Function Parameters

Parameters are named inputs declared in the function definition. Arguments are the actual values supplied by a caller.

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

A parameter annotation applies to calls and to expressions inside the body. Multiple parameters are separated by commas; their positions matter for positional calls. TypeScript can often infer types for callback parameters, but explicit declarations are useful for public function contracts.

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 area(width: number, height: number): number {
  return width * height;
}
console.log(area(4, 7));
console.log(area(2, 9));

Verified output

28
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.

In the first call width=4 and height=7, giving 28. In the second they are 2 and 9, giving 18. A parameter binding is local to each invocation.

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.

Passing "4" to a number parameter is a compile-time error; parameter names alone do not enforce runtime conversion.

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.

Create a perimeter(width, height) function. Test with a 3-by-5 rectangle and explain why argument order matters for noncommutative operations.

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 Parameters: Parameters are named inputs declared in the function definition.

In the first call width=4 and height=7, giving 28. In the second they are 2 and 9, giving 18. A parameter binding is local to each invocation.

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