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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.

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.

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.

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.

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.

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

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).

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.

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.

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?

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!