promise

回调嵌套

let fs = require('fs');
fs.readFile('./template.txt', 'utf8', function (err, template) {
  fs.readFile('./data.txt', 'utf8', function (err, data) {
   console.log(template + ' ' + data);
  })
})

如果我们需要template中的数据，又需要data中的数据。 这样就会出现回调嵌套的问题。如果还有很多这样的请求，将会一层一层嵌套下去，这样代码将变得非常难看和难以维护。
如何解决这个问题？

promise的简单使用

promise为承诺，承诺的是未来的事情。可能成功，也可能失败。

let p1 = new Promise(function(resolve,reject){
  setTimeout(function(){
    resolve('成功');
  },1000);
});
p1.then(function(value){
  console.log('成功',value);
},function(reason){
  console.log('失败',reason);
});

new一个promise的实例p1，传入一个函数，这个函数中有两个参数，resolve和reject这是两个方法，resolve为成功执行的方法，reject为失败执行的方法。

promise原理分析

简单实现promise

先简单实现一个promise，然后逐步完善。
Promise是一个构造函数，所以先创建一个构造函数。里面有resolve和reject两个方法。因为then方法是通过promise的实例调用的。在Promise的实例的原型上有一个then方法。

//构造函数的参数是一个异步任务
function Promise(task) {
  let that = this;//缓存this
  //默认状态为pending
  that.status = 'pending';
  //此变量里放着此promise的结果
  that.value = undefined;
  //存放的着所有成功的回调函数
  that.onResolvedCallbacks = [];
  //存放着所有的失败的回调函数
  that.onRejectedCallbacks = [];
  //调用此方法可以把promise变成成功态
  function resolve(value) {
    if (that.status == 'pending') {
      that.status = 'fulfilled';
      that.value = value;
      that.onResolvedCallbacks.forEach(item=>item(that.value));
    }
  }

  //调用此方法可以把当前的promise变成失败态
  function reject(reason) {
    //如果当前状态是初始态，则转成失败态
    if (that.status == 'pending') {
      that.status = 'rejected';
      that.value = reason;
      that.onRejectedCallbacks.forEach(item=>item(that.value));
    }
  }

  //立即执行传入的任务
  try {
    task(resolve, reject);
  } catch (e) {
    reject(e);
  }
}
//onFulfilled成功的回调，onReject失败的回调
Promise.prototype.then = function (onFulfilled, onReject) {
  let that = this;
  if(that.status == 'fulfilled'){
    onFulfilled(that.value);
  }
  if(that.status == 'rejected'){
    onReject(that.value);
  }
  if(that.status == 'pending'){
    that.onResolvedCallbacks.push(onFulfilled);
    that.onRejectedCallbacks.push(onReject);
  }

}

module.exports = Promise;

完善then方法

//onFulfilled 是用来接收promise成功的值或者失败的原因
Promise.prototype.then = function(onFulfilled,onRejected){
  //如果成功和失败的回调没有传，则表示这个then没有任何逻辑，只会把值往后抛
  onFulfilled = typeof onFulfilled == 'function'?onFulfilled:function(value){return  value};
  onRejected = typeof onRejected == 'function'?onRejected:reason=>{throw reason};
  //如果当前promise状态已经是成功态了，onFulfilled直接取值
  let self = this;
  let promise2;
  if(self.status == "fulfilled"){
    return promise2 = new Promise(function(resolve,reject){
      setTimeout(function(){
        try{
          let x =onFulfilled(self.value);
          resolvePromise(promise2,x,resolve,reject);
        }catch(e){
          //如果执行成功的回调过程中出错了，用错误原因把promise2 reject
          reject(e);
        }
      })
    });
  }
  if(self.status == "rejected"){
    return promise2 = new Promise(function(resolve,reject){
      setTimeout(function(){
        try{
          let x =onRejected(self.value);
          resolvePromise(promise2,x,resolve,reject);
        }catch(e){
          reject(e);
        }
      })
    });
  }
  if(self.status == "pending"){
   return promise2 = new Promise(function(resolve,reject){
     self.onResolvedCallbacks.push(function(){
         try{
           let x =onFulfilled(self.value);
           //如果获取到了返回值x,会走解析promise的过程
           resolvePromise(promise2,x,resolve,reject);
         }catch(e){
           reject(e);
         }

     });
     self.onRejectedCallbacks.push(function(){
         try{
           let x =onRejected(self.value);
           resolvePromise(promise2,x,resolve,reject);
         }catch(e){
           reject(e);
         }
     });
   });
  }
}

添加resolvePromise方法

function resolvePromise(promise2,x,resolve,reject){
  if(promise2 === x){
    return reject(new TypeError('循环引用'));
  }
  let called = false;//promise2是否已经resolve 或reject了
  if(x instanceof Promise){
    if(x.status == "pending"){
      x.then(function(y){
        resolvePromise(promise2,y,resolve,reject);
      },reject);
    }else{
      x.then(resolve,reject);
    }
  //x是一个thenable对象或函数，只要有then方法的对象，
  }else if(x!= null &&((typeof x=='object')||(typeof x == 'function'))){
    //当我们的promise和别的promise进行交互
   try{
     let then = x.then;
     if(typeof then == 'function'){
       //有些promise会同时执行成功和失败的回调
       then.call(x,function(y){
         //如果promise2已经成功或失败了，则不会再处理了
          if(called)return;
          called = true;
          resolvePromise(promise2,y,resolve,reject)
       },function(err){
         if(called)return;
         called = true;
         reject(err);
       });
     }else{
       //到此的话x不是一个thenable对象，那直接把它当成值resolve promise2就可以了
       resolve(x);
     }
   }catch(e){
     if(called)return;
     called = true;
     reject(e);
   }

  }else{
    //如果X是一个普通 的值，则用x的值去resolve promise2
    resolve(x);
  }
}

Promise.all

Promise.all = function(promises){
 return new Promise(function(resolve,reject){
   let done = gen(promises.length,resolve);
   for(let i=0;i<promises.length;i++){
     promises[i].then(function(data){
       done(i,data);
     },reject);
   }
 });
}

Promise.race

Promise.race = function(promises){
  return new Promise(function(resolve,reject){
    for(let i=0;i<promises.length;i++){
      promises[i].then(resolve,reject);
    }
  });

Promise.resolve

Promise.resolve = function(value){
  return new Promise(function(resolve){
    resolve(value);
  });
}

Promise.reject

Promise.reject = function(reason){
  return new Promise(function(resolve,reject){
    reject(reason);
  });
}