Developing with TypeScript in Next.js 14

Anton Ioffe - November 11th 2023 - 10 minutes read

As the landscape of web development evolves with ever-greater velocity, harnessing the power of TypeScript within your Next.js 14 projects can be like giving your codebase a sixth sense. It's not just about writing code—it's about architecting a resilient foundation that precognitively wards off bugs and enhances developer collaboration. In this article, we'll stride through the corridors of type safety, rethinking project configuration for the better, and peppering your server-side marvels with typed precision. We'll also finesse the delicate dance of frontend-backend choreography with TypeScript’s articulate assurances and finally, transcend to higher-level techniques that mold complexity into an art form. Prepare to fortify your Next.js applications with the foresight and finesse that TypeScript bestows.

Enhancing Next.js with TypeScript's Type Safety

Integrating TypeScript into Next.js 14 projects brings the robustness of static typing to the dynamic world of JavaScript, creating a formidable combination for web application development. The adoption of TypeScript as a primary language for Next.js projects adds a layer of early error detection, often catching sneaky bugs that could otherwise slip into production. As developers write code, TypeScript’s real-time type checking flags inconsistencies and potential issues straight in the IDE, substantially reducing the debug time. This preemptive sifting not only leads to cleaner code but also provides fellow developers with self-documenting type annotations that clarify intent and usage.

TypeScript's benefits are particularly noticeable when dealing with complex data structures or integrating third-party libraries. JavaScript's dynamic typing can be a double-edged sword, allowing for flexible code but also making it easy to introduce type-related bugs. TypeScript, by contrast, enforces type correctness at compile time, and with Next.js's optimized build pipeline, these checks are seamlessly integrated into the development workflow. When a developer mistakenly assigns a wrong type, the compiler will emit a warning, allowing the issue to be addressed long before the code is deployed.

The interplay between TypeScript and Next.js enhances the developer experience by supporting features like auto-completion and intelligent code navigation. These capabilities stem from TypeScript's understanding of types, enabling it to offer suggestions that match expected properties and methods on typed objects. This not only saves keystrokes but also minimizes the cognitive load during development, allowing developers to maintain focus on implementing business logic rather than recalling API specifics.

While the JavaScript ecosystem is renowned for its rapid evolution and vibrant community contributions, this sometimes entails breaking changes or deprecated APIs. TypeScript shields developers from a subset of these challenges by serving as a safeguard against incompatible library updates. When a dependency update introduces breaking changes, TypeScript surfaces these discrepancies immediately, making it easier for developers to assess and adapt to the changes.

Despite TypeScript's rigidity, it strikes a balance with JavaScript's dynamic nature through advanced type features like unions, generics, and intersections. These features provide developers with the flexibility to accurately describe the shapes and behaviors of complex and variable data entities. As a result, developers can enjoy the best of both worlds: the dynamism and expressiveness of JavaScript, bolstered by the steadfast reliability of static typing that TypeScript offers, culminating in a robust development experience within Next.js 14.

Project Configuration and Structure with TypeScript

Incorporating TypeScript into a Next.js project begins with the configuration of tsconfig.json, which serves as the roadmap for the TypeScript compiler. This file is generated automatically when you rename any project file to .ts or .tsx and subsequently run next dev or next build. It's a common pitfall to overlook the compiler options within this config file. For maintainability, ensure that options like strict are enabled to enforce type checking, and leverage baseUrl and paths for managing custom module paths. This not only helps in avoiding relative path hell but also keeps imports cleaner and more manageable across large-scale applications.

For a scalable codebase, the project should adhere to the conventions outlined by Next.js project structure. Top-level folders such as pages, components, and public should be complemented by a types directory for holding global type definitions. Common mistakes include defining types inline and repeatedly across files, which can be mitigated by using shared interfaces and types defined in the types directory. Additionally, employing module augmentation to extend existing type declarations can eliminate type duplications and inconsistencies in the codebase.

When designing the project structure, it's essential to consider the separation of concerns. Utilize the pages directory for routing and page components, the components directory for reusable UI components, and the lib or utils directory for utility functions and custom hooks, each with corresponding TypeScript definitions. Keeping these elements compartmentalized enhances modularity and aids in developing a clear mental model of the project's architecture. A typical mistake is to conflate logic and UI components, which can lead to reduced readability and difficulties with unit testing and reuse.

