实现一个简单的promise

const Fulfilled = 'fulfilled'
const Rejected = 'rejected'
const Pending = 'pending'
const promiseError = val => `(in promise) ${val}`

function P(fn) {
  this.state = Pending
  this.value = undefined
  this.onFulfilledCb = []
  this.onRejectedCb = []

  const resolve = val => {
    if (this.state === Pending) {
      this.state = Fulfilled
      this.value = val

      while (this.onFulfilledCb.length) {
        const fulfiledFn = this.onFulfilledCb.shift()
        fulfiledFn()
      }
    }
  }

  const reject = val => {
    if (this.state === Pending) {
      this.state = Rejected
      this.value = val

      // if (!this.onRejectedCb.length) {
      //   throw promiseError(val)
      // }

      while (this.onRejectedCb.length) {
        const rejectedFn = this.onRejectedCb.shift()
        rejectedFn()
      }
    }
  }

  fn(resolve, reject)
}

P.prototype.then = function(onFulfilled, onRejected) {
  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v
  onRejected = typeof onRejected === 'function' ? onRejected : r => r

  return new P((resolve, reject) => {
    const fulfilledHandler = () => {
      try {
        resolve(onFulfilled(this.value))
      } catch (e) {
        reject(e)
      }
    }

    const rejectedHandler = () => {
      // if (typeof onRejected === 'function') {
        try {
          resolve(onRejected(this.value))
        } catch (e) {
          reject(e)
        }
      // } else {
      //   throw promiseError(this.value)
      // }
    }

    if (this.state === Pending) {
      this.onFulfilledCb.push(fulfilledHandler)

      this.onRejectedCb.push(rejectedHandler)
     }

    if (this.state === Fulfilled) {
      fulfilledHandler()
    }

    if (this.state === Rejected) {
      rejectedHandler()
    }
  })
}

P.prototype.catch = function(callback) {
  return this.then(undefined, callback)
}

P.prototype.finally = function (callback) {
  return this.then(callback, undefined)
}

P.resolve = function(val) {
  return new P(resolve => resolve(val))
}

P.reject = function(val) {
  return new P((_, reject) => reject(val))
}

P.all = function(arr) {
  return new P((resolve, reject) => {
    const values = []
    let count = 0
    for (let [i, p] of arr.entries()) {
      p.then(res => {
        values[i] = res
        count++

        if (count === arr.length) {
          resolve(values)
        }
      }, err => {
        reject(err)
      })
    }
  })
}

P.race = function(arr) {
  return new P((resolve, reject) => {
    for (let p of arr) {
      p.then(res => {
        resolve(res)
      }, err => {
        reject(err)
      })
    }
  })
}