What are common TypeScript mistakes in Next.js?

When working with Next.js and TypeScript together, developers often hit a few common pitfalls that can slow down development or introduce subtle bugs. Having used both extensively in production, I’ve seen these mistakes crop up repeatedly in interviews and real projects. Understanding these traps not only helps you write cleaner, more maintainable code but also prepares you to explain your reasoning clearly during technical interviews.

Next.js and TypeScript are a powerful combo: Next.js offers server-side rendering, static site generation, and API routes, while TypeScript adds type safety and better developer tooling. However, combining them requires a solid grasp of both frameworks’ quirks and how they interact.

Common TypeScript Mistakes in Next.js Projects

1. Misunderstanding Next.js Data Fetching Types

One of the most frequent issues is incorrectly typing the data fetching functions like getStaticProps, getServerSideProps, and getStaticPaths. These functions have specific return types that Next.js expects, and if you don’t type them properly, you lose type safety or cause runtime errors.

For example, a common mistake is returning an object that doesn’t conform to the expected shape, or forgetting to type the props object:

export const getStaticProps = async (): Promise<{ props: { posts: Post[] } }> => {
  const posts = await fetchPosts();
  return {
    props: {
      posts,
    },
  };
};

Here, explicitly typing the return value ensures that TypeScript catches if you accidentally omit props or return something unexpected. Many developers skip this and rely on implicit typing, which can lead to subtle bugs when the data shape changes.

2. Ignoring the NextPage Type and Its Generics

Next.js provides a NextPage type for typing page components, which supports generics for typing props. A common mistake is to use plain React.FC or not type the props at all, which defeats the purpose of TypeScript.

Using NextPage properly looks like this:

import { NextPage } from 'next';

interface Props {
  posts: Post[];
}

const BlogPage: NextPage<Props> = ({ posts }) => {
  return (
    <div>
      {posts.map(post => (
        <article key={post.id}>{post.title}</article>
      ))}
    </div>
  );
};

export default BlogPage;

This approach helps with autocomplete and ensures the props passed from getStaticProps or getServerSideProps match the component’s expected props. Skipping this typing can cause runtime errors if the props don’t align.

3. Overusing any or Disabling Type Checking

When TypeScript errors become overwhelming, some developers fall back on any or disable strict mode in tsconfig.json. This is a slippery slope because it defeats the purpose of TypeScript and leads to fragile code.

Instead, it’s better to invest time in defining proper interfaces or types, even if it means writing a few extra lines. For example, when dealing with API responses, define the expected shape explicitly:

interface ApiResponse {
  id: string;
  title: string;
  content: string;
}

const fetchPosts = async (): Promise<ApiResponse[]> => {
  const res = await fetch('/api/posts');
  const data = await res.json();
  return data;
};

This way, you get type safety throughout your app, and your IDE can help catch mistakes early.

4. Not Handling Optional or Nullable Props Correctly

Next.js pages often receive props that may be optional or nullable, especially when using dynamic routes or fallback pages. A common mistake is to assume props are always present, leading to runtime errors like “cannot read property of undefined.”

For example, when using getStaticPaths with fallback pages, props might be undefined initially:

interface PostPageProps {
  post?: Post;
}

const PostPage: NextPage<PostPageProps> = ({ post }) => {
  if (!post) {
    return <p>Loading...</p>;
  }

  return <article>{post.title}</article>;
};

Typing props as optional and handling the loading state explicitly prevents crashes and improves user experience.

Best Practices for Using TypeScript in Next.js

• Enable strict mode: Turn on strict in tsconfig.json to catch more errors early.
• Use Next.js provided types: Types like NextPage, GetStaticProps, and GetServerSideProps help keep your code aligned with Next.js expectations.
• Define clear interfaces for props and API data: This improves maintainability and helps onboard new developers faster.
• Handle optional and nullable data explicitly: Avoid assumptions about data presence, especially in dynamic routes or fallback pages.
• Avoid any and type assertions: Use them sparingly and only when absolutely necessary.

