Promise是什么？

处理异步函数的状态机，拥有链式调用、错误捕获、状态不可逆等特性。为了解决地狱回调而设计的解决方案

A+规范概括

官方中文链接

初始化

• 初始化 status, value ,reason , fullfilledQueue,rejectedQueue值
• 接收一个fn，接收resolve,reject函数作为参数
• 内部实现resovle,reject时，判断status是否为Pending，若是，则转为指定状态，并更新当前value/reason，遍历回调。

then的实现

• 首先判断接收的参数是否为函数类型，不是则转函数v=>v，当然onReject需要抛出值r=>{throw r};
• 需要返回一个promise，这个promise的resolve/reject由状态、onFullFilled/onRejected计算结果来决定。

状态之分

• fullfiled/rejected: 计算结果x，再将x传入resolvePromise作结果解析
• pending：两个队列推入 上一步的逻辑。

resovlePromise

• 这一步主要是处理 x和then中返回的promise是否相同，相同则报错(循环引用报错)。
• 判断x的类型，普通类型则直接resovle解决
• 若是object / function,若存在then，试图try&catch执行，根据then结果递归调用resovlePromise。 （注意，这里可能同时触发 resovle,reject。因此需要用标识符 called来防止多处调用。) (还要注意then的上下文为x,需要用call来纠正)
• 若不存在then，也直接resolve

catch的实现

• 在then的基础上，只传第二个参数并返回

finally

• 返回一个then执行，两个参数传入回调为返回promise.resolve(cb).then的Promise执行结果。

代码实现

const RESOLVED = 'RESOLVED'
const REJECTED = 'REJECTED'
const PENDING = 'PENDING'
const resolvePromise = (promise2, x, resolve, reject) => {
  if (promise2 === x) {
    return reject(new TypeError('循环引用报错'))
  }
  if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
    let called;
    try {
      let then = x.then
      if (typeof then === 'function') {
        then.call(x, y => {
          if (called) return
          called = true
          resolvePromise(promise2, y, resolve, reject)
        }, e => {
          if (called) return
          called = true
          reject(e)
        })
      } else {
        resolve(x)
      }
    } catch (e) {
      if (called) return
      called = true
      reject(e)
    }
  } else {
    resolve(x)
  }
}
class Promise {
  constructor(executor) {
    this.status = PENDING
    this.value = undefined
    this.reason = undefined
    this.onfulfilledCallbacks = []
    this.onrejectedCallbacks = []
    let resolve = (value) => {
      if (value instanceof Promise) {
        return value.then(resolve, reject)
      }
      if (this.status === PENDING) {
        this.value = value
        this.status = RESOLVED
        this.onfulfilledCallbacks.forEach(fn => fn())
      }
    }
    let reject = (reason) => {
      if (this.status === PENDING) {
        this.reason = reason
        this.status = REJECTED
        this.onrejectedCallbacks.forEach(fn => fn())
      }
    }

    try {
      executor(resolve, reject)
    } catch (e) {
      reject(e)
    }
  }
  then(onfulfilled, onrejected) {
    onfulfilled = typeof onfulfilled === 'function' ? onfulfilled : v => v;
    onrejected = typeof onrejected === 'function' ? onrejected : err => {
      throw err
    };
    let promise2 = new Promise((resolve, reject) => {
      if (this.status === RESOLVED) {
        setTimeout(() => {
          try {
            let x = onfulfilled(this.value)
            resolvePromise(promise2, x, resolve, reject)
          } catch (e) {
            reject(e)
          }
        })
      }
      if (this.status === REJECTED) {
        setTimeout(() => {
          try {
            let x = onrejected(this.reason)
            resolvePromise(promise2, x, resolve, reject)
          } catch (e) {
            reject(e)
          }
        })
      }
      if (this.status === PENDING) {
        this.onfulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              let x = onfulfilled(this.value)
              resolvePromise(promise2, x, resolve, reject)
            } catch (e) {
              reject(e)
            }
          })
        })
        this.onrejectedCallbacks.push(() => {
          setTimeout(() => {
            try {
              let x = onrejected(this.reason)
              resolvePromise(promise2, x, resolve, reject)
            } catch (e) {
              reject(e)
            }
          })
        })
      }
    })
    return promise2
  }
  catch (onrejected) {
    return this.then(null, onrejected)
  } 
  finally(cb) {
    return this.then(
      (data) => {
        return Promise.resolve(cb()).then(() => data)
      }, (err) => {
        return Promise.resolve(cb()).then(() => { throw err })
      })
  }
  static resolve(value) {
    return new Promise((resolve, reject) => {
      resolve(value)
    })
  }
  static reject(reason) {
    return new Promise((resolve, reject) => {
      reject(reason)
    })
  }
  static all(promises) {
    return new Promise((resolve, reject) => {
      let result = []
      let times = 0
      const processSuccess = (index, val) => {
        result[index] = val
        if (++times === promises.length) {
          resolve(result)
        }
      }
      for (let i = 0; i < promises.length; i++) {
        let p = promises[i];
        if (p && typeof p.then === 'function') {
          p.then(data => {
            processSuccess(i, data)
          }, reject)
        } else {
          processSuccess(i, p)
        }
      }
    })
  }
  static race(promises) {
    return new Promise((resolve, reject) => {
      for (let i = 0; i < promises.length; i++) {
        let p = promises[i];
        if (p && typeof p.then === 'function') {
          p.then(data => {
            resolve(data)
          }, reject)
        } else {
          resolve(p)
        }
      }
    })
  }
}

Promise.defer = Promise.deferred = function () {
  let dfd = {}
  dfd.promise = new Promise((resolve, reject) => {
    dfd.resolve = resolve;
    dfd.reject = reject
  })
  return dfd;
}

module.exports = Promise