Functions

Typing a function means annotating both its parameters and, usually, its return value — so a mistake in what a function is called with, or what it hands back, gets caught the same way any other type mismatch does.

Parameter and return types

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

console.log(add(3, 4));
console.log(add("3", 4));

add.ts:6:14 - error TS2345: Argument of type 'string' is not
assignable to parameter of type 'number'.

The : number after the closing parenthesis is the return type — if the function body ever returned something other than a number, that line itself would fail to compile too.

Optional and default parameters

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

function tag(text: string, category?: string): string {
  return category ? `[${category}] ${text}` : text;
}

console.log(greet("Sam"));
console.log(greet("Sam", "Hey"));
console.log(tag("Server down"));
console.log(tag("Server down", "urgent"));

Hello, Sam!
Hey, Sam!
Server down
[urgent] Server down

greeting: string = "Hello" supplies a fallback used whenever the caller leaves that argument out. category?: string instead makes the parameter genuinely optional, with no default — inside the function it's either a string or undefined.

Function types

function transform(value: number, fn: (n: number) => number): number {
  return fn(value);
}

const doubled = transform(5, n => n * 2);
console.log(doubled);

10

fn: (n: number) => number describes a function type directly — a callback that takes a number and returns a number — the same style of annotation you'd use for a variable.