wft-utils/just-play/hand-writing/prom.js

The commonly used tool functions in daily development

// 简单手写一个Promise
class MyPromise {
  static PENDING = 'pending'
  static FULFILLED = 'fulfilled'
  static REJECTED = 'rejected'
  constructor(fn) {
    this.status = MyPromise.PENDING
    this.result = null
    this.resolveCallbacks = []
    this.rejectCallbacks = []
    try {
      fn(this.resolve.bind(this), this.reject.bind(this))
    } catch (e) {
      this.reject(e)
    }
  }
  resolve(data) {
    setTimeout(() => {
      if (this.status === MyPromise.PENDING) {
        this.status = MyPromise.FULFILLED
        this.result = data
        if (this.resolveCallbacks.length) {
          this.resolveCallbacks.forEach(callback => {
            callback(data)
          })
        }
      }
    })
  }
  reject(err) {
    setTimeout(() => {
      if (this.status === MyPromise.PENDING) {
        this.status = MyPromise.REJECTED
        this.result = err
        if (this.rejectCallbacks.length) {
          this.rejectCallbacks.forEach(callback => {
            callback(err)
          })
        }
      }
    })
  }
  then(ONFULFILLED, ONREJECTED) {
    return new MyPromise((resolve, reject) => {
      ONFULFILLED = typeof ONFULFILLED === 'function' ? ONFULFILLED : () => { }
      ONREJECTED = typeof ONREJECTED === 'function' ? ONREJECTED : () => { }
      if (this.status === MyPromise.PENDING) {
        this.resolveCallbacks.push(ONFULFILLED)
        this.rejectCallbacks.push(ONREJECTED)
      }
      if (this.status === MyPromise.FULFILLED) {
        setTimeout(() => {
          ONFULFILLED(this.result)
        })
      }
      if (this.status === MyPromise.REJECTED) {
        setTimeout(() => {
          ONREJECTED(this.result)
        })
      }
    })
  }
}