Detailed explanation of Proxy in JavaScript

What is Proxy

First of all, we need to understand what Proxy means. When translated, this word means acting.

It can be understood that there is a very popular star who has opened a Weibo account. This account is very active, replying to fans, giving likes everywhere, etc., but it may not really be maintained by him. .

But there is another person or team behind the operation, we can call them agents, because the Weibo they publish represents the meaning of the star himself.

P.S. Forcibly give an example, because I am not chasing stars, I just guess that there may be such an operation team

This is substituted into JavaScript, It can be understood as a proxy operation on object or function.

Proxy in JavaScript

Proxy is a new API provided in ES6, which can be used to define custom behaviors for various basic operations of objects (discussed in the document It's called traps, I think it can be understood as a hook for various behaviors of objects), you can do a lot of interesting things with it, and it will become very effective when we need to control the behavior of some objects.

Proxy syntax

Creating an instance of Proxy requires passing in two parameters

1. target The object to be proxied can be an object or function
2. handlers Various operating behaviors of the proxy object Processing

let target = {}
let handlers = {} // do nothing
let proxy = new Proxy(target, handlers)

proxy.a = 123

console.log(target.a) // 123

When the second parameter is an empty object, it can basically be understood as a shallow copy of the first parameter
(Proxy must be a shallow copy , if it is a deep copy, the meaning of the agent will be lost)

Traps (agent for various behaviors)

Just like the sample code above, If the corresponding trap is not defined, it will not have any effect, which is equivalent to directly operating target.

When we write a certain trap, when we perform the corresponding action, our callback function will be triggered, and we will control the behavior of the proxy object.

The two most commonly used trap should be get and set.

Early YearsJavaScriptThere is a way to set a certain property when defining an objectgetter、setter：

let obj = {
  _age: 18,
  get age ()  {
    return `I'm ${this._age} years old`
  },
  set age (val) {
    this._age = Number(val)
  }
}

console.log(obj.age) // I'm 18 years old
obj.age = 19
console.log(obj.age) // I'm 19 years old

Like This code describes the same, we set a property _age, and then set a get age and set age.

Then we can directly call obj.age to get a return value, or assign it.

There are several disadvantages to doing this:

1. For each attribute to be proxied, the corresponding getter and setter must be written.
2. There must also be a key that stores the real value (if we call this.age directly in the getter, a stack overflow will occur, because whenever we call this .age will trigger getter when taking value).

Proxy solves these two problems very well:

let target = { age: 18, name: 'Niko Bellic' }
let handlers = {
  get (target, property) {
    return `${property}: ${target[property]}`
  },
  set (target, property, value) {
    target[property] = value
  }
}
let proxy = new Proxy(target, handlers)

proxy.age = 19
console.log(target.age, proxy.age)   // 19,          age : 19
console.log(target.name, proxy.name) // Niko Bellic, name: Niko Bellic

We create get, setTwo trap to manage all operations uniformly. You can see that while modifying proxy, the content of target is also modified, and we have # The behavior of ##proxy has been treated with some special processing.

And we don’t need to use an additional

key to store the real value, because we operate the target object inside trap, and Not a proxy object.

What to do with Proxy

Because after using

Proxy, the behavior of the object is basically controllable, so we It can be used to do some things that were more complicated to implement before.

A few simple applicable scenarios are listed below.

Solving the problem of undefined object properties

In some deep-level object property acquisitions, how to deal with

undefined has always been a painful process. If we use Proxy can be well compatible with this situation.

(() => {
  let target = {}
  let handlers = {
    get: (target, property) => {
      target[property] = (property in target) ? target[property] : {}
      if (typeof target[property] === 'object') {
        return new Proxy(target[property], handlers)
      }
      return target[property]
    }
  }
  let proxy = new Proxy(target, handlers)
  console.log('z' in proxy.x.y) // false (其实这一步已经针对`target`创建了一个x.y的属性)
  proxy.x.y.z = 'hello'
  console.log('z' in proxy.x.y) // true
  console.log(target.x.y.z)     // hello
})()

We proxy

get and perform logical processing inside it. If we want to perform get, the value comes from a non-existent key , then we will create a corresponding key in target, and then return a proxy object for this key.