React components and hooks in Next.js applications should have their prop types and return types explicitly defined. While TypeScript's inference capabilities are powerful, explicitly typing these crucial parts of a component prevent props drilling errors and maintain component API contracts. Aim for single-responsibility and type-safety by breaking down complex components into smaller, well-typed ones. Be cautious of over-engineering types; creating too granular or complex types can obfuscate the intended use and may lead to the opposite effect of decreasing type readability and maintainability.

Lastly, it is vital to regularly run the TypeScript compiler and address any type errors. This habit ensures your codebase remains clean and reflects the true state of your types. Avoid using any as it bypasses TypeScript's type checking, negating the benefits TypeScript brings to your Next.js application. Where necessary, leverage more descriptive types such as unknown or never, and use type casting and type guards judiciously to ensure that your application remains as type-safe as possible. Remember that a well-typed application is more predictable, easier to refactor, and facilitates better developer collaboration.

Typed Server-Side Rendering and API Routes

Leveraging TypeScript's type system in server-side rendering functions such as getStaticProps and getServerSideProps can enhance code clarity and reduce runtime errors. When defining getStaticProps, the use of GetStaticProps type ensures that your function parameters and return value conform to what Next.js expects. Similarly, GetServerSideProps enforces correct typing for the context parameter and return type in getServerSideProps. These constraints not only facilitate safer data handling but also improve the developer experience by enabling better tooling support such as auto-completion and refactoring capabilities.

export const getStaticProps: GetStaticProps = async (context) => {
    // Logic to fetch data
    const data = await fetchData();
    return {
        props: { data }, // Must match the expected return type

For API routes, proper typing with NextApiRequest and NextApiResponse is crucial. This guarantees that the request and response objects you interact with in your handlers are well-understood by the TypeScript compiler. For example, specifying the expected JSON response shape via a generic type can prevent mistakenly sending an incorrect response format, thus improving the maintainability of the codebase.

type Data = {
    name: string;
    age: number;

export default function handler(
    req: NextApiRequest, 
    res: NextApiResponse<Data>
) {
    res.status(200).json({ name: 'John Doe', age: 30 }); // Response adheres to the Data type

Performance considerations also come into play when heavily utilizing typed server-side rendering. Extraneous data fetching or complex data transformations in getStaticProps or getServerSideProps can lead to increased response times. Therefore, it's essential to keep these functions lean and focused only on what's necessary for rendering, offloading non-essential operations to the client-side or background processes.

In the context of API routes, performance typically relies on the efficiency of the data layer interaction. However, the use of TypeScript adds a negligible overhead since the type checking occurs at build time and does not impact the runtime execution. Thus, the primary concerns are the actual logic within the API routes and the potential latency when interfacing with databases or external services.

When integrating types into your server-side logic, common mistakes include assigning incorrect types that don't reflect the actual data structure or neglecting to type the return objects of API handlers. These can lead to unforeseen type errors down the road when the application scales in complexity. A diligent approach to typing, alongside incremental type checking, ensures the robustness of your server-side code as the application evolves.

Are the types in your server-side functions leading you down a path of maintainability and safety, or do they require augmentation to better capture the nuances of your application's data contract? Reflect on the tightness of your type usage—could it be a double-edged sword where overly strict typing hinders development, or is it the safety net that catches potential pitfalls before they become runtime issues?

Optimizing Frontend and Backend Interactions

When developing a full-stack application using Next.js and TypeScript, optimizing frontend and backend interactions is paramount. The static typing provided by TypeScript forms contracts between the frontend interfaces and the backend logic. To streamline data fetching and display, developers should leverage shared TypeScript interfaces. These not only clarify the data structure but also ensure that the components receive the right data shape, a practice that minimizes runtime errors and simplifies debugging.

In Next.js, API routes serve as the conduit for frontend and backend communication. Defining the input and output types precisely for these routes creates a robust boundary. For instance, when using fetch() to retrieve server-side data, creating and implementing an interface for the expected response ensures that the frontend can handle the data correctly and with full awareness of its structure. This approach eliminates the guesswork and enables developers to build more predictive UI components.

Type assertions at the API boundaries further solidify the interaction between the two ends. By enforcing types on the requests sent to the backend and the responses received, you ensure consistency and reliability in the data flow. This can be achieved through utility functions that act as gatekeepers, validating and transforming data to conform to predefined interfaces before consumption by the frontend.

