Callbacks

Callbacks are functions passed as arguments to other functions and invoked at a later time, typically to complete a task or respond to an event. They enable decoupling of the operation from the code that initiates it, making it possible to customize behavior without modifying the caller. Callbacks are a core mechanism in many programming languages and are implemented as function values or references to executable code.

In practice, callbacks are used in synchronous contexts (for example, when an array method applies a function to each element) and asynchronous contexts (for example, when an I/O operation finishes or a timer fires). Common patterns include event listeners, where a callback runs in response to an event, and continuation-style flows, where a callback processes the result of another function. In languages with explicit error handling, callbacks may take multiple parameters, with the first often representing an error.

A frequent design in asynchronous environments is the error-first callback, common in Node.js, where the first parameter reports an error and subsequent parameters provide results. This convention helps centralize error handling but can lead to deeply nested code, sometimes called callback hell. To mitigate complexity, many ecosystems offer alternatives such as promises, futures, or async/await, which represent the eventual result and reduce nesting.

Potential pitfalls include accidental retention of objects via closures, inadvertent multiple invocations, and difficulties canceling pending callbacks. Well-designed APIs document the callback’s contract and lifecycle, and may provide cancellation mechanisms.

See also: function pointers, delegates, promises, futures, asynchronous programming, event-driven programming, continuation-passing style.