手写promise

const PENDING = 'pending'
const FULFILLED = 'fullfilled'
const REJECTED = 'rejected'

class MyPromise {
  state = PENDING
  result = undefined
  handlers = []

  constructor(executor) {
    const resolve = (data) => {
      this.changeState(FULFILLED, data)
    }
    const reject = (reson) => {
      this.changeState(REJECTED, reson)
    }
    try {
      executor(resolve, reject)
    } catch (error) {
      reject(error)
    }

  }

  changeState(state, result) {
    if (this.state !== PENDING) return
    this.state = state
    this.result = result
    this.run()
  }

  isPromiseLike = (value) => {
    if (value !== null && (typeof value === 'object' || typeof value === 'function')) {
      return typeof value.then === 'function'
    }
    return false
  }

  runMocroTask = (func) => {
    if (typeof process === 'object' && typeof process.nextTick === 'function') {
      process.nextTick(func)
    } else if (typeof MutationObserver === 'function') {
      const ob = new MutationObserver(func)
      const textNode = document.createTextNode('1')
      ob.observe(textNode, {
        characterData: true
      })
      textNode.data = '2'
    } else {
      setTimeout(func, 0)
    }
  }

  runOne = (callback, resolve, reject) => {
    this.runMocroTask(() => {
      // 1、对应回调函数不是一个函数 直接把结果返回
      if (typeof callback !== 'function') {
        const settled = this.state === FULFILLED ? resolve : reject
        settled(this.result)
        return
      }
      // 2、回调函数是一个函数
      try {
        const data = callback(this.result)
        // 3、返回的是不是promise（满足promiseA+规范就行）
        if (this.isPromiseLike(data)) {
          data.then(resolve, reject)
        } else {
          resolve(data)
        }
      } catch (error) {
        reject(error)
      }
    })
  }

  run() {
    if (this.state === PENDING) return
    while (this.handlers.length) {
      const {
        onFulfilled,
        onRejected,
        reject,
        resolve
      } = this.handlers.shift()
      if (this.state === FULFILLED) {
        this.runOne(onFulfilled, resolve, reject)
      } else {
        this.runOne(onRejected, resolve, reject)
      }
    }
  }

  then(onFulfilled, onRejected) {
    return new MyPromise((resolve, reject) => {
      this.handlers.push({
        onFulfilled,
        onRejected,
        resolve,
        reject
      })
      this.run()
    })
  }
}

// 测试
function delay(duration = 1000) {
    return new MyPromise((resolve, reject) => {
      setTimeout(resolve, duration)
    })
}
async function test() {
    await delay(1000)
    console.log('ok1');
    await delay(1000)
    console.log('ok2');
}
test()

// const p = new MyPromise((resolve, reject) => {
//   setTimeout(() => {
//     resolve(1)
//   }, 1000);
// })
// p.then((data) => {
//   console.log('ok1', data);
//   return new Promise((resolve, reject) => {
//     setTimeout(() => {
//       resolve(data * 2)
//     }, 1000);
//   })
// }).then((data) => {
//   console.log('ok2', data);
// })