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Next.js

NextJS App Types: Component & pageProps Typescript

By Filip on 10/05/2024

Learn how to correctly type the Component and pageProps in your Next.js _app.tsx file for improved code clarity and type safety.

NextJS App Types: Component & pageProps Typescript

Table of Contents

Introduction

This article will guide you through understanding and properly typing the _app.tsx file in Next.js applications that use TypeScript. We'll cover the basic structure and typing, customizing props, handling getInitialProps, and additional considerations for layout components and third-party libraries. By implementing these practices, you'll ensure type safety, catch potential errors early, and improve code readability and maintainability in your Next.js projects.

Step-by-Step Guide

The _app.tsx file in Next.js serves as the entry point for your application, rendering every page and providing a consistent layout. When using TypeScript, properly typing this component and its props is crucial for ensuring type safety and catching potential errors early. Let's explore the steps involved:

1. Basic Structure and Typing:

  • Create _app.tsx: If it doesn't exist, create the _app.tsx file in your project's root directory.
  • Import necessary components: Import React and the AppProps type from Next.js.
import React from 'react';
import type { AppProps } from 'next/app';

function MyApp({ Component, pageProps }: AppProps) {
  return <Component {...pageProps} />;
}

export default MyApp;
  • Define the MyApp component: This component receives two props:
    • Component: The active page component being rendered.
    • pageProps: An object containing props for the page component.
  • Render the page component: Use the Component prop to render the active page and pass pageProps to it.

2. Customizing AppProps:

  • Create a custom type: If your pages require additional props beyond the default AppProps, define a custom type that extends it.
type CustomAppProps = AppProps & {
  myCustomProp: string;
};
  • Update the MyApp component: Modify the type annotation for the props to use your custom type.
function MyApp({ Component, pageProps, myCustomProp }: CustomAppProps) {
  // ...
}

3. Handling getInitialProps (if used):

  • Define return type: If your _app.tsx uses getInitialProps to fetch data, specify the return type for the function.
MyApp.getInitialProps = async (appContext) => {
  // ... your logic
  return {
    pageProps: {
      // ...
    },
    myCustomProp: 'value',
  };
};
  • Ensure type consistency: Make sure the returned object matches the structure of your custom AppProps type.

4. Additional Considerations:

  • Layout components: If you use layout components, ensure they are also properly typed and receive the necessary props.
  • Third-party libraries: When integrating libraries, check for available type definitions or create your own to maintain type safety.

Benefits of Typing:

  • Early error detection: TypeScript helps catch type errors during development, preventing runtime issues.
  • Improved code readability: Explicit types make code easier to understand and maintain.
  • Enhanced IDE support: IDEs can provide better code completion and refactoring suggestions with type information.

By following these steps and understanding the typing mechanisms, you can ensure a robust and type-safe Next.js application with TypeScript.

Code Example

This JavaScript code defines a basic custom App component for a Next.js application. It demonstrates how to render pages and fetch data using getInitialProps. The code lacks type annotations and type safety compared to a TypeScript version but maintains similar functionality.

While the provided article focuses on using TypeScript with _app.tsx, here's how you can achieve a similar structure and functionality using plain JavaScript in _app.js:

import React from 'react';

function MyApp({ Component, pageProps }) {
  return <Component {...pageProps} />;
}

MyApp.getInitialProps = async (appContext) => {
  // Perform any data fetching or logic here
  const appProps = await App.getInitialProps(appContext);

  return { ...appProps };
};

export default MyApp;

Explanation:

  1. Import React: We import the React library to work with JSX and components.

  2. Define MyApp Component:

    • This functional component receives Component and pageProps as props.
    • Component represents the active page component to be rendered.
    • pageProps is an object containing any props for the page component.
    • The component simply renders the Component with the provided pageProps.

  3. getInitialProps (Optional):

    • This static method is used for data fetching or any logic that needs to be executed before the page renders.
    • It receives an appContext object with information about the request and application.
    • Here, we demonstrate fetching initial props from the App component (if it has any) and returning them along with any additional data.

Key Differences from TypeScript Version:

  • No Type Annotations: JavaScript doesn't require explicit type definitions, so we don't have AppProps or custom types.
  • Type Safety: Without TypeScript, we lose the benefits of static type checking and potential error detection during development.

Additional Considerations:

  • Prop Validation: You can use libraries like prop-types to add runtime type checking and validation for props.
  • Code Clarity: While not enforced, maintaining consistent naming conventions and code structure can improve readability.

In conclusion, while using TypeScript provides advantages in terms of type safety and code clarity, you can still achieve a similar structure and functionality in your Next.js application using plain JavaScript.

Additional Notes

Advanced Typing Techniques:

  • Generics: Utilize generics to create reusable components that can work with different data types. This is particularly useful for layout components or wrappers that need to adapt to various page props.
  • Utility Types: Leverage TypeScript's built-in utility types like Partial, Pick, and Omit to create more specific and flexible type definitions.
  • Conditional Types: Employ conditional types to define types that change based on certain conditions, allowing for dynamic type behavior.

Error Handling and Debugging:

  • TypeScript Compiler Errors: Pay close attention to TypeScript compiler errors, as they often point to type mismatches or potential issues in your code.
  • IDE Support: Take advantage of your IDE's TypeScript support for features like type checking, code completion, and refactoring.
  • Runtime Type Checks: Consider using libraries like io-ts or runtypes for runtime type validation, especially when dealing with data from external sources.

Best Practices:

  • Start with Basic Types: Begin by defining basic types for your props and gradually add complexity as needed.
  • Use Interfaces for Props: Prefer interfaces over type aliases for defining props, as they offer better readability and extensibility.
  • Document Your Types: Provide clear comments and documentation for your custom types to improve code understanding and maintainability.
  • Keep Types Consistent: Ensure consistency in your type definitions throughout your application to avoid confusion and errors.

Additional Resources:

  • Next.js TypeScript Documentation: Refer to the official Next.js documentation for detailed guidance on using TypeScript with Next.js.
  • TypeScript Handbook: Explore the TypeScript Handbook for comprehensive information on TypeScript features and best practices.
  • Community Resources: Engage with the Next.js and TypeScript communities for support, discussions, and solutions to common challenges.

By incorporating these additional notes and exploring advanced typing techniques, you can further enhance the type safety, maintainability, and overall quality of your Next.js applications built with TypeScript.

Summary

Step Description
1 Basic Structure and Typing: Create _app.tsx, import necessary components, define MyApp component to render pages.
2 Customizing AppProps: Extend AppProps with a custom type if needed and update MyApp component accordingly.
3 Handling getInitialProps: Define return type for getInitialProps if used, ensuring consistency with AppProps.
4 Additional Considerations: Ensure proper typing for layout components and third-party libraries.

Benefits of Typing:

  • Early error detection
  • Improved code readability
  • Enhanced IDE support

Conclusion

By understanding and effectively typing the _app.tsx file, you unlock the full potential of TypeScript within your Next.js projects. The benefits extend beyond simple type checking, leading to more robust, maintainable, and error-free applications. Remember, the key lies in defining clear types, leveraging TypeScript features, and adhering to best practices. With these tools at your disposal, you'll be well-equipped to build exceptional Next.js applications that are both powerful and reliable.

References

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