how to make callback function javascript

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})

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 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 functions - are functions that are called AFTER something happened
2
3  const foo = (number, callbackFunction) => {
4    //first 
5    console.log(number)
6    
7    //second - runs AFTER console.log() happened
8    callbackFunction()
9  }
10