How to write a callback function in javascript?

A callback function is a function that is passed to another function as an argument and is executed after some operation has been completed.
for example of a simple callback function that logs to the console after some operations have been completed.

function extendArray(arr, callback) {
  arr.push(100);
  // then execute the callback function that was passed
  callback();
}

var arr = [1, 2, 3, 4, 5];

extendArray(arr, function() {
  console.log("array has been modified", arr);
});