Errorfirst

Errorfirst is a term used to describe the error-first callback pattern common in asynchronous programming, especially in JavaScript and Node.js. In this convention, callbacks receive an error value as their first argument, followed by any successful result arguments. An operation that succeeds typically passes null or undefined as the first parameter, while a failure passes an error object.

In practice, the typical signature is function (err, result) { if (err) { handleError(err); return; } // use result }. For example, a file read operation might call back as function (err, data) { if (err) { … } else { process(data); } }. This uniform structure aims to centralize error handling and avoid try/catch within nested asynchronous flows.

Historically, the pattern became a cornerstone of Node.js API design and influenced many JavaScript libraries. It provided a simple, consistent way to propagate errors up the call stack in asynchronous code, enabling early exit on errors and straightforward error handling at call sites.

Benefits of the errorfirst pattern include predictable error propagation and compatibility with a broad range of libraries. Drawbacks include deeply nested callbacks in complex flows (often termed “callback hell”), and the need to check the first argument at each step, which can clutter code. The rise of promises and async/await has reduced reliance on errorfirst in newer code, offering alternative, more linear error handling while preserving similar error semantics.

See also: Node.js, callback pattern, promises, async/await, error handling.