Function vs Callback Function

Callbacks are just functions, but instead of being called by the programmer explicitly, they are called by the computer in specific circumstances.

function callbackFunction() {
  //...
}
function executeCallback( callbackParm ) {
  //...
  callbackParm();
  //...
}
executeCallback(callbackFunction);

executeCallback( callbackFunction );
Notice there are no parentheses, because we are not calling the function, we are passing it because we want the computer to call it.
Any function can become a callback function if it is passed into another function as a parameter for later use.

Simple callback example

// Normal function used as callback
function sayHello() {
  console.log('Hello');
}
// Function will invoke callback function
function executeCalleback( callback ){
  callback();
}
// Run function
executeCallback(sayHello);

// output: 'Hello'

Other examples

onClick(showHint);
everySecond(tickClock);
readFile("storyFile.txt", printStory);

showHint, tickClock, and printStory are all examples of callback functions passed as arguments to functions.

Callback Use Cases

• Timers, one-offs and repeating ones (e.g. screen clocks).
• Triggered by a user interaction (e.g. interaction events).
• Loading data from a server (e.g. using AJAX).
• Opening and complete reading a file (e.g. in Node.js).
• And many more…

Synchronous & Asynchronous Programming

Synchronous Programming with Normal Functions – When you call or invoke a function within your code block, you’re asking for the function’s code to be executed immediately.

Asynchronous Programming – When you use the function as a callback, it gets executed or invoked when it’s supposed to sometime in the future!