实现一个promise方法

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看到好多次面试题有要求实现一个promise了，打算实现以下，理清思路，一步一步实现，其实也不难。

定义promise内部三个状态

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const PENDING =  'pending';//进行中
const FULFILLED =  'fulfilled';//成功或者“resolved”
const REJECTED =  'rejected';//失败

定义promise主函数

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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 定义resolve方法;
  function resolve(value){  
    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
    //为什么要把它用setTimeout包起来
    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));
      }
    })
  }
   //2.1.2 定义reject方法;
  function reject(reason){ 
    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);
  };
}

实现promise相关API

then()

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//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);
         }
     });
   });
  }

}

resolve()

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

reject()

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//返回一个立刻失败的promise
Promise.reject = function(reason){
  return new Promise(function(resolve,reject){
    reject(reason);
  });
}

all()

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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);
   }
 });
}

function gen(times,cb){
  let result = [],count=0;
  return function(i,data){
    result[i] = data;
    if(++count==times){
      cb(result);
    }
  }
}

catch()

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//catch原理就是只传失败的回调
Promise.prototype.catch = function(onRejected){
  this.then(null,onRejected);
}

finally()

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Promise.prototype.finally = function (callback) {
  let P = this.constructor;
  return this.then(
    value  => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  );
};

参考资料

[1] https://juejin.im/entry/597ddade5188257f98295191

[2] https://juejin.im/post/5c763869e51d4569013c11f5#heading-5