Functions

Annotate parameters and returns

Types on parameters are checked at every call. A return type documents the output.

function add(a: number, b: number): number {
  return a + b;
}

console.log(add(2, 3));

Use optional and default params

Optional parameters may be omitted. Defaults provide a value when the argument is missing.

function greet(name: string, greeting = "Hello"): string {
  return `${greeting}, ${name}`;
}

console.log(greet("World"));
console.log(greet("World", "Hi"));

Type arrow functions

The same annotations work on arrow functions and callbacks.

const double = (n: number): number => n * 2;
const nums = [1, 2, 3].map(double);
console.log(nums);

Void and never briefly

• void means the function does not return a useful value
• never means it never completes normally (throw or infinite loop)
• Prefer explicit return types on public helpers
• Rest parameters use ...args: number[]