tRPC End-to-End Type Safe APIs · Lesson

Authentication Middleware

Implement authentication checks using tRPC middleware to protect your API procedures.

Authentication Middleware is a free tRPC End-to-End Type Safe APIs lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the tRPC End-to-End Type Safe APIs learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Protect Your API with Auth

Welcome to this lesson on tRPC authentication middleware! Securing your API is crucial to ensure only authorized users can access sensitive data or perform critical actions.

Middleware in tRPC provides an elegant way to centralize these security checks before any procedure runs.

Why Auth Middleware?

Using middleware for authentication offers significant advantages:

Centralized Logic: Define authentication rules once and apply them everywhere.
Reduced Duplication: Avoid writing the same security checks in every API procedure.
Clean Code: Keep your business logic separate from security concerns.
Consistency: Ensure all protected endpoints adhere to the same security standards.

Authentication Basics

Before implementing, let's briefly recall common authentication methods:

Tokens: Such as JWTs (JSON Web Tokens) or API keys, typically sent in an Authorization header.
Sessions: Often managed with cookies, where the server stores session data and the client sends a session ID.

Our middleware will be responsible for validating these credentials.

Context for User Data

Remember that the tRPC context is an object available to all procedures, carrying request-specific data. For authentication, this means our createContext function (from a previous lesson) should parse incoming authentication information (e.g., from headers) and populate the context with user data if available.

Our middleware will then *read* this user data from the context.

Building Auth Middleware

tRPC's t.middleware() function is where the magic happens. It takes an asynchronous function that receives an object with ctx (the context) and next (a function to call the next middleware or the procedure itself).

Inside, you'll check for authentication. If successful, you call next(). If not, you throw a TRPCError.

Simple Authentication Middleware

Here's a runnable TypeScript example that simulates a basic authentication middleware. It checks if a user object exists in the context.

class TRPCError extends Error {
  code: string;
  constructor(opts: { code: string }) {
    super(`TRPCError: ${opts.code}`);
    this.code = opts.code;
  }
}

type MockContext = { user?: { id: string; name: string } };
type MiddlewareFn = (opts: { ctx: MockContext; next: Function }) => Promise<any>;

const isAuthenticated: MiddlewareFn = async ({ ctx, next }) => {
  if (!ctx.user) {
    throw new TRPCError({ code: 'UNAUTHORIZED' });
  }
  return next({
    ctx: {
      ...ctx,
      user: ctx.user,
    },
  });
};

async function runMiddlewareDemo() {
  console.log("--- Test with authenticated user ---");
  try {
    await isAuthenticated({
      ctx: { user: { id: "123", name: "Alice" } },
      next: async (opts: { ctx: MockContext }) => {
        console.log("Middleware passed. User:", opts.ctx.user?.name);
        return "Success";
      }
    });
  } catch (error) {
    console.error("Error:", error instanceof TRPCError ? error.code : String(error));
  }

  console.log("\n--- Test with unauthenticated user ---");
  try {
    await isAuthenticated({
      ctx: {}, // No user in context
      next: async (opts: { ctx: MockContext }) => {
        console.log("Middleware passed (should not happen)");
        return "Success";
      }
    });
  } catch (error) {
    console.error("Error:", error instanceof TRPCError ? error.code : String(error));
  }
}

runMiddlewareDemo();

Applying Middleware to Procedures

Once defined, you can apply middleware using the .use() method. This can be done on individual procedures or even entire routers to protect multiple procedures at once.

Middleware can also be chained together, allowing you to combine multiple checks (e.g., authentication then authorization).

A Protected Query Example

Here's how you might apply the isAuthenticated middleware to a specific query procedure. The ctx.user will be guaranteed to exist inside the procedure if the middleware passes.

import { t } from './trpc'; // Your tRPC instance
import { isAuthenticated } from './middleware'; // Your auth middleware

// Imagine 'z' is imported for input validation from Zod
// import { z } from 'zod';

const appRouter = t.router({
  publicGreeting: t.procedure
    .query(() => {
      return "Hello, stranger!";
    }),
  
  protectedGreeting: t.procedure
    .use(isAuthenticated) // Apply the middleware here
    .query(({ ctx }) => {
      // ctx.user is guaranteed to exist here due to middleware
      return `Welcome, ${ctx.user.name}! You are authenticated.`;
    }),
});

// This is a conceptual snippet and not runnable standalone.

Handling Unauthorized Access

When the middleware detects an unauthenticated request and throws a TRPCError (e.g., with code: 'UNAUTHORIZED'), tRPC automatically catches this error.

It then sends a standardized error response to the client, allowing your frontend application to gracefully handle the unauthorized access, perhaps by redirecting the user to a login page.

Test Your Auth Middleware Knowledge

You have an isAdmin middleware. You want to protect all procedures within an adminRouter so only administrators can access them. Which is the correct way to apply the middleware?

Authentication Middleware Recap

You've learned how to implement authentication checks using tRPC middleware!

Authentication middleware centralizes security logic.
It leverages the tRPC context to access user information.
You define it using t.middleware().
You apply it to procedures or entire routers using .use().
TRPCError ensures proper error handling for unauthorized requests.

Next, explore how to build custom middleware chains for more complex scenarios!

Frequently asked questions

Is the “Authentication Middleware” lesson free?

Yes — the full text of “Authentication Middleware” is free to read here on the web, and the tRPC End-to-End Type Safe APIs course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the tRPC End-to-End Type Safe APIs course, upgrade to CoddyKit PRO.

What will I learn in “Authentication Middleware”?

Implement authentication checks using tRPC middleware to protect your API procedures. You practise tRPC End-to-End Type Safe APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start tRPC End-to-End Type Safe APIs?

No prior experience is required. tRPC End-to-End Type Safe APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Authentication Middleware” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this tRPC End-to-End Type Safe APIs lesson?

Yes. Every tRPC End-to-End Type Safe APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Creating tRPC Context
Authentication Middleware
Custom Middleware Chains
Logging and Performance Timing Middleware

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