custom.md

Custom Middleware

Create your own middleware to handle cross-cutting concerns like authentication, logging, and response handling.

Unified Middleware Pattern

All middleware uses the same intercept pattern where you can modify requests before they're sent and process responses after they're received:

client.use(async (request, next) => {
  // 1. Modify request before sending
  request.headers = {
    ...request.headers,
    'X-Custom-Header': 'value',
  };

  // 2. Call next middleware/make request
  const response = await next(request);

  // 3. Process response after receiving
  if (response.status >= 400) {
    console.error(`Request failed: ${response.status}`);
  }

  return response;
});

Execution Order

Middleware executes in an onion-like pattern:

1. Middleware 1 (request processing)
2. Middleware 2 (request processing)
3. HTTP request
4. Middleware 2 (response processing, reverse order)
5. Middleware 1 (response processing, reverse order)

Advanced Patterns

Conditional Processing

client.use(async (request, next) => {
  const response = await next(request);

  if (response.status >= 400) {
    // Only process error responses
    logError(response);
  }
  return response;
});

Middleware Composition

function createAuthMiddleware(token: string) {
  return async (request, next) => {
    request.headers = { ...request.headers, Authorization: `Bearer ${token}` };
    return next(request);
  };
}

client.use(createAuthMiddleware(userToken));

See also: Error Handling