What is call-back function

What is call back function:

Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name ‘call back’.

More complexly put: In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions. Functions that do this are called higher-order functions. Any function that is passed as an argument is called a callback function.

Why call back:

For one very important reason — JavaScript is an event driven language. This means that instead of waiting for a response before moving on, JavaScript will keep executing while listening for other events.

eg.

function first(){
  // Simulate a code delay
  setTimeout( function(){
    console.log(1);
  }, 500 );
}
function second(){
  console.log(2);
}
first();
second();

and get

// 2
// 1

It’s not that JavaScript didn’t execute our functions in the order we wanted it to, it’s instead that JavaScript didn’t wait for a response from first() before moving on to execute second().
So why show you this? Because you can’t just call one function after another and hope they execute in the right order. Callbacks are a way to make sure certain code doesn’t execute until other code has already finished execution.
Another example:

function doHomework(subject, callback) {
  alert(`Starting my ${subject} homework.`);
  callback();
}

doHomework('math', function() {
  alert('Finished my homework');
});

ref link: https://codeburst.io/javascript-what-the-heck-is-a-callback-aba4da2deced