Usefulness of callback functions

Question

In Javascript there is a possibility to define a function X and pass it as an argument to another function Y.

Such a function X is called a callback function

Answer 1

Callbacks are inevitable if you do any kind of event listening. JavaScript and event-driven paradigm are brothers in arms. User interaction as presented in web browsers is the reason.

Whether you listen to DOM events, Ajax completion, file system access or HTTP requests in Node.js, you must define a procedure to be called upon event occurring.

More advanced form of callbacks are promises, for instance.

---

Also, callbacks are handy as iteration processors:

[ 1, 2, 3 ].forEach(function callback(val) { console.log(val); });

The following could replace all letters with their code-points:

'hey hop hello'.replace(/./, function callback(s) { return s.charCodeAt(0); });