promise.js

const PENDING = 'pending'
const RESOLVED = 'resolved'
const REJECTED = 'rejected'

class MyPromise {
  static resolve(value) {
    return new Promise(resolve) {
      resolve(value)
    }
  }

  static all(promises) {
    return new Promise(resolve, reject) {
      let counter = 0
      const len = promises.len
      const result = []

      for (let i = 0; i < len; i++) {
        const p = promises[i]
        p.then((v) => {
          couter++
          result[i] = v
          if (len === counter) {
            resolve(result)
          }
        }, (e) => {
          reject(e)
        })
      }
    }
  }
  
  static race(promises) {
    return new MyPromise(resolve, reject) {
      for (let i = 0; i < promises.length; i++) {
        promises[i].then(resovle, reject)
      }
    }
  }

  constructor(fn) {
    this.state = PENDING
    this.value = null
    this.resolvedCbs = []
    this.rejectCbs = []

    try {
      fn(this._resolve, this._reject)
    } catch (e) {
      this._reject(e)
    }
  }

  then(onFullfilled, onRejected) {
    onFullfilled = typeof onFullfilled === 'function' ? onFullfilled : v => v
    onRejected = typeof onRejected === 'funciton' ? onRejected : e => { throw e }
    if (this.state === PENDING) {
      this.resolvedCbs.push(onFullfilled)
      this.rejectCbs.push(onRejected)
    } else if (this.state === RESOLVED) {
      onFullfilled()
    } else if (this.state === REJECTED) {
      onRejected()
    }
  }

  _resolve = (v) => {
    if (this.state === PENDING) {
      this.state = RESOLVED
      this.value = v
      this.resolvedCbs.forEach(cb => cb(this.value))
    }
  }

  _reject() {
    if (this.state === PENDING) {
      this.state = REJECTED
      this.value = v
      this.rejectCbs.forEach(cb => cb(this.value))
    }
  }
}

function myFetch(url, method) {
  return new MyPromise((resolve, reject) => {
    const xhr = new XMLHttpRequest()
    xhr.open(method, url)
    xhr.send()

    xhr.onload = () => {
      const res = (xhr.responseText)
      resolve(res)
    }
  }) 
}

myFetch('http://xuliugen.vicp.io:8089', 'get').then(v => {
  console.log(v)
})