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

Source

https://coderbyte.com/algorithm/10-common-javascript-interview-questions

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function modifyArray(arr, callback) {

  // do something to arr here

  arr.push(100);

  // then execute the callback function that was passed

  callback();

}

​

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

​

modifyArray(arr, function() {

  console.log("array has been modified", arr);

});