This will ensure that our value operation will not throw

can not get xxx from undefinedBut this will have a small disadvantage, that is, if you really want to judge this Whether
key exists can only be judged through the in operator, and cannot be judged directly through get.

普通函数与构造函数的兼容处理

如果我们提供了一个Class对象给其他人，或者说一个ES5版本的构造函数。
如果没有使用new关键字来调用的话，Class对象会直接抛出异常，而ES5中的构造函数this指向则会变为调用函数时的作用域。
我们可以使用apply这个trap来兼容这种情况：

class Test {
  constructor (a, b) {
    console.log('constructor', a, b)
  }
}

// Test(1, 2) // throw an error
let proxyClass = new Proxy(Test, {
  apply (target, thisArg, argumentsList) {
    // 如果想要禁止使用非new的方式来调用函数，直接抛出异常即可
    // throw new Error(`Function ${target.name} cannot be invoked without 'new'`)
    return new (target.bind(thisArg, ...argumentsList))()
  }
})

proxyClass(1, 2) // constructor 1 2

我们使用了apply来代理一些行为，在函数调用时会被触发，因为我们明确的知道，代理的是一个Class或构造函数，所以我们直接在apply中使用new关键字来调用被代理的函数。

以及如果我们想要对函数进行限制，禁止使用new关键字来调用，可以用另一个trap:construct

function add (a, b) {
  return a + b
}

let proxy = new Proxy(add, {
  construct (target, argumentsList, newTarget) {
    throw new Error(`Function ${target.name} cannot be invoked with 'new'`)
  }
})

proxy(1, 2)     // 3
new proxy(1, 2) // throw an error

用Proxy来包装fetch

在前端发送请求，我们现在经常用到的应该就是fetch了，一个原生提供的API。
我们可以用Proxy来包装它，使其变得更易用。

let handlers = {
  get (target, property) {
    if (!target.init) {
      // 初始化对象
      ['GET', 'POST'].forEach(method => {
        target[method] = (url, params = {}) => {
          return fetch(url, {
            headers: {
              'content-type': 'application/json'
            },
            mode: 'cors',
            credentials: 'same-origin',
            method,
            ...params
          }).then(response => response.json())
        }
      })
    }

    return target[property]
  }
}
let API = new Proxy({}, handlers)

await API.GET('XXX')
await API.POST('XXX', {
  body: JSON.stringify({name: 1})
})

对GET、POST进行了一层封装，可以直接通过.GET这种方式来调用，并设置一些通用的参数。

实现一个简易的断言工具

写过测试的各位童鞋，应该都会知道断言这个东西
console.assert就是一个断言工具，接受两个参数，如果第一个为false，则会将第二个参数作为Error message抛出。
我们可以使用Proxy来做一个直接赋值就能实现断言的工具。

let assert = new Proxy({}, {
  set (target, message, value) {
    if (!value) console.error(message)
  }
})

assert['Isn\'t true'] = false      // Error: Isn't true
assert['Less than 18'] = 18 >= 19  // Error: Less than 18

统计函数调用次数

在做服务端时，我们可以用Proxy代理一些函数，来统计一段时间内调用的次数。
在后期做性能分析时可能会能够用上：

function orginFunction () {}
let proxyFunction = new Proxy(orginFunction, {
  apply (target, thisArg. argumentsList) {
    log(XXX)

    return target.apply(thisArg, argumentsList)
  }
})

全部的traps

这里列出了handlers所有可以定义的行为 (traps)：

具体的可以查看MDN-Proxy
里边同样有一些例子

traps	description
get	获取某个key值
set	设置某个key值
has	使用in操作符判断某个key是否存在
apply	函数调用，仅在代理对象为function时有效
ownKeys	获取目标对象所有的key
construct	函数通过实例化调用，仅在代理对象为function时有效
isExtensible	判断对象是否可扩展，Object.isExtensible的代理
deleteProperty	删除一个property
defineProperty	定义一个新的property
getPrototypeOf	获取原型对象
setPrototypeOf	设置原型对象
preventExtensions	设置对象为不可扩展
getOwnPropertyDescriptor	获取一个自有属性 （不会去原型链查找） 的属性描述

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