手写基于Promise A+规范的Promise

const PENDING =  ‘pending‘;//初始态const FULFILLED =  ‘fulfilled‘;//初始态const REJECTED =  ‘rejected‘;//初始态function Promise(executor){  let self = this;//先缓存当前promise实例  self.status = PENDING;//设置状态  //定义存放成功的回调的数组  self.onResolvedCallbacks = [];  //定义存放失败回调的数组  self.onRejectedCallbacks = [];  //当调用此方法的时候，如果promise状态为pending,的话可以转成成功态,如果已经是成功态或者失败态了，则什么都不做  //2.1  function resolve(value){ //2.1.1    if(value!=null &&value.then&&typeof value.then == ‘function‘){      return value.then(resolve,reject);    }    //如果是初始态，则转成成功态    setTimeout(function(){      if(self.status == PENDING){        self.status = FULFILLED;        self.value = value;//成功后会得到一个值，这个值不能改        //调用所有成功的回调        self.onResolvedCallbacks.forEach(cb=>cb(self.value));      }    })

}  function reject(reason){ //2.1.2    setTimeout(function(){      //如果是初始态，则转成失败态      if(self.status == PENDING){        self.status = REJECTED;        self.value = reason;//失败的原因给了value        self.onRejectedCallbacks.forEach(cb=>cb(self.value));      }    });

}  try{    //因为此函数执行可能会异常，所以需要捕获，如果出错了，需要用错误 对象reject    executor(resolve,reject);  }catch(e){    //如果这函数执行失败了，则用失败的原因reject这个promise    reject(e);  };}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，编写这段代码的时候尽量的考虑兼容性，允许别人瞎写   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);  }}//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  //2.2.1  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);          //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){          //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);         }     });   });  }

}//catch原理就是只传失败的回调Promise.prototype.catch = function(onRejected){    this.then(null,onRejected);}Promise.deferred = Promise.defer = function(){  let defer = {};  defer.promise = new Promise(function(resolve,reject){    defer.resolve = resolve;    defer.reject = reject;  });  return defer;}function gen(times,cb){  let result = [],count=0;  return function(i,data){    result[i] = data;    if(++count==times){      cb(result);    }  }}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 = function(promises){  return new Promise(function(resolve,reject){    for(let i=0;i<promises.length;i++){      promises[i].then(resolve,reject);    }  });}//返回一个立刻成功的promise//别人提供 给你一个方法，需要你传入一个promise,但你只有一个普通的值，你就可以通过这个方法把这个普通的值(string number object)转成一个promise对象Promise.resolve = function(value){  return new Promise(function(resolve){    resolve(value);  });}//返回一个立刻失败的promisePromise.reject = function(reason){  return new Promise(function(resolve,reject){    reject(reason);  });}module.exports = Promise;

原文地址：https://www.cnblogs.com/qinmengjiao123-123/p/8280973.html