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class Prom {
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  static resolve (value) {
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    if (value && value.then) {
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      return value
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    }
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    return new Prom(resolve => resolve(value))
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  }
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  constructor (fn) {
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    this.value = undefined
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    this.reason = undefined
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    this.status = 'PENDING'
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    // 维护一个 resolve/pending 的函数队列
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    this.resolveFns = []
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    this.rejectFns = []
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    const resolve = (value) => {
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      // 注意此处的 setTimeout
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      setTimeout(() => {
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        this.status = 'RESOLVED'
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        this.value = value
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        this.resolveFns.forEach(({ fn, resolve: res, reject: rej }) => res(fn(value)))
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      })
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    }
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    const reject = (e) => {
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      setTimeout(() => {
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        this.status = 'REJECTED'
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        this.reason = e
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        this.rejectFns.forEach(({ fn, resolve: res, reject: rej }) => rej(fn(e)))
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      })
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    }
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    fn(resolve, reject)
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  }
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  then (fn) {
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    if (this.status === 'RESOLVED') {
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      const result = fn(this.value)
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      // 需要返回一个 Promise
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      // 如果状态为 resolved，直接执行
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      return Prom.resolve(result)
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    }
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    if (this.status === 'PENDING') {
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      // 也是返回一个 Promise
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      return new Prom((resolve, reject) => {
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        // 推进队列中，resolved 后统一执行
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        this.resolveFns.push({ fn, resolve, reject })
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      })
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    }
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  }
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  catch (fn) {
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    if (this.status === 'REJECTED') {
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      const result = fn(this.value)
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      return Prom.resolve(result)
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    }
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    if (this.status === 'PENDING') {
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      return new Prom((resolve, reject) => {
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        this.rejectFns.push({ fn, resolve, reject })
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      })
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    }
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  }
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}
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Prom.resolve(10).then(o => o * 10).then(o => o + 10).then(o => {
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  console.log(o)
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})
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return new Prom((resolve, reject) => reject('Error')).catch(e => {
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  console.log('Error', e)
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})