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Some tips when using T3 Stack: Part 1

orignally published here.

May be these tips are not specifically related to T3 Stack but concern the stacks it uses

In my last article, I introduced the side project that I'm building and the tools (stacks) I'm using. And explain why I choose T3 Stack to build the project. Good, in this article I'll share some tips I discovered using T3 Stack when building the project. Let's go!


  • NextAuth needs/uses for authentication a model User with some pre-configured columns like name, image, email, etc.

  • With T3 Stack we use Prisma as ORM

  • We have added some columns to the User model like the username

NextAuth and Module augmentation

NextAuth is used for authentication in Next.js applications and is packed in T3 Stack. When using NextAuth, the hook useSession returns session data that can be checked to verify if someone is signed like this:

const index = () => {
    const { data: session } = useSession()
    if (session) {
        return <div>You're signed as {}</div>    
    return <div>Not signed, we'll redirect you</div>
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But for security reasons, the session data returned (by default) contained just

export interface DefaultSession {
    user?: {
        name?: string | null;
        email?: string | null;
        image?: string | null;
    expires: ISODateString;
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So what can we do if we want to add a new property like the username which is a column of our User model that we define with Prisma?

T3 dedicated's section to NextAuth and NextAuth itself already documented this part. To do it, we'll use module augmentation offered by TypeScript which allows a user to override classes, modules,... types that he/she doesn't access (maybe from a lib). To do it,

  1. We can override the Session interface in the next-auth module declaration inside the file types/next-auth.d.ts.

    import { DefaultSession } from "next-auth";
    declare module "next-auth" {
      interface Session {
        user?: {
          id: string;
          username: string | null  
        } & DefaultSession["user"];

    We import next-auth module and override the Session interface by adding not only a username but also merging the type of user in DefaultSession so we still have the default properties of the user provided by NextAuth.

  2. Then in the /pages/api/auth/[...nextauth].ts file we can override session callback by adding username like this

    callbacks: {
        session({ session, user }) {
          if (session.user) {
            session.user.username = user.username;
          return session;

    But if you leave it like this, you'll get this warning from TS: Property 'username' does not exist on type 'User | AdapterUser'. Why?

    Because, if you go to the TS definition of User (used by Nextauth) you'll see that it extends DefaultUser which has the following definition (the minimum)

    export interface DefaultUser {
        id: string;
        name?: string | null;
        email?: string | null;
        image?: string | null;

    So to fix this error, I think you guess the solution, yes: override the declaration of User as we did it with Session little above.

  3. So back to the types/next-auth.d.ts, we can override User after the Session interface (inside next-auth declaration) like this

      interface User extends DefaultUser{
        username: string | null

    We've just override User interface by firstly extending DefaultUser (provided by NextAuth) and secondly adding a username.

Let's move to another tip.

Models types deductions (with tRPC)

With Prisma, we define our tables as models inside schema.prisma file but we don't have a direct access to their types in TypeScript because it generates types definitions in node_modules/.prisma/client/index.d.ts by default (read this link if you want to change this output file location). But there are some ways to get these types:

  1. Use the return type of Prisma client methods like this

    const getById = async () => await{
    type Post = ReturnType<typeof getById>
  2. Or you can use the GetPayload version of the model type where we can specify all relations between this model and others

    import { PrismaClient, Prisma } from "@prisma/client";
    type User = Prisma.UserGetPayload<{
        // here you can specify all relations like: select, include, ...

    You can read more here and on this link.

I don't recommend you these solutions because we have tRPC (don't blame me).

In our case, we use Prisma client through tRPC. And you'll be happy to know that tRPC has a utility to infer routers' input/return types. Let's suppose that we have a router named user and we define inside this router a procedure retrieve. To get the return type in plain tRPC of this procedure we can do:

import type { inferRouterInputs } from "@trpc/server"

type User = inferRouterInputs<AppRouter>['user']['retrieve']
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Nice! But how will you be if I say that in T3 Stack there is a little helper to make this operation easier:

import {RouterOutputs} from "@utils/api";
type User = RouterOutputs['project']['getById']
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And yes, if you go to look inside the @utils/api you'll see that

export type RouterOutputs = inferRouterOutputs<AppRouter>;
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Anyway, it's soft to use.


With this approach, TS can warm you that User type could be null, so to avoid it you can use the NonNullable utility:

import {RouterOutputs} from "@utils/api";
type User = NonNullable<RouterOutputs['project']['getById']>
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I love the end-to-end typesafe that we get from tRPC and it's very cool to work with T3 Stack where they put together some great libs and frameworks to make full-stack development with JS much easier.

We are at the end of this article. I hope you enjoy it. If I get another tip while building my SaaS, I'll share them. Feedback or anything that could help me to improve this post is welcome.

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Top comments (2)

mikahfsmusic profile image
Mikah Feldman-Stein

The [...nextAuth].ts file doesn't have a callback section in the most current version of create-t3-app. There is a file there that is just the following:

import NextAuth from "next-auth";
import { authOptions } from "~/server/auth";

export default NextAuth(authOptions);

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would you be willing to update this doc with the most recent version in mind?

tawaliou profile image

Oh yes thank you for your feedback. Yes I will do it very soon. I'll upgrade my app and come back to update the post.

Thanks 🤗