callback4.js

find([1,2], response);

function response(err, result) {
    if(err) {
       alert(err);
    }
    else {
       alert("operation successful : "+result);
    }
}

alert("meanwhile I show up");

function find(elements, callback) {
    //The time consuming operation
    setTimeout(function() {
        if(elements) {
            elements.push(3,4,5);
            callback(null, elements);
        }
        else {
            callback("Elements array should not be null", null);
        }
        
    } ,2000);
}