JavaScript Callbacks

Any function that is passed as an argument to another function so that it can be executed in that another function is called as a callback function.

Need of callback function

• we need callback functions because many JavaScript actions are asynchronous, which means they do not really stop the program from running until they are completed.
• A callback’s primary purpose is to execute code in response to an event. These events might be user-initiated, such as mouse clicks or typing.
• e.g. button.addEventListener('click', callback)

Example code

function firstFunction(){
  // Simulate a code delay
  setTimeout( function(){
    console.log('First function called');
  }, 1000 );
}
function secondFunction(){
  console.log('Second function called');
}
firstFunction();
secondFunction();

Output
// Second function called
// First function called