深入理解ES6之《代理和反射》

摘要：使用陷阱验证属性用于接收属性代理的目标的对象要写入的属性键被写入的属性的值操作发生的对象通常是代理属性必须是数字抛错用陷阱验证对象结构属性不存在抛出错误使用陷阱隐藏已有属性可以用操作符来检测给定对象中是否包含有某个属性，如果自有属性或原型属

let target = {
    name: "target"
}
let proxy = new Proxy(target, {
    /**
     * 
     * 
     * @param {any} trapTarget 用于接收属性（代理的目标）的对象
     * @param {any} key 要写入的属性键
     * @param {any} value 被写入的属性的值
     * @param {any} receiver 操作发生的对象（通常是代理）
     */
    set(trapTarget, key, value, receiver) {
        if (!trapTarget.hasOwnProperty(key)) {
            if (isNaN(value)) {
                throw new TypeError("属性必须是数字")
            }
        }
        return Reflect.set(trapTarget, key, value, receiver)
    }
})
proxy.count = 1;
console.log(proxy.count)//1
console.log(target.count)//1

proxy.name = "proxy"
console.log(proxy.name)//proxy
console.log(target.name)//proxy

proxy.anthorName = "test"// 抛错

let proxy = new Proxy({}, {
    get(trapTarget, key, receiver) {
        if (!(key in receiver)) {
            throw new TypeError("属性" + key + "不存在")
        }
        return Reflect.get(trapTarget,key,receiver)
    }
})
proxy.name="proxy"
console.log(proxy.name)//proxy
console.log(proxy.nme)//抛出错误

可以用in操作符来检测给定对象中是否包含有某个属性，如果自有属性或原型属性匹配这个名称或Symbol就返回true

let target = {
    name: "target",
    value: 42
}
let proxy = new Proxy(target, {
    has(trapTarget, key) {
        if (key === "value") {
            return false
        }
        return Reflect.has(trapTarget, key)
    }
})
console.log("value" in proxy)//false
console.log("name" in proxy)//true
console.log("toString" in proxy)//true

不可配置属性name用delete操作返回的是false，如果在严格模式下还会抛出错误
可以通过deleteProperty陷阱来改变这个行为

let target = {
    name: "target",
    value: 42
}
let proxy = new Proxy(target, {
    deleteProperty(trapTarget, key) {
        if (key === "value") {
            return false
        }
        return Reflect.deleteProperty(trapTarget, key)
    }
})
console.log("value" in proxy)//true
let result1 = delete proxy.value
console.log(result1)//false
console.log("value" in proxy)//true
//尝试删除不可配置属性name 如果没有使用代理则会返回false并且删除不成功
console.log("name" in proxy)//true
let result2 = delete proxy.name;
console.log(result2)//true
console.log("name" in proxy)//false

let target = {}
let proxy = new Proxy(target, {
    getPrototypeOf(trapTarget) {
        //必须返回对象或null，只要返回的是值类型必将导致运行时错误
        return null;
    },
    setPrototypeOf(trapTarget, proto) {
        // 如果操作失败则返回false  如果setPrototypeOf返回了任何不是false的值，那么Object.setPrototypeOf便设置成功
        return false
    }
})
let targetProto = Object.getPrototypeOf(target);
let proxyProto = Object.getPrototypeOf(proxy)
console.log(targetProto === Object.prototype)//true
console.log(proxyProto === Object.prototype)//false
Object.setPrototypeOf(target, {})//成功
Object.setPrototypeOf(proxy, {})//抛出错误

再看一下下面的代码

let target = {}
let proxy = new Proxy(target, {
    getPrototypeOf(trapTarget) {
        return Reflect.getPrototypeOf(trapTarget);
    },
    setPrototypeOf(trapTarget, proto) {
        return Reflect.setPrototypeOf(trapTarget,proto)
    }
})
let targetProto = Object.getPrototypeOf(target);
let proxyProto = Object.getPrototypeOf(proxy)
console.log(targetProto === Object.prototype)//true
console.log(proxyProto === Object.prototype)//true
Object.setPrototypeOf(target, {})//成功
Object.setPrototypeOf(proxy, {})//成功