Performance and Scalability Considerations

TypeScript itself doesn’t impact runtime performance, but how you type your Next.js app can affect developer productivity and scalability. For example, well-typed data fetching functions reduce bugs in server-side rendering and static generation, which can prevent costly production issues.

On the flip side, overly complex or deeply nested types can slow down your IDE and build times. It’s a balance—avoid unnecessary complexity in your types and keep them as flat and reusable as possible.

Security Considerations

TypeScript can help prevent certain classes of bugs that lead to security issues, like injection attacks or data leaks, by enforcing strict typing on inputs and outputs. However, it’s not a silver bullet. You still need to validate and sanitize data on the server side, especially in API routes.

For example, when typing API request handlers, define the expected request body shape and validate it before processing:

import type { NextApiRequest, NextApiResponse } from 'next';

interface CreateUserRequest {
  username: string;
  email: string;
}

export default function handler(
  req: NextApiRequest,
  res: NextApiResponse
) {
  const body: CreateUserRequest = req.body;

  if (!body.username || !body.email) {
    res.status(400).json({ error: 'Missing fields' });
    return;
  }

  // Proceed with creating user
}

TypeScript helps here by making sure you don’t accidentally miss required fields, but runtime validation is still necessary.

Common Mistakes Summary Table

Mistake	Description	Impact	How to Avoid
Incorrect typing of data fetching functions	Returning wrong shape or missing props in getStaticProps	Runtime errors, loss of type safety	Use Next.js types like GetStaticProps and explicitly type return values
Not using NextPage type	Using plain React.FC or untyped components	Props mismatch, harder to maintain	Type pages with NextPage<Props>
Overusing any	Disabling strict mode or using any to silence errors	Loss of type safety, fragile code	Define proper interfaces and enable strict mode
Ignoring optional/nullable props	Assuming props are always present	Runtime crashes	Mark props optional and handle loading/fallback states

Interview Tips for Discussing TypeScript in Next.js

• Explain your typing strategy: Interviewers appreciate when you can articulate why you use certain types or interfaces, not just that you use them.
• Show awareness of Next.js lifecycle: Mention how you type data fetching methods and why that matters for server-side rendering or static generation.
• Discuss trade-offs: For example, strict typing vs. developer speed, or when you might use any temporarily during prototyping.
• Bring up real bugs you’ve caught: Sharing stories about how TypeScript helped you avoid production issues makes your experience tangible.
• Be honest about challenges: It’s okay to mention that typing complex API responses or third-party libraries can be tricky and how you handle those cases.

Practical Production Scenario

Imagine you’re building a blog with Next.js and TypeScript. You use getStaticProps to fetch posts from a headless CMS. If you don’t type the response correctly, you might accidentally access a property that doesn’t exist, causing your build to fail or your page to crash.

By defining an interface for your posts and typing getStaticProps accordingly, you get immediate feedback if the CMS schema changes or if you mistype a property. This reduces debugging time and improves confidence in your deployment.

Also, when you add dynamic routes (e.g., /posts/[id]), typing getStaticPaths and handling fallback states properly ensures your app doesn’t break when a user visits a new post that hasn’t been statically generated yet.

Comparison with Alternatives

Some developers choose to use plain JavaScript with JSDoc comments or PropTypes in Next.js projects instead of TypeScript. While these approaches can provide some level of type checking or runtime validation, they don’t offer the same level of static analysis and tooling support.

Approach	Pros	Cons
TypeScript	Static typing, better IDE support, catches errors early	Steeper learning curve, longer initial setup
PropTypes	Runtime type checking, easy to add incrementally	Only runtime checks, no compile-time safety
JSDoc	Lightweight, no build step needed	Limited type inference, less tooling support

For serious Next.js projects, especially those expected to scale or be maintained long-term, TypeScript is generally the better choice despite the upfront cost.