TypeScript Basics›Core· 35 min read

Typing Functions

Type a function fully: its parameters, its return value, plus optional and default arguments.

What you will learn

Type parameters and the return value
Use optional parameters with ?
Give parameters default values

Parameters in, a value out

A fully typed function says what goes in (each parameter) and what comes out (the return type, written after the parentheses).

Typed parameter (string) and typed return (string)

function greet(name: string): string {
  return 'Hi, ' + name + '!';
}

console.log(greet('Asha'));

Note: Output: Hi, Asha! name: string types the input; : string after the parentheses types the output. Returning a number here would be an error.

Functions that return nothing: void

If a function just does something and returns no value, its return type is void:

A void function returns no value

function logError(message: string): void {
  console.log('ERROR: ' + message);
}

logError('Disk full');

Note: Output: ERROR: Disk full void means "this returns nothing". It is the right type for functions that only have a side effect, like logging.

Optional parameters with ?

Add a ? after a parameter name to make it optional. Inside the function it might be missing, so TypeScript treats it as "the type, or undefined":

title is optional thanks to the ?

function greetUser(name: string, title?: string): string {
  if (title) {
    return 'Hello, ' + title + ' ' + name;
  }
  return 'Hello, ' + name;
}

console.log(greetUser('Asha'));
console.log(greetUser('Asha', 'Dr'));

Note: Output: Hello, Asha Hello, Dr Asha The first call skips title entirely — allowed because of the ?. The second supplies it.

Default values

Instead of optional, you can give a parameter a default value used when the caller leaves it out:

b defaults to 2 when not provided

function multiply(a: number, b: number = 2): number {
  return a * b;
}

console.log(multiply(5));      // b defaults to 2
console.log(multiply(5, 10));  // b is 10

Note: Output: 10 50 In the first call b was missing, so it became 2 (giving 5 × 2). In the second, b was 10. TypeScript infers b is a number from the default.

Watch out: An optional parameter must come after all required ones. function f(a?: string, b: number) is an error — the optional one cannot sit before a required one.

Tip: Use a default value when there is a sensible fallback (like b = 2). Use ? when "missing" is genuinely meaningful and you handle it yourself.

Q. What does the ? in function f(x: number, y?: number) mean?

Answer: A ? after a parameter name marks it optional. Inside the function its type becomes number | undefined, so you should check before using it.

✍️ Practice

Write a typed area(width: number, height: number): number and call it.
Add an optional unit?: string parameter to a function and handle the missing case.

🏠 Homework

Write a makeLabel function with one required and one default parameter, and show two calls with their outputs.