再来说说Object.getPrototypeOf和Reflect.getPrototypeOf的异同点吧
1如果传入的参数不是对象，则Reflect.getPrototypeOf方法会抛出错误，而Object.getPrototypeOf方法则会在操作执行前先将参数强制转换为一个对象（对于Object.getPrototypeOf也是同样）

let result = Object.getPrototypeOf(1)
console.log(result === Number.prototype)//true
Reflect.getPrototypeOf(1)//抛错

let target = {}
let proxy = new Proxy(target, {
    isExtensible(trapTarget) {
        return Reflect.isExtensible(trapTarget)
    },
    preventExtensions(trapTarget) {
        return Reflect.preventExtensions(trapTarget)
    }
})
console.log(Object.isExtensible(target))//true
console.log(Object.isExtensible(proxy))//true
Object.preventExtensions(proxy)
console.log(Object.isExtensible(target))//false
console.log(Object.isExtensible(proxy))//false

比方说你想让Object.preventExtensions失败，可返回false,看下面的例子

let target = {}
let proxy = new Proxy(target, {
    isExtensible(trapTarget) {
        return Reflect.isExtensible(trapTarget)
    },
    preventExtensions(trapTarget) {
        return false
    }
})
console.log(Object.isExtensible(target))//true
console.log(Object.isExtensible(proxy))//true
Object.preventExtensions(proxy)
console.log(Object.isExtensible(target))//true  //书上说这里会返回true，可是我自己运行的时候就已经抛出错误了
console.log(Object.isExtensible(proxy))//true

Object.isExtensible和Reflect.isExtensible方法非常相似，只有当传入非对象值时，Object.isExtensible返回false而Reflect.isExtensible则抛出错误

let result1 = Object.isExtensible(2)
console.log(result1)//false
let result2 = Reflect.isExtensible(2)

Object.preventExtensions和Reflect.preventExtensions非常类似，无论传入Object.preventExtensions方法的参数是否为一个对象，它总是返回该参数，而如果Reflect.preventExtensions方法的参数不是一个对象则会抛出错误，如果参数是一个对象，操作成功时Reflect.preventExtensions会返回true否则返回false

let result1 = Object.preventExtensions(2)
console.log(result1)//2
let target = {}
let result2 = Reflect.preventExtensions(target)
console.log(result2)//true
let result3 = Reflect.preventExtensions(3)///抛出错误

let proxy = new Proxy({}, {
    defineProperty(trapTarget, key, descriptor) {
        if (typeof key === "symbol") {
            return false
        }
        return Reflect.defineProperty(trapTarget, key, descriptor)
    }
})
Object.defineProperty(proxy, "name", {
    value: "proxy"
})
console.log(proxy.name)//proxy
let nameSymbol = Symbol("name")
//抛错
Object.defineProperty(proxy, nameSymbol, {
    value: "proxy"
})

如果让陷阱返回true并且不调用Reflect.defineProperty方法，则可以让Object.defineProperty方法静默失效，这既消除了错误又不会真正定义属性
无论将什么参数作为第三个参数传递给Object.defineProperty方法都只有属性enumerable、configurable、value、writable、get和set将出现在传递给defineProperty陷阱的描述符对象中

let proxy = new Proxy({}, {
    defineProperty(trapTarget, key, descriptor) {
        console.log(descriptor)
        console.log(descriptor.value)
        console.log(descriptor.name)
        return Reflect.defineProperty(trapTarget, key, descriptor)
    }
})
Object.defineProperty(proxy, "name", {
    value: "proxy",
    name: "custom"
})

getOwnPropertyDescriptor它的返回值必须是null、undefined或是一个对象，如果返回对象，则对象自己的属性只能是enumerable、configurable、value、writable、get和set,在返回的对象中使用不被允许的属性则会抛出一个错误

let proxy = new Proxy({}, {
    getOwnPropertyDescriptor(trapTarget, key) {
        //在返回的对象中使用不被允许的属性则会抛出一个错误
        return {
            name: "proxy"
        }
    }
})
let descriptor = Object.getOwnPropertyDescriptor(proxy, "name")

Object.defineProperty和Reflect.defineProperty只有返回值不同
Object.defineProperty返回第一个参数
Reflect.defineProperty的返回值与操作有关，成功则返回true,失败则返回false

let target = {}
let result1 = Object.defineProperty(target, "name", { value: "target" })
console.log(target === result1)//true
let result2 = Reflect.defineProperty(target, "name", { value: "refelct" })
console.log(result2)//false

