Anonymous Functions

An anonymous function is a function expression without its own declared name. It is often supplied directly where a callback is required.

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

Contextual typing lets TypeScript infer a parameter type from the surrounding API. Anonymous does not mean untyped: the array map call here gives the callback parameter a number type.

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

const prices: number[] = [10, 20, 30];
const afterTax = prices.map(function (price) {
  return price * 1.1;
});
console.log(afterTax.map(value => value.toFixed(2)).join(", "));

Verified output

11.00, 22.00, 33.00

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.

map calls the anonymous function once for each price, receiving 11, 22, and 33. toFixed(2) converts each number to a two-decimal string before joining.

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 mistake the anonymous function for a call that happens immediately on definition: map is the operation that invokes it.

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.

Replace the function expression with an arrow callback. Explain which values and types remain unchanged.

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.

Anonymous Functions: An anonymous function is a function expression without its own declared name.

map calls the anonymous function once for each price, receiving 11, 22, and 33. toFixed(2) converts each number to a two-decimal string before joining.

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