手写Promise

代码比较短的实现

const states = {
  0: 'pending',
  1: 'fulfilled',
  2: 'rejected'
}
function MyPromise (cb) {
  if (typeof cb!== 'function') {
    throw new TypeError('callback must be a function')
  }
  let state = states[0]
  let value = null
  let handlers = []
  function fulfill (result) {
    state = states[1]
    value = result
    handlers.forEach(handle)
    handlers = null
  }
  function reject (error) {
    state = states[2]
    value = error
    handlers.forEach(handle)
    handlers = null
  }
  function resolve (value) {
    try {
      let then = getThen(value)
      if (then) {
        resolveAnotherPromise(then.bind(value), resolve, reject)
        return
      }
      fulfill(value)
    } catch (err) {
      reject(err) 
    }
  }
  function handle (handler) {
    if (state === states[0]) handlers.push(handler)
    else {
      if (state === states[1] && 
          typeof handler.onFulfill === 'function') {
        handler.onFulfill(value)
      }
      if (state === states[2] && 
          typeof handler.onReject === 'function') {
        handler.onReject(value)
      }
    }
  }
  this.done = function (onFulfill, onReject) {
    setTimeout(() => handle(onFulfill, onReject), 0)
  }
  this.then = function (onFulfill, onReject) {
    let self = this
    return new Promise((resolve, reject) => {
      return self.done(result => {
        if (typeof onFulfill === 'function') {
          try {
            return resolve(onFulfill(result))
          } catch (err) {
            return reject(err)
          }
        } else {
          return resolve(result)
        }
      }, error => {
        if (typeof onReject === 'function') {
          try {
            return resolve(onReject(error))
          } catch (err) {
            return reject(err)
          }
        } else {
          return reject(error)
        }
      })
    })
  }
}  
function getThen (value) {
  if (typeof(value) === 'object' 
      || typeof(value) === 'function') {
    let then = value.then
    if (typeof(then) === 'function') return then
  }
  return null
}
 
function resolveAnotherPromise (cb, Onfulfill, Onreject) {
  let finished = false
  try {
    cb(value => {
      if (finished) return
      finished = true
      Onfulfill(value)
    }, reason => {
      if (finished) return
      finished = true
      Onreject(reason)
    })
  } catch (err) {
    if (finished) return
    finished = true
    Onreject(err)
  }
}
}