5. Parameter Types, Default Values & Return Signatures

Typing Functions in TypeScript

// Explicit parameter types and return type:
function calculateTimeout(retries: number, baseTimeout: number = 5000): number {
  return baseTimeout * (retries + 1);
}
// Optional parameter (?):
function logResult(testName: string, durationMs?: number): void {
  if (durationMs !== undefined) {
    console.log(${testName} finished in ${durationMs}ms);
  } else {
    console.log(${testName} finished);
  }
}
// Async function return signature: always Promise<T>
async function fetchToken(apiUrl: string): Promise<string> {
  return "auth_token_xyz";
}