promiseA+中的 then方法

1. promise必须返回一个then方法
   

promise2 = promise1. then(onFulfilled, onRejected);

2. 执行成功/失败的回调函数，返回值为x
   

If either onFulfilled or onRejected returns a value x

3. 如果一旦回调函数中报错，抛出异常，则被catch捕获

1. 链式调用返回普通值情况处理

let fs = require('fs')
let Promise = require('. /promise)

let p1 = new Promise((resolve, reject) => {
  resolve(100)
})
let p2 = p1. then((data) => {
  return 1
})
p2. then((data) => {
  console. log(data, 'p2----data')
})

链式调用返回普通值得部分源码

then(onFulfilled, onRejected) {
  let promise2 = new Promise((resolve, reject) => {

    if(this.status == RESOLVED) {
      let x = onFulfilled(this.value); // 1 普通值
      resolve(x);  // 执行p2.resolve
    }
    if(this.status == REJECTED) {
      onRejected(this.reason);
      resolve(x);  // 执行p2.resolve
    }
    if(this.status == PENDING) {
      this.onResolvedCallbacks.push(_ => {
        // todo...
        let x = onFulfilled(this.value);
        resolve(x);
      })
      this.onRejectedCallbacks.push(_ => {
        // todo...
        let x = onRejected(this.reason);
        resolve(x);
      })
    }

  })
}

2. 链式调用返回promise情况处理

let fs = require('fs')
let Promise = require('. /promise)

// p2的返回值是 promise
let p1 = new Promise((resolve, reject) => {
  resolve(100)
})
let p2 = p1. then((data) => {
  return new Promise((resolve, reject) => {

    setTimeout(_ => {
      resolve('ok')
    }, 1000)

  })
}, err => {
  return '失败'; 
})
p2. then((data) => {
  console. log(data, 'p2----data')
})

// p2的回调中 抛了异常
let p1 = new Promise((resolve, reject) => {
  resolve(100)
})
let p2 = p1. then((data) => {
  throw new Error(); 
}, err => {
  return '失败'; 
})
p2. then((data) => {
  console. log(data, 'p2----data')
})

const PENDING = 'PENDING'; 
const RESOLVED = 'RESOLVED'; 
const REJECTED = 'REJECTED'; 
const resolvePromise = (promise2, x, resolve, reject) => {

}

class Promise{
  constructor(executor){

    this.status = PENDING;
    this.value = undefined;
    this.reason = undefined;
    this.onResolvedCallbacks = []; // 存储成功时需要执行的回调函数
    this.onRejectedCallbacks = []; // 存储失败时需要执行的回调函数
    
    let resolve = (value) => {
      if(this.status === PENDING) {
        this.status = RESOLVED;
        this.value = value;
        // 将 pending状态时，存储的成功的回调 依次执行
        this.onResolvedCallbacks.forEach( fn => fn()); 
      }
    }
    let reject = (reason) => {
      if(this.status === PENDING) {
        this.status = REJECTED;
        this.reason = reason;
        this.onRejectedCallbacks.forEach( fn => fn());
      }
    }
    try{
      executor(resolve, reject); // 立即执行
    } catch(e) {
      reject(e)
    }

  }
  then(onFulfilled, onRejected) {

    let promise2 = new Promise((resolve, reject) => {
      if(this.status == RESOLVED) {
        setTimeout(_ => {
          try {
            let x = onFulfilled(this.value); 
            // x 可能是一个promise
            resolvePromise(promise2, x, resolve, reject); 
          } catch (e) {
            reject(e)
          }
        }, 0)
        
      }
      if(this.status == REJECTED) {
        setTimeout(_ => {
          try {
            let x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);    
          } catch (e) {
            reject(e)
          }
        }, 0)
      }
      if(this.status == PENDING) {
        this.onResolvedCallbacks.push(_ => {
          setTimeout(_ => {
            try {
               // todo...
              let x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);  
            } catch (e) {
              reject(e)
            }
          }, 0)
        })
        this.onRejectedCallbacks.push(_ => {
          setTimeout(_ => {
            try {
               // todo...
              let x = onRejected(this.reason);
              resolvePromise(promise2, x, resolve, reject);  
            } catch (e) {
              reject(e)
            }
          }, 0)
        })
      }
    })

  }
}

promise2和x引用的同一个变量

const resolvePromise = (promise2, x, resolve, reject) => {
  // 1、循环引用 自己等待自己完成
  if(promise2 === x) { // 用一个类型错误 结束掉promise

    return reject(new TypeError('Chainig cycle detected for promise...'))

  }
}