what is callback function in javascript with example

Example 1: javascript callback

/*
A callback function is a function passed into another function
as an argument, which is then invoked inside the outer function
to complete some kind of routine or action. 
*/
function greeting(name) {
  alert('Hello ' + name);
}

function processUserInput(callback) {
  var name = prompt('Please enter your name.');
  callback(name);
}

processUserInput(greeting);
// The above example is a synchronous callback, as it is executed immediately.

Example 2: javascript callback

// Create a callback in the probs, in this case we call it 'callback'
function newCallback(callback) {
  callback('This can be any value you want to return')
}

// Do something with callback (in this case, we console log it)
function actionAferCallback (callbackData) {
  console.log(callbackData)
}

// Function that asks for a callback from the newCallback function, then parses the value to actionAferCallback
function requestCallback() {
  newCallback(actionAferCallback)
}

Example 3: create callback function javascript

function add(a, b, callback) {
  if (callback && typeof(callback) === "function") {
    callback(a + b);
  }
}

add(5, 3, function(answer) {
  console.log(answer);
});

Example 4: javascript callback function

// Callback Example 1: note, fn=function
/* 
In JavaScript, callback fn is:

- a function based into another functions as an argument to be
	executed LATER
- would be a Synchronous OR Asynchronous callback.
- hint: 

	Synchronous: processing from top to bottom, 
	stop until current code finished.
        
	Asynchronous: no wait, process the next block if there
    
*/

let numbers = [1, 2, 4, 7, 3, 5, 6];

/* To find all the odd numbers in the array, 
you can use the filter() method of the Array object.
- The filter() method creates a new array with the elements that
pass the test implemented by a fn.
- The following test fn returns true if a number is an odd
number:   */


function isOddNumber(number) { //to be the callback fn
    return number % 2;
}

// callback fn passed into another fn by its reference, No ()
const oddNumbers = numbers.filter(isOddNumber);
console.log(oddNumbers); // [ 1, 7, 3, 5 ]