手写Promise

/**
 *  @description MyPromise
 *  @author TsingYi
 */

class MyPromise {
  state = 'pending' //状态，'pending', fulfilled','rejected'
  value = undefined //成功后的值
  reason = undefined //失败后的原因

  resolveCallbacks = [] //pending状态下，储存成功的回调
  rejectCallbacks = [] //pending状态下，储存失败的回调

  constructor(fn) {
    const resolveHandler = (value) => {
      this.state = this.state === 'pending' ? 'fulfilled' : this.state
      this.value = value
      this.resolveCallbacks.forEach((fn) => fn(this.value))
    }
    const rejectHandler = (reason) => {
      this.state = this.state === 'pending' ? 'rejected' : this.state
      this.reason = reason
      this.rejectCallbacks.forEach((fn) => fn(this.reason))
    }

    try {
      fn(resolveHandler, rejectHandler)
    } catch (err) {
      rejectHandler(err)
    }
  }
  then(fn1, fn2) {
    fn1 = typeof fn1 === 'function' ? fn1 : (v) => v
    fn2 =
      typeof fn2 === 'function'
        ? fn2
        : (err) => {
            throw err
          }

    if (this.state === 'pending') {
      const p = new MyPromise((resolve, reject) => {
        this.resolveCallbacks.push(() => {
          try {
            const newValue = fn1(this.value)
            resolve(newValue)
          } catch (err) {
            reject(err)
          }
        })
        this.rejectCallbacks.push(() => {
          try {
            const newReason = fn2(this.reason)
            resolve(newReason)
          } catch (err) {
            reject(err)
          }
        })
      })
      return p
    }
    if (this.state === 'fulfilled') {
      const p = new MyPromise((resolve, reject) => {
        try {
          const newValue = fn1(this.value)
          resolve(newValue)
        } catch (err) {
          reject(err)
        }
      })
      return p
    }
    if (this.state === 'rejected') {
      const p = new MyPromise((resolve, reject) => {
        try {
          const newReason = fn2(this.reason)
          resolve(newReason)
        } catch (err) {
          reject(err)
        }
      })
      return p
    }
  }
  catch(fn) {
    return this.then(null, fn)
  }
}

const p1 = new MyPromise((resolve, reject) => {
  resolve(100)
  // reject('err123')
  // setTimeout(() => {
  //   resolve(100)
  // }, 500)
  // setTimeout(() => {
  //   reject('err123')
  // }, 500)
})
p1.then((data1) => {
  console.log('data1', data1)
  return data1 + 2
})
  .catch((err2) => {
    console.error(err2)
    return err2 + 4
  })
  .then((data3) => {
    console.log('data3', data3)
    return data3
  })