Building on the idea of type contracts, the use of TypeScript enables developers to enforce a certain level of discipline across asynchronous operations. Async and Await in conjunction with typed promises make handling fetched data more intuitive. For example, when you declare a function async, you can also specify the return type as a Promise of a specific interface, ensuring that the resolved value is of the expected type. This adds a layer of predictability and makes asynchronous data handling in React components more robust and less prone to errors.

Lastly, developers should also focus on creating reusable data-fetching hooks that are agnostic to the components consuming them. By abstracting the data-fetching logic and ensuring it returns a specific interface or set of interfaces, frontend components can reuse these hooks with confidence, knowing exactly what type of data they will receive. This not only aids in modularity and reusability but also upholds the integrity of type contracts throughout the application, leading to more scalable and maintainable codebases.

Advanced TypeScript Patterns in Next.js

In leveraging the full potential of TypeScript in a Next.js project, conditional types offer exquisite power for defining types that can vary under specific conditions. Consider a scenario where a Next.js page should render different types of content based on the data received from an API. Conditional types can articulate this requirement gracefully:

type ContentData = {
  type: 'article' | 'video';
  content: string;

type ConditionalContentType<T> = 
  T extends { type: 'article' }
  ? { articleContent: string }
  : T extends { type: 'video' }
  ? { videoUrl: string }
  : never;

function renderContent<T extends ContentData>(data: T): ConditionalContentType<T> {
  if (data.type === 'article') {
    return { articleContent: data.content }; 
  } else {
    return { videoUrl: data.content }; 

Here, the ConditionalContentType type adapts based on the input 'type', ensuring our renderContent function returns a properly shaped object. Yet, one must consider the balance between the expressiveness of conditional types and the risk of introducing a complex type hierarchy that might be hard to debug or understand—would a simpler type suffice, or is the complexity justified by the use case?

Mapped types are another powerful tool, particularly when dealing with transformations of existing types. For instance, when creating an API that should respond only with readable fields, thereby omitting sensitive data like passwords:

type User = {
  id: number;
  name: string;
  password: string;

type ReadOnlyUser = {
  readonly [P in keyof User]: User[P];

function getUserData(): ReadOnlyUser {
  const user: User = {
    id: 1, 
    name: 'John Doe', 
    password: 'secret'
  // Perform operations to strip sensitive fields
  delete user.password;

  return user as ReadOnlyUser;

We've crafted a ReadOnlyUser that safely exposes user data without allowing changes to the object's properties, but consider the implications—does it add unnecessary complexity, or does it increase the codebase's robustness?

Utility types, such as Partial, Readonly, and Record, are indispensable for reducing repetition and making common type transformations succinct. For example, to modify a subset of an object's properties safely, without mutating the original object, you could use the Partial utility:

type UserUpdate = Partial<User>;

function updateUser(id: number, updates: UserUpdate): User {
  let user = getUserById(id);
  return { ...user, ...updates };

Using a Partial type here provides flexibility, but when is this approach overkill? Have you evaluated if making the entire object properties optional might lead to logic errors?

Lastly, let's confront a common coding mistake: the misuse of any, a type that can lead to unchecked runtime errors. The correct approach is to use a type assertion or generic type when the type is unknown or could change:

// Incorrect: Reliance on 'any' type, which circumvents TypeScript's checks
function getItem(key: string): any {
  return JSON.parse(localStorage.getItem(key));

// Correct: Using a generic to provide a type to the function at the call site
function getItem<T>(key: string): T {
  return JSON.parse(localStorage.getItem(key)) as T;

Ponder on when to use type assertions wisely, ensuring they don't undermine TypeScript's type safety. Are you using them as a shortcut to bypass complex types, or genuinely to inform TypeScript of something you know that it can’t infer?


In this article about developing with TypeScript in Next.js 14, the writer explores the benefits of incorporating TypeScript into web development projects. They discuss how TypeScript enhances Next.js applications through type safety, improved project configuration and structure, and optimized frontend-backend interactions. The article also highlights advanced TypeScript patterns and offers a challenging task for readers to reflect on their own use of types in their server-side functions. The task encourages readers to evaluate the tightness of their type usage and consider whether it hinders development or acts as a safety net.

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