TypeScript

Add parameter and return types to a function.

function formatPrice(
  amount: number,
  currency: string
): string {
  return `${currency} ${amount.toFixed(2)}`;
}

const total = formatPrice(24.5, "USD");

Describe and format a structured user record.

interface User {
  id: number;
  name: string;
  roles: string[];
}

function displayUser(user: User): string {
  return `${user.id}: ${user.name}`;
}

Return the first element while preserving its type.

function firstItem<T>(items: T[]): T | undefined {
  return items[0];
}

const firstNumber = firstItem([4, 8, 15]);
const firstName = firstItem(["Ada", "Lin"]);