javascript callback function

1/*
2A callback function is a function passed into another function
3as an argument, which is then invoked inside the outer function
4to complete some kind of routine or action. 
5*/
6function greeting(name) {
7  alert('Hello ' + name);
8}
9
10function processUserInput(callback) {
11  var name = prompt('Please enter your name.');
12  callback(name);
13}
14
15processUserInput(greeting);
16// The above example is a synchronous callback, as it is executed immediately.

1function startWith(message, callback){
2	console.log("Clearly written messages is: " + message);
3  
4  	//check if the callback variable is a function..
5  	if(typeof callback == "function"){
6    	callback(); //execute function if function is truly a function.
7    }
8}
9//finally execute function at the end 
10startWith("This Messsage", function mySpecialCallback(){
11	console.log("Message from callback function");
12})

1function greeting(name) {
2  alert('Hello ' + name);
3}
4
5function processUserInput(callback) {
6  var name = prompt('Please enter your name.');
7  callback(name);
8}
9
10processUserInput(greeting);

1function add(a, b, callback) {
2  if (callback && typeof(callback) === "function") {
3    callback(a + b);
4  }
5}
6
7add(5, 3, function(answer) {
8  console.log(answer);
9});

1// Callback Example 1: note, fn=function
2/* 
3In JavaScript, callback fn is:
4
5- a function based into another functions as an argument to be
6	executed LATER
7- would be a Synchronous OR Asynchronous callback.
8- hint: 
9
10	Synchronous: processing from top to bottom, 
11	stop until current code finished.
12        
13	Asynchronous: no wait, process the next block if there
14    
15*/
16
17let numbers = [1, 2, 4, 7, 3, 5, 6];
18
19/* To find all the odd numbers in the array, 
20you can use the filter() method of the Array object.
21- The filter() method creates a new array with the elements that
22pass the test implemented by a fn.
23- The following test fn returns true if a number is an odd
24number:   */
25
26
27function isOddNumber(number) { //to be the callback fn
28    return number % 2;
29}
30
31// callback fn passed into another fn by its reference, No ()
32const oddNumbers = numbers.filter(isOddNumber);
33console.log(oddNumbers); // [ 1, 7, 3, 5 ]
34

1const message = function() {  
2    console.log("This message is shown after 3 seconds");
3}
4 
5setTimeout(message, 3000);