ES6 - Promise

Promise 是异步编程的一种解决方案，比传统的解决方案——回调函数和事件——更合理和更强大。它由社区最早提出和实现，ES6 将其写进了语言标准，统一了用法，原生提供了Promise对象。

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实现一个基本的Promise

class Promise {
  constructor (fn) {
    // 三个状态
    this.state = 'pending' // 状态
    this.value = undefined // 成功值
    this.reason = undefined // 失败值

    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled'
        this.value = value
      }
    }

    let reject = value => {
      if (this.state === 'pending') {
        this.state = 'rejected'
        this.reason = value
      }
    }

    // 自动执行函数
    try {
      fn(resolve, reject)
    } catch (e) {
      reject(e)
    }
  }

  // then
  then(onFulfilled, onRejected) {
    switch (this.state) {
      case 'fulfilled':
        onFulfilled()
        break
      case 'rejected':
        onRejected()
        break
      default:
    }
  }
}

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