Generic Functions

A generic function can retain element types across reusable operations, such as retrieving or transforming values.

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

In many calls, the compiler infers the type arguments from actual parameters, so you do not have to write <number> or <string> explicitly. Return types can depend on multiple type parameters when an operation combines differently typed inputs.

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 pair<A, B>(first: A, second: B): [A, B] {
  return [first, second];
}
const data = pair("score", 42);
console.log(data[0].toUpperCase());
console.log(data[1] + 8);

Verified output

SCORE
50

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.

pair receives a string and a number. A is inferred as string, B as number, so the tuple preserves both positions. The first expression prints SCORE and the second prints 50.

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 return any[] when the function promises position-specific results; a tuple maintains the types of the individual slots.

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.

Build swap<A,B>([a,b]: [A,B]): [B,A] and call it with a boolean and number.

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.

Generic Functions: A generic function can retain element types across reusable operations, such as retrieving or transforming values.

pair receives a string and a number. A is inferred as string, B as number, so the tuple preserves both positions. The first expression prints SCORE and the second prints 50.

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