callback

A callback is a function that is passed as an argument to another function and is invoked after a specified operation completes. Callbacks enable a form of continuation or event handling by letting the caller supply the code that should run later.

In asynchronous programming, a callback is invoked when an operation finishes, such as reading a file, making a network request, or responding to an event. Many languages support callbacks; JavaScript is especially known for using callbacks in APIs, notably in browser and server environments. A common pattern is the error-first callback, where the first parameter indicates an error if one occurred and the second carries the result.

Callbacks can be synchronous, meaning the on-callback code runs before the outer function returns, or asynchronous, where the outer function returns immediately and the callback runs later. They embody inversion of control, since the callee decides when to run the callback.

Common uses include event listeners, timer callbacks, and asynchronous I/O. An example pattern is a function fetchData(url, cb) that initiates a request and eventually calls cb(null, data) to report success or cb(err) to report failure. Callbacks also appear in GUI event handling and various API designs.

Complex chains of callbacks can lead to reduced readability, a problem sometimes called callback hell. Modern languages offer alternatives such as promises, futures, or async/await, which help structure asynchronous flows while retaining the concept of executable code invoked upon completion.