What are callbacks?

Learn the callback function concept and usage.

What are callbacks?

A callback is a function passed as an argument to another function, to be called later. The call does not happen right away; it comes when an event completes.

Syntax: function run(cb) { cb(); }. Here cb is a function, and run calls it at the right moment. You define the function; another function runs it for you.

Example: setTimeout(() => console.log('hello'), 1000). The arrow function is a callback; setTimeout calls it after one second. Callbacks are common in delayed and event-driven work.

Call time

The power of callbacks comes from flexible call time. You define the function; the caller decides when and how to invoke it. This is a fundamental pattern in event-driven programming.

Example: function withGreeting(name, cb) { cb('Hello ' + name); }. Here withGreeting passes a value to cb. The caller receives and uses it: withGreeting('Ayşe', msg => console.log(msg)).

This pattern separates the side that prepares the data from the side that uses it. When preparation is done, the callback fires; you can then change each side independently.

Exercise Greet with a callback

Define a function withGreeting(name, cb). Its body should call cb('Hello ' + name). Call it as withGreeting('Ayşe', msg => console.log(msg)) so the result prints.

Show example solution

function withGreeting(name, cb) {
  cb('Hello ' + name);
}
withGreeting('Ayşe', msg => console.log(msg));

Test yourself

What does callback mean?
- A function called later
- A returned value
- An error
- A loop body
Show answer

A function called later — a callback is a function passed in and called later.

Invoke the callback

function run(cb) {
  ____;
}

Show answer

function run(cb) {
  cb();
}

Match the callback parts

cb

Callback parameter
cb()

Callback invocation
() => {}

Function passed as callback
setTimeout

Function using a callback

Next lesson: Higher-order functions

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies