When developers speak about api functions, they typically refer to one of two things: the backend controller functions that handle incoming HTTP routes, or the client-side software development kit (SDK) functions that wrap network requests in ergonomic syntax.

1. Server-Side API Functions (Route Handlers)

On the backend, an api function is the executable routine bound to a route. In Express, Next.js, or FastAPI, it accepts a request object, performs business logic, and returns a response:

// Server-side API Function in TypeScript
export async function handleCreateInvoice(req: Request, res: Response) {
  try {
    const { customerId, items, currency } = req.body;
    
    // 1. Validation function
    validateInvoicePayload({ customerId, items });

    // 2. Database business logic
    const invoice = await db.invoices.create({
      data: { customerId, items, currency, status: 'unpaid' }
    });

    // 3. Return response function
    return res.status(201).json({ success: true, invoice });
  } catch (err) {
    return res.status(400).json({ error: err.message });
  }
}

2. Client-Side API Functions (SDK Wrappers)

On the frontend, calling raw fetch() repeatedly is error-prone. Developers encapsulate network calls into reusable api functions:

// Client API function SDK
export const api = {
  getProfile: (userId: string) => 
    fetch(`/api/users/${userId}`).then(r => r.json()),
    
  updateAvatar: (userId: string, file: File) => {
    const form = new FormData();
    form.append('avatar', file);
    return fetch(`/api/users/${userId}/avatar`, { method: 'POST', body: form });
  }
};

Well-organized api functions keep frontend code clean, reduce boilerplate, and centralize error handling and authentication token refreshes in one place.