Typed Function Parameters

Params & return type

Annotate each parameter, and optionally the return type after ).

function area(width: number, height: number): number {
  return width * height;
}
area(4, 5);   // 20

Optional & default params

? marks a parameter optional; a default value makes it optional too and sets its type from that value.

function greet(name: string, greeting: string = "Hello"): string {
  return `${greeting}, ${name}!`;
}
function findUser(id: number, cache?: Map<number, string>) { /* ... */ }

Calling it wrong

Wrong argument count or type is rejected before the code ever runs.

area(4);
// Error: Expected 2 arguments, but got 1.
area(4, "5");
// Error: Argument of type 'string' is not assignable to parameter of type 'number'.

Quick check: Which makes a function parameter optional?

• A trailing `?` after its name
• Wrapping it in `unknown`
• Putting it first in the parameter list