Functions

A function's parameters and return value can each be typed. TypeScript checks every call site against these types, so passing the wrong kind of argument is caught immediately, not when the function happens to run.

main.ts

Try it yourself →

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

console.log(greet("Alice"));
console.log(greet("Bob"));

Output

Hello, Alice!
Hello, Bob!

void, and optional/default parameters

void marks a function that doesn't return a meaningful value. A parameter followed by ? is optional — callers can leave it out — and a parameter can also have a default value, making it optional in a different way: it's filled in automatically when omitted.

main.ts

Try it yourself →

function logScore(name: string, score: number = 0): void {
  console.log(`${name}: ${score}`);
}

logScore("Alice", 95);
logScore("Bob");

Output

Alice: 95
Bob: 0

Next: arrow functions — a shorter syntax, with the same typing rules.

Practice exercise

Write a function multiply(a: number, b: number = 2): number that returns a * b, then print multiply(5, 3) and multiply(5).

← Type Aliases and Union Types Arrow Functions →