Function Overloading

Overload signatures describe multiple supported call forms for one implementation. Callers see the overload signatures rather than the implementation signature.

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

An implementation often accepts a union and narrows it inside the body. Use overloads when caller-visible return types depend on distinct argument shapes; a union parameter is simpler when one return type suffices.

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 formatValue(value: number): string;
function formatValue(value: string): string;
function formatValue(value: number | string): string {
  return typeof value === "number" ? value.toFixed(1) : value.trim();
}
console.log(formatValue(2));
console.log(formatValue("  hello  "));

Verified output

2.0
hello

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.

For 2, the first overload applies and the number branch uses toFixed(1), producing 2.0. The string argument selects the other overload and trim removes surrounding spaces.

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 expect the implementation signature to add new callable overload forms. Overloads must be compatible with the one implementation.

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.

Add an overload accepting boolean and returning "yes" or "no". Explain why one shared implementation body is still enough.

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.

Function Overloading: Overload signatures describe multiple supported call forms for one implementation.

For 2, the first overload applies and the number branch uses toFixed(1), producing 2.0. The string argument selects the other overload and trim removes surrounding spaces.

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