SOURCE

new Promise(function(resolve,reject){
    let i = 3;
    setTimeout(function(){
        if(i>5){
            resolve(i);
        }else{
            reject(i);
        }
    },2000);
}).then(function(index){
    console.log(index);
},function(index){
    console.log(index);
});

function demo(callback,callback2){
    let i = 3;
    setTimeout(function(){
        if(i>5){
            callback(i);
        }else{
            callback2(i);
        }
    },1000);
}

demo(function(index){
    console.log(index);
},function(index){
    console.log(index);
});