index.js

let names = ['Rahul','Rohit','Rohan']
function addNewName(name, cb){
    setTimeout(function(){
        names.push(name)
        cb()
    },5000)
}
function printNames(){
    setTimeout(function(){
        console.log(names)
    },3000)
}
addNewName('Alex',printNames)
// Output:
// [ 'Rahul', 'Rohit', 'Rohan', 'Alex' ]