原生 ES6 实现 Promise

const PENDING = Symbol('pending')
const FULLFILLED = Symbol('fullfilled')
const REJECTED = Symbol('rejected')

class Promise {
  constructor (fn) {
    this.status = PENDING
    this.value = undefined
    this.resolveFunc = () => {
    }
    this.rejectFunc = () => {

    }

    fn.call(this, this.resolve.bind(this), this.reject.bind(this))
  }

  resolve (value) {
    if (this.status == PENDING) {
      this.status = FULLFILLED
      this.value = value
      this.resolveFunc(value)
    }
  }

  reject (value) {
    if (this.status == PENDING) {
      this.status = REJECTED
      this.value = value
      this.rejectFunc(value)
    }
  }

  then (resolveFunc, rejectFunc) {
    return new Promise((resolve_next, reject_next) => {
      const resolveFuncWrap = () => {
        try {
          let result = resolveFunc(this.value)
          if (result && typeof result.then == 'function') {
            result.then(resolve_next, reject_next)
          } else {
            resolve_next(result)
          }
        } catch (e) {
          reject_next(e)
        }
      }

      const rejectFuncWrap = () => {
        let result = rejectFunc(this.value)
        reject_next(result)
      }

      this.resolveFunc = resolveFuncWrap
      this.rejectFunc = rejectFuncWrap
    })
  }

  catch (errCallback) {
    this.rejectFunc = errCallback
  }
}



let hello = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('hello')
    }, 1000)
  })
}

hello().then(res1 => {
  console.log('res1: ', res1)
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('world')
    })
  })
}).then(res2 => {
  console.log('res2：', res2)
  // 测试出 error 情况
  console.log('error：', hello.i.b)
}).catch(err => {
  console.log('error: ', err)
})