Optional Default and Rest Parameters · Functions in TypeScript | TypeScript

Optional and Default Parameters

param?: T makes a parameter optional (its type becomes T | undefined). param = value gives a default and makes it optional automatically. Optional parameters must come after required ones.

Example

function formatPrice(amount: number, currency = "INR", decimals?: number): string {
  const fixed = decimals === undefined ? amount.toString() : amount.toFixed(decimals);
  return currency + " " + fixed;
}

console.log(formatPrice(499));
console.log(formatPrice(499, "USD"));
console.log(formatPrice(499, "INR", 2));

Output

INR 499
USD 499
INR 499.00

Rest Parameters

Example

A rest parameter collects any number of arguments into an array. Its type is always an array type.

function cartTotal(...prices: number[]): number {
  return prices.reduce((sum, p) => sum + p, 0);
}

function tagLine(label: string, ...tags: string[]): string {
  return label + ": " + tags.map((t) => "#" + t).join(" ");
}

console.log(cartTotal());
console.log(cartTotal(199, 349, 899));
console.log(tagLine("Product", "sale", "electronics"));

Output

0
1447
Product: #sale #electronics

Object Parameters with Defaults

Example

For functions with many options, pass one object and destructure it with defaults. Callers can then pass options in any order.

type SearchOptions = { query: string; page?: number; pageSize?: number; sort?: "price" | "rating" };

function search({ query, page = 1, pageSize = 20, sort = "rating" }: SearchOptions): string {
  return `q=${query}&page=${page}&size=${pageSize}&sort=${sort}`;
}

console.log(search({ query: "shoes" }));
console.log(search({ query: "laptop", sort: "price", page: 3 }));

Output

q=shoes&page=1&size=20&sort=rating
q=laptop&page=3&size=20&sort=price