TypeScript Functions, Default Parameters, Rest Parameters, and async/await

Functions are one of the most common places to use TypeScript types. Parameter types, return types, and async return types should usually be explicit.

Basic Function Types

function calculateDiscount(price: number, rate: number): number {
  return price * rate;
}

Return types can often be inferred, but public or complex functions are clearer with explicit return types.

Default Parameters

function calculateDiscount(price: number, rate: number = 0.5): number {
  return price * rate;
}
 
calculateDiscount(1000);
calculateDiscount(1000, 0.3);

Default parameters reduce repeated configuration.

Rest Parameters

function joinNames(prefix: string, ...names: string[]): string {
  return `${prefix}: ${names.join(", ")}`;
}

...names means the function accepts any number of string arguments.

Promise Basics

Async functions usually return Promise<T>.

async function fetchUser(): Promise<{ id: string; name: string }> {
  return { id: "1", name: "Alice" };
}

async automatically wraps returned values in a Promise.

await

await waits for a Promise to resolve.

async function main() {
  try {
    const user = await fetchUser();
    console.log(user.name);
  } catch (error) {
    console.error("Failed to fetch user", error);
  }
}

Practical Advice

For async service functions, make return values explicit.

async function pullContacts(args: {
  limit?: number;
  page?: number;
}): Promise<ContactList> {
  return { contacts: [] };
}

This gives callers better type hints and catches interface changes earlier.

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