Application Middleware

Learn how to use application-level middleware in Fastro to modify requests and responses globally

Application middleware in Fastro runs for every request before reaching your route handlers. It's perfect for adding common functionality like authentication, logging, or request modification across your entire application.

Basic Usage

import fastro, { Context, HttpRequest } from "https://fastro.deno.dev/mod.ts";

const f = new fastro();

// Define middleware function
const middleware = (req: HttpRequest, ctx: Context) => {
  // Modify request object
  req.ok = true;

  // Add properties to context
  ctx.msg = "hello";
  ctx.getTitle = () => "Application Title";

  // Always call ctx.next() to continue to the next middleware or route
  return ctx.next();
};

// Register middleware globally
f.use(middleware);

// Route handler can access modified request and context
f.get("/", (req: HttpRequest, ctx: Context) => {
  return {
    ok: req.ok,
    msg: ctx.msg,
    title: ctx.getTitle(),
  };
});

await f.serve();

Multiple Middleware

You can register multiple middleware functions that will execute in the order they were added:

const logger = (req: HttpRequest, ctx: Context) => {
  console.log(`${req.method} ${req.url}`);
  return ctx.next();
};

const auth = (req: HttpRequest, ctx: Context) => {
  req.user = { id: 1, name: "John" };
  return ctx.next();
};

f.use(logger);
f.use(auth);

Error Handling in Middleware

Middleware can also handle errors and prevent further execution:

const errorHandler = (req: HttpRequest, ctx: Context) => {
  try {
    // Some validation logic
    if (!req.headers.get("authorization")) {
      return ctx.json({ error: "Unauthorized" }, 401);
    }
    return ctx.next();
  } catch (error) {
    return ctx.json({ error: "Internal Server Error" }, 500);
  }
};

f.use(errorHandler);

Key Points

Middleware functions receive HttpRequest and Context objects
Always call ctx.next() to continue to the next middleware or route handler
Middleware executes in the order it was registered with f.use()
You can modify both request and context objects that will be available in subsequent middleware and route handlers
Middleware can return early to prevent further execution (useful for authentication, validation, etc.)