callbackFunction.js

function third(val, callback){
    callback(val*2, false);
}
function second(val, callback){
    callback(val*2, false);
}
function first(val, callback){
    callback(val*2, false);
}

first (2, function(result1, err) { //calling first function with parameters, val 2 and a callback function.
    if(!err) { // if execution of first function is successfull.
		second(result1, function(result2, err) { // calling second function, in parameter we are passing the result of first function and a callback
			if(!err){ // if execution of second function is successfull.
				third(result2, function(result3, err) { // calling third function, in parameter we are passing the result of second function and a callback
					if(!err){ // if execution of third function is successfull.
						console.log(result3) //printing the result from third function on console.
					}
				});
			}
		});
	}
});