一、代理

Proxy 用于修改某些操作的默认行为，等同于在语言层面做出修改，所以属于一种“元编程”（meta programming），即对编程语言进行编程。

Proxy 可以理解成，在目标对象之前架设一层“拦截”，外界对该对象的访问，都必须先通过这层拦截，因此提供了一种机制，可以对外界的访问进行过滤和改写。Proxy 这个词的原意是代理，用在这里表示由它来“代理”某些操作，可以译为“代理器”。

二、常用拦截方法

// ES5
let obj = {}
let newVal = ''
Object.defineProperty(obj, 'name', {get () {console.log('get')return newVal},set (val) {console.log('set')newVal = val}
})
obj.name = 'zz'
console.log(obj.name )// ES6 proxy
let obj = {}
let p = new Proxy(obj, {})
p.name = 'zzzzz'
console.log(p, obj.name)// get
let arr = [11,22,33]
arr = new Proxy(arr, {get(target, key) {console.log(target, key)if (key in target) {return target[key]} else {return 'err'}}
})
console.log(arr[5])// set
let arr = []
arr = new Proxy(arr, {set(target, key, val) {if (typeof val === 'number') {target[key] = valreturn true} else {return false}}
})
arr.push(1)
console.log(arr, arr[0], arr[1])// has
let range = {min: 1,max: 5
}
range = new Proxy(range, {has(target, key) {return key >= target.min && key <= target.max}
})
console.log( 2 in range)
console.log( 6 in range)// ownKeys
let obj = {name: 'zz',[Symbol('66')]: 66
}
Object.getOwnPropertyNames(obj)
Object.getOwnPropertySymbols(obj)
Object.keys(obj)
Reflect.ownKeys(obj)
let userInfo = {name: 'zz',age: 25,_password: '123456'
}
userInfo = new Proxy(userInfo, {ownKeys(target) {return Object.keys(target).filter(key => !key.startsWith('_'))}
})
for (let key in userInfo) {console.log(key)
}
console.log(Object.keys(userInfo))// apply
let sum = (...args) => {let sum = 0args.forEach(item => {sum += item})return sum
}
sum = new Proxy(sum, {apply(target, ctx, args) {return target(...args) * 2}
})
console.log(sum(1,2))
console.log(sum.call(null, 1,2,3));// construct
let User = class {constructor (name) {this.name = name}
}
User = new Proxy(User, {construct(target, args, newTarger) {return new target(...args)}
})
console.log(new User('zzz'))// 
let user = {name: 'zz',age: 25,_password: '123456'
}user = new Proxy(user, {get(target, key) {if (key.startsWith('_')) {throw new Error('不可访问')} else {return target[key]}},set(target, key, val) {if (key.startsWith('_')) {throw new Error('不可访问')} else {target[key] = valreturn true}},deleteProperty(target, key) {if (key.startsWith('_')) {throw new Error('不可删除')} else {delete target[key]return true}},ownKeys(target) {return Object.keys(target).filter(key => !key.startsWith('_'))},// apply
})
try {console.log(user._password)
} catch(e) {console.log(e.message)
}
console.log(user.name)
user.age = 18
console.log(user)
for (let key in user) {console.log(key)
}