javascript callback tutorial code 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: 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 3: what are callbacks

/*
tl;dr: Callbacks are a way to force one function to complete,
before running another. Used for asynchronus programming.
*/

// Consider this example:
  
names = ['Anthony', 'Betty', 'Camie']

function addName(name, callback){
	setTimeout(function(){
    	names.append(name)
      	callback()
    }, 200)
}

function getNames(){
	setTimeout(function(){
    	console.log(names)
    }, 100)
}

addName('Daniel', getNames)
/* 
addName('Daniel', getNames) is saying:
'finish the function addName() BEFORE running the function getNames()'

> 'getNames()' here is the callback

Without the call back in this example, getNames would finish 
before addName. (setTimeout is used to force one function to go slower)

Callbacks are given as arguments to other functions

*/

Example 4: javascript callback function

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