Promise.resolve的实现

promise.resolve 把给定的值转换为promise对象

const PENDING = 'pending' // 等待
const FULFILLED = 'fulfilled' // 成功
const REJECTED = 'rejected'// 失败
class MyPromise {
  constructor(executor) {
    // 捕获执行器错误
    try{
      executor(this.resolve, this.reject)
    } catch (e) {
      this.reject(e)
    }
  }
  status = PENDING
  value = undefined
  reason = undefined
  successCallback = []
  failCallback = []
  resolve = value => {
    if (this.status !== PENDING) return
    // 将状态修改为成功
    this.status = FULFILLED
    // 保存成功之后的值
    this.value = value
    // this.successCallback && this.successCallback(this.value)
    while(this.successCallback.length) {
      this.successCallback.shift()()
    }
  }
  reject = reason => {
    if (this.status !== PENDING) return
    this.status = REJECTED
    this.reason = reason
    // this.failCallback && this.failCallback(this.reason)
    while(this.failCallback.length) {
      this.failCallback.shift()()
    }
  }
  then(successCallback, failCallback) {
    successCallback = successCallback ? successCallback : value => value 
    failCallback = failCallback ? failCallback : reason => {throw reason}
    let promise2 = new MyPromise((resolve, reject) => {
      if (this.status === FULFILLED) {
        setTimeout(() => {
          try {
            let x = successCallback(this.value)
            // 判断x的值是普通值还是promise对象
            // 如果是普通值，直接调用resolve
            // 如果是promise对象，查看promise对象返回的结果
            // 再根据promise对象返回的结果，决定调用resolve还是reject
            resolvePromise(promise2, x, resolve, reject)
          } catch(e) {
            reject(e)
          }
        }, 0)
      } else if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = failCallback(this.reason)
            // 判断x的值是普通值还是promise对象
            // 如果是普通值，直接调用resolve
            // 如果是promise对象，查看promise对象返回的结果
            // 再根据promise对象返回的结果，决定调用resolve还是reject
            resolvePromise(promise2, x, resolve, reject)
          } catch(e) {
            reject(e)
          }
        }, 0)
      } else {
        // 等待
        // 讲成功回调和失败回调进行存储
        this.successCallback.push(() => {
          setTimeout(() => {
            try {
              let x = successCallback(this.value)
              // 判断x的值是普通值还是promise对象
              // 如果是普通值，直接调用resolve
              // 如果是promise对象，查看promise对象返回的结果
              // 再根据promise对象返回的结果，决定调用resolve还是reject
              resolvePromise(promise2, x, resolve, reject)
            } catch(e) {
              reject(e)
            }
          }, 0)
        })
        this.failCallback.push(() => {
          setTimeout(() => {
            try {
              let x = failCallback(this.reason)
              // 判断x的值是普通值还是promise对象
              // 如果是普通值，直接调用resolve
              // 如果是promise对象，查看promise对象返回的结果
              // 再根据promise对象返回的结果，决定调用resolve还是reject
              resolvePromise(promise2, x, resolve, reject)
            } catch(e) {
              reject(e)
            }
          }, 0)
        })
      }
    })
    return promise2
  }
  static all (array) {
    let result = [];
    let index = 0;
    
    return new MyPromise((resolve, reject) => {
      function addData(key, value) {
        result[key] = value;
        index++;
        if (index === array.length) {
          resolve(result);
        }
      }
      for (let i = 0; i < array.length; i++) {
        let current = array[i];
        if (current instanceof MyPromise) {
          current.then(value => addData(i, value), reason => reject(reason))
        }else {
          addData(i, array[i]);
        }
      }
      // resolve(result)
    })
  }
  static resolve (value) {
    if (value instanceof MyPromise) return value;
    return new MyPromise(resolve => resolve(value))
  }
}

function resolvePromise(promise2, x, resolve, reject) {
  if (promise2 === x) {
   return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
  }
  if (x instanceof MyPromise) {
    // x.then(value => resolve(value), reason => reject(reason))
    x.then(resolve, reject)
  } else {
    resolve(x)
  }
}

module.exports = MyPromise

const MyPromise = require('./myPromise')
function p1 () {
  return new MyPromise(function(resolve, reject) {
    resolve('p1')
  })
}
MyPromise.resolve(10).then(value => console.log(value))
MyPromise.resolve(p1()).then(value => console.log(value))