Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

// Basic function type annotation
function add(a: number, b: number): number {
    return a + b;
}

// Function type alias
type BinaryOp = (a: number, b: number) => number;
const multiply: BinaryOp = (a, b) => a * b;

// Optional and default parameters
function greet(name: string, greeting: string = "Hello"): string {
    return `${greeting}, ${name}!`;
}
console.log(greet("Alice"));           // Hello, Alice!
console.log(greet("Bob", "Hi"));       // Hi, Bob!

// Rest parameters
function sum(...numbers: number[]): number {
    return numbers.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3, 4)); // 10

// Function overloads
function format(value: string): string;
function format(value: number, decimals: number): string;
function format(value: string | number, decimals?: number): string {
    if (typeof value === "string") return value.trim();
    return value.toFixed(decimals ?? 2);
}

console.log(format("  hello  "));   // "hello"
console.log(format(3.14159, 2));    // "3.14"

// void vs never return types
function logInfo(msg: string): void {
    console.log("[INFO]", msg);
}

function panic(msg: string): never {
    throw new Error(msg);
}