理解JavaScript中的Proxy 与 Reflection API

理解JavaScript中的Proxy 与 Reflection API

一、创建 Proxy

let target = {}
let proxy = new Proxy(target, {})

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

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

在上面的例子中,由 Proxy 构造器创建的 proxy 对象会将自身的所有操作直接转发给 target
proxy.name 被赋值为 "proxy" 时,target 对象也会创建 name 属性并获得同样的值。实际上 proxy 对象本身并不创建和存储 name 属性,它只是转发对应的操作给 target

类似的,proxy.name target.name 的值始终保持一致,因为它们实际上都指向了 target.name。这也意味着给 target.name 赋予一个新的值时,该变化也会反映到 proxy.name 上。

使用 set Trap 验证属性

Proxy 允许开发者主动拦截本该转发给 target 对象的底层操作,这些拦截行为通过 trap 实现。每个 trap 都可以覆盖 JavaScript 对象的某些内置行为,即 proxy 允许通过 trap 拦截并修改指向 target 对象的操作。

假设需要创建一个新添加的属性值只能是数字类型的对象,就可以借助 set trap 覆盖默认的赋值行为。代码如下:

let target = {
 name: "target"
}

let proxy = new Proxy(target, {
 set(trapTarget, key, value, receiver) {
  if (!trapTarget.hasOwnProperty(key)) {
   if (isNaN(value)) {
    throw new TypeError("New property must be a number.")
   }
  }
  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.anotherName = "proxy"
// TypeError: New property must be a number.

set trap 中的四个参数含义如下:

  • trapTarget:接收新属性的对象(即 proxy 指向的 target)
  • key:新属性对应的 key
  • value:新属性对应的 value
  • receiver:通常为 proxy 自身

Reflect.set() 是与 set trap 相对应的原始方法,表示被覆盖前的默认的赋值行为。

使用 get Trap 令程序读取不存在属性时报错

JavaScript 在读取不存在的属性时并不会报错,而是返回 undefined

let target = {}
console.log(target.name) // undefined

可以借助 get trap 修改读取对象属性时的默认行为:

let proxy = new Proxy({}, {
 get(trapTarget, key, receiver) {
  if (!(key in receiver)) {
   throw new TypeError("Property " + key + " doesn"t exist.")
  }
  return Reflect.get(trapTarget, key, receiver)
 }
})

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

console.log(proxy.nme)
// TypeError: Property nme doesn"t exist.

通过 deleteProperty Trap 防止删除属性

JavaScript 中使用 delete 操作符删除对象的属性:

let target = {
 name: "target",
 value: 42
}

Object.defineProperty(target, "name", { configurable: false })
console.log("value" in target)  // true

let result1 = delete target.value
console.log(result1)       // true
console.log("value" in target)  // false

let result2 = delete target.name
console.log(result2)       // false
console.log("name" in target)   // true

使用 deleteProxy Trap 防止属性被意外删除:

let target = {
 name: "target",
 value: 42
}

let proxy = new Proxy(target, {
 deleteProperty(trapTarget, key) {
  if (key === "value") {
   return false
  } else {
   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

let result2 = delete proxy.name
console.log(result2)       // true
console.log("name" in proxy)   // false

二、Proxy 的现实应用

logging

function makeLoggable(target) {
 return new Proxy(target, {
  get: (target, property) => {
   console.log("Reading " + property)
   return target[property]
  },

  set: (target, property, value) => {
   console.log("Writing value " + value + " to " + property)
   target[property] = value
  }
 })
}

let ninja = { name: "Yoshi" }
ninja = makeLoggable(ninja)

console.log(ninja.name)
ninja.weapon = "sword"
// Reading name
// Yoshi
// Writing value sword to weapon

性能测试

function isPrime(number) {
 if (number < 2) { return false }

 for (let i = 2; i < number; i++) {
  if (number % i === 0) { return false }
 }
 return true
}

isPrime = new Proxy(isPrime, {
 apply: (target, thisArg, args) => {
  console.time("isPrime")
  const result = target.apply(thisArg, args)
  console.timeEnd("isPrime")
  return result
 }
})

console.log(isPrime(1358765377))
// isPrime: 6815.107ms
// true

自动添加属性

function Folder() {
 return new Proxy({}, {
  get: (target, property) => {
   console.log("Reading " + property)

   if(!(property in target)) {
    target[property] = new Folder()
   }
   return target[property]
  }
 })
}

const rootFolder = new Folder()
rootFolder.ninjasDir.firstNinjaDir.ninjaFile = "yoshi.txt"
// Reading ninjasDir
// Reading firstNinjaDir
console.log(rootFolder.ninjasDir.firstNinjaDir.ninjaFile)
// Reading ninjasDir
// Reading firstNinjaDir
// Reading ninjaFile
// yoshi.txt

参考资料

https://leanpub.com/understandinges6

https://www.manning.com/books/secrets-of-the-javascript-ninja-second-edition

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原文链接:https://rollingstarky.github.io/2020/08/19/understanding-javascript-ecmascript6-proxy-and-reflection-api/?utm_source=tuicool&utm_medium=referral