Programming Fundamentals › Concurrency & Async

Callback

A function passed in to be called when work finishes.

Also known as: callbacks, callback function, completion handler

A callback is a function you pass to another function, to be called later, when something finishes or happens.

setTimeout(() => console.log("a second later"), 1000);

button.addEventListener("click", () => save());

fs.readFile("data.txt", (err, text) => {       // Node style: (error, result)
  if (err) return console.error(err);
  console.log(text);
});

The caller says “here’s what to do when you’re done”, and gets on with other work. That’s how JavaScript handles waiting without freezing (event loop).

Callbacks also appear in synchronous code: array.map(fn), sorted(key=fn).

Rules to know

• Pass the function, don’t call it: setTimeout(save, 1000), not setTimeout(save(), 1000).
• Errors need a plan. An exception inside a callback doesn’t reach the code that registered it. Node’s convention is to pass the error as the first argument.
• this can change inside a callback (this and self).
• Call it once. A callback invoked twice, or never, causes subtle bugs.
• Order: don’t assume that a callback runs before the code after the call. It usually runs later.

The downside

Chaining several dependent steps makes nested code that’s hard to follow and hard to handle errors in (callback hell). Modern code usually wraps asynchronous work in promises and uses async/await, keeping callbacks for events and simple hooks.