Object.getOwnPropertyDescriptor如果传入原始值作为第一个参数，内部会将这个值强制转换成一个对象，若调用Reflect.getOwnPropertyDescriptor传入原始值作为第一个参数，则会抛出错误

let proxy = new Proxy({}, {
    ownKeys(trapTarget) {
        return Reflect.ownKeys(trapTarget).filter(key => {
            return typeof key !== "string" || key[0] !== "_"
        })
    }
})
let nameSymbol = Symbol("name")
proxy.name = "proxy"
proxy._name = "private"
proxy[nameSymbol] = "symbol"

let names = Object.getOwnPropertyNames(proxy),
    keys = Object.keys(proxy),
    symbols = Object.getOwnPropertySymbols(proxy)
console.log(names)//["name"]
console.log(keys)//["name"]
console.log(symbols)//[Symbol(name)]

尽管ownKeys代理陷阱可以修改一小部分操作返回的键，但不影响更常用的操作，例如for of循环，这些不能使用代理为更改，ownKeys陷阱也会影响for in循环，当确定循环内部使用的键时会调用陷阱

let target = function () { return 42; },
    proxy = new Proxy(target, {
        apply: function (trapTarget, thisArg, argumentList) {
            return Reflect.apply(trapTarget, thisArg, argumentList)
        },
        construct: function (trapTarget, argumentList) {
            return Reflect.construct(trapTarget, argumentList)
        }
    });
    //一个目标是函数的代理看起来也像是一个函数
console.log(typeof proxy)//function
console.log(proxy())//42
let instance=new proxy();
//用new创建一个instance对象，它同时是代理和目标的实例，因为instanceof通过原型链来确定此信息，而原型链查找不受代理影响，这也就是代理和目标好像有相同原型的原因
console.log(instance instanceof proxy)//true
console.log(instance instanceof target)//true

可以在apply陷阱中检查参数，在construct陷阱中来确认函数不会被new调用

function sum(...values) {
    return values.reduce((pre, cur) => pre + cur, 0)
}
let sumProxy = new Proxy(sum, {
    apply: function (trapTarget, thisArg, argumentList) {
        argumentList.forEach(arg => {
            if (typeof arg !== "number") {
                throw new TypeError("所有参数必须是数字。")
            }
        });
        return Reflect.apply(trapTarget, thisArg, argumentList)
    },
    construct: function (trapTarget, argumentList) {
        throw new TypeError("该函数不可通过new来调用")
    }
})
console.log(sumProxy(1, 2, 3, 4, 5))//15
console.log(sumProxy(1, 2, "3", 4, 5))//抛出错误
let result = new sumProxy()//抛出错误

以下例子是确保用new来调用函数并验证其参数为数字

function Numbers(...values) {
    this.values = values
}
let NumberProxy = new Proxy(Numbers, {
    apply: function (trapTarget, thisArg, argumentList) {
        throw new TypeError("该函数必须通过new来调用")
    },
    construct: function (trapTarget, argumentList) {
        argumentList.forEach(arg => {
            if (typeof arg !== "number") {
                throw new TypeError("所有参数必须是数字")
            }
        })
        return Reflect.construct(trapTarget, argumentList)
    }
})
let instance = new NumberProxy(12, 3, 4, 8)
console.log(instance.values)// [12, 3, 4, 8]
NumberProxy(1, 2, 3, 4)//报错

看一个不用new调用构造函数的例子：

function Numbers(...values) {
    if (typeof new.target === "undefined") {
        throw new TypeError("该函数必须通过new来调用")
    }
    this.values = values
}
let NumberProxy = new Proxy(Numbers, {
    apply: function (trapTarget, thisArg, argumentList) {
        return Reflect.construct(trapTarget, argumentList)
    }
})
let instance = NumberProxy(1, 2, 3, 4)
console.log(instance.values)//[1,2,3,4]

覆写抽象基类构造函数

class AbstractNumbers {
    constructor(...values) {
        if (new.target === AbstractNumbers) {
            throw new TypeError("此函数必须被继承")
        }
        this.values = values
    }
}
class Numbers extends AbstractNumbers{}
let instance = new Numbers(1,2,3,4,5)
console.log(instance.values)//[1, 2, 3, 4, 5]
new AbstractNumbers(1,2,3,4,5)//报错  此函数必须被继承

