promise

 simply to handle callback hell

 function multiply(h){

    return new Promise(function(resolve,reject){

        setTimeout(()=>{

            resolve(h*2)

        },2000)

    })

};

function add(a,b){

    return new Promise(function(resolve,reject){

        setTimeout(()=>{

            console.log("hello")

            resolve(a+b)

        },2000)

    })

};

function show(ok){

    console.log("sfhljl")

    console.log(ok)

}

add(10,20)

.then((add)=>multiply(add))

.then((zz)=>show(zz));

console.log("end")