Callback Functions

A callback is a function passed to another function for the other function to invoke at the appropriate moment.

What you will learn: the purpose of callback functions, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

The callback type should describe how the receiving function actually invokes it. Mark a callback parameter optional only if the receiver sometimes calls the callback without that argument; otherwise the callback would be forced to handle undefined needlessly.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

function visit(values: number[], action: (value: number) => void): void {
  for (const value of values) action(value);
}
const seen: number[] = [];
visit([3, 6, 9], value => seen.push(value * 2));
console.log(seen.join(", "));

Verified output

6, 12, 18

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

visit receives both the array and a function. The loop calls action for 3, 6, and 9. Each call appends twice its argument to seen; the array becomes [6,12,18].

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not write action: number for something you intend to call. Use a function signature such as (value: number) => void.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Change visit to accept strings and append the uppercase form of each value to an array.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Callback Functions: A callback is a function passed to another function for the other function to invoke at the appropriate moment.

visit receives both the array and a function. The loop calls action for 3, 6, and 9. Each call appends twice its argument to seen; the array becomes [6,12,18].

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.