手动用代理给new.target赋值来绕过构造函数限制

class AbstractNumbers {
    constructor(...values) {
        if (new.target === AbstractNumbers) {
            throw new TypeError("此函数必须被继承")
        }
        this.values = values
    }
}
let AbstractNumbersProxy = new Proxy(AbstractNumbers, {
    construct: function (trapTarget, argumentList) {
        return Reflect.construct(trapTarget, argumentList, function () { })
    }
})
let instance = new AbstractNumbersProxy(1, 2, 3, 4)
console.log(instance.values)//[1, 2, 3, 4]

可调用的类构造函数

class Person {
    constructor(name) {
        this.name = name;
    }
}
let PersonProxy = new Proxy(Person, {
    apply: function (trapTarget, thisArg, argumentList) {
        return new trapTarget(...argumentList)
    }
})
let me = PersonProxy("angela")
console.log(me.name)//angela
console.log(me instanceof Person)//true
console.log(me instanceof PersonProxy)//true

可撤销代理

let target = {
    name: "target"
}
let { proxy, revoke } = Proxy.revocable(target, {})
console.log(proxy.name)//traget
revoke()
console.log(proxy.name)//报错

function toUint32(value) {
    return Math.floor(Math.abs(Number(value))) % Math.pow(2, 32)
}
function isArrayIndex(key) {
    let numericKey = toUint32(key)
    return String(numericKey) == key && numericKey < (Math.pow(2, 32) - 1)
}
function createMyArray(length = 0) {
    return new Proxy({ length }, {
        set(trapTarget, key, value) {
            let currentLength = Reflect.get(trapTarget, "length")
            if (isArrayIndex(key)) {
                let numericKey = Number(key)
                if (numericKey >= currentLength) {
                    Reflect.set(trapTarget, "length", numericKey + 1)
                }
            } else if (key === "length") {
                if (value < currentLength) {
                    for (let index = currentLength - 1; index >= value; index--) {
                        Reflect.deleteProperty(trapTarget, index)
                    }
                }
            }
            Reflect.set(trapTarget, key, value)
        }
    })
}
let colors = createMyArray(3)
colors[0] = "red"
colors[1] = "green"
colors[2] = "blue"
console.log(colors.length)//3
colors[3] = "black"
console.log(colors[3])//black
console.log(colors.length)//4
colors.length = 1
console.log(colors)//{0: "red", length: 1}

如果代理是原型，仅当默认操作继续执行到原型上时才调用代理陷阱，这会限制代理作为原型的能力
在原型上使用get陷阱

let target={}
let thing=Object.create(new Proxy(target,{
    /**
     * 
     * 
     * @param {any} trapTarget  原型对象
     * @param {any} key 
     * @param {any} receiver 实例对象
     */
    get(trapTarget,key,receiver){
        throw new ReferenceError(`${key} doesn"t exist`)
    }
}))
thing.name="thing"
console.log(thing.name)//thing
let unknown=thing.unknown//抛出错误

在原型上使用set陷阱

let target={}
let thing=Object.create(new Proxy(target,{
    set(trapTarget,key,value,receiver){
        return Reflect.set(trapTarget,key,value,receiver)
    }
}))
console.log(thing.hasOwnProperty("name"))
//触发set代理陷阱
thing.name="thing"
console.log(thing.name)
console.log(thing.hasOwnProperty("name"))
//不触发set代理陷阱
thing.name="boo"
console.log(thing.name)//boo

在原型上使用has陷阱

let target = {}
let thing = Object.create(new Proxy(target, {
    has(trapTarget, key) {
        return Reflect.has(trapTarget, key)
    }
}))
//触发has代理陷阱
console.log("name" in thing)//false
thing.name = "thing"
//不触发has代理陷阱
console.log("name" in thing)//true

将代理用作类的原型

function NoSuchProperty(){}
NoSuchProperty.prototype=new Proxy({},{
    get(trapTarget,key,receiver){
        throw new ReferenceError(`${key} doesn"t exist`)
    }
})
let thing=new NoSuchProperty()
//在get代理陷阱中抛出错误
let result=thing.name

function NoSuchProperty() { }
NoSuchProperty.prototype = new Proxy({}, {
    get(trapTarget, key, receiver) {
        throw new ReferenceError(`${key} doesn"t exist`)
    }
})
class Square extends NoSuchProperty {
    constructor(length, width) {
        super()
        this.length = length;
        this.width = width
    }
}
let shape = new Square(2, 6)
let area1 = shape.length * shape.width
console.log(area1)//12
let area2 = shape.length * shape.wdth//抛出错误