promisify,promisifyAll,promise.all实现原理

1.promisify

function toPrimisify (fn){
return function (...args){
      return new Promise(function(resolve,reject){
           fn(...args,function(err,data){
            err?reject(err):resolve(data)
          })
     })
}
let read2 = toPrimisify(fs.readFile);
fs.readFile'./2.txt','utf8').then(res=>{console.log(res)});

2.promisifyAll

function toPromisifyAll(obj){

Object.keys(obj).forEach((item,index)=>{

if(typeof obj[item]=='function')
obj[item+'Async'] =toPrimisify(obj[item])
})
}
toPromisifyAll(fs);
fs.readFileAsync('./2.txt','utf8').then(res=>{console.log(res)});

3.promise.all

function promiseAll(promises){
     return new Promise(function(resolve,reject){
            if(!Array.isArray(promises)){
             return reject(new TypeError("argument must be anarray"))
           }
    var countNum=0;
    var promiseNum=promises.length;
    var resolvedvalue=new Array(promiseNum);
    for(var i=0;i<promiseNum;i++){
      (function(i){
         Promise.resolve(promises[i]).then(function(value){
            countNum++;
           resolvedvalue[i]=value;
          if(countNum===promiseNum){
              return resolve(resolvedvalue)
          }
       },function(reason){
        return reject(reason)
      )
     })(i)
    }
})
}

var p1=Promise.resolve(1),
p2=Promise.resolve(2),
p3=Promise.resolve(3);

promiseAll([p1,p2,p3]).then(function(value){
console.log(value)

})