.
  • The result List must contain the keys of all non-configurable own properties of the target object.
  • If the target object is not extensible, then the result List must contain all the keys of the own properties of the target object and no other values.
handler.apply() proxy(..args)
Reflect.apply()
There are no invariants for the handler.apply method.
handler.construct() new proxy(...args)
Reflect.construct()
结果一定是一个Object

撤销 Proxy

TypeError

var revocable = Proxy.revocable({}, {
  get: function(target, name) {
    return "[[" + name + "]]";
  }
});
var proxy = revocable.proxy;
console.log(proxy.foo); // "[[foo]]"

revocable.revoke();

console.log(proxy.foo); // TypeError is thrown
proxy.foo = 1           // TypeError again
delete proxy.foo;       // still TypeError
typeof proxy            // "object", typeof doesn't trigger any trap

反射

代理句柄类似,但 Reflect 方法并不是一个函数对象。

Reflect 有助于将默认操作从处理程序转发到目标。

Reflect.has() 为例,你可以将 in 运算符作为函数:

Reflect.has(Object, "assign"); // true

更好的 apply 函数

在 ES5 中,我们通常使用 Function.prototype.apply() 方法调用一个具有给定 this 值和 arguments 数组(或类数组对象)的函数。

Function.prototype.apply.call(Math.floor, undefined, [1.75]);

使用 Reflect.apply,这变得不那么冗长和容易理解:

Reflect.apply(Math.floor, undefined, [1.75]); 
// 1;

Reflect.apply(String.fromCharCode, undefined, [104, 101, 108, 108, 111]);
// "hello"

Reflect.apply(RegExp.prototype.exec, /ab/, ['confabulation']).index;
// 4

Reflect.apply(''.charAt, 'ponies', [3]);
// "i"

检查属性定义是否成功

使用 if...else 块:

if (Reflect.defineProperty(target, property, attributes)) {
  // success
} else {
  // failure
}