Complete Next.js Interview Guide
Questions
What is the `_app.js` file and what is it used for in Next.js?
The Scenario
You are a frontend engineer at a social media company. You are building a new Next.js application and you want to add a global stylesheet that will be applied to the entire application.
You are not sure where to put the global stylesheet.
The Challenge
Explain what the _app.js file is in Next.js and how you would use it to solve this problem. What are the key benefits of using the _app.js file?
A junior engineer might not be aware of the `_app.js` file. They might try to solve this problem by adding a `<link>` tag to each page, which would not be a very good solution.
A senior engineer would know that the `_app.js` file is the perfect tool for this job. They would be able to explain what the `_app.js` file is and how to use it to add a global stylesheet to the application.
Step 1: Understand What the _app.js File Is
The pages/_app.js file is a special file in Next.js that allows you to override the default App component.
Step 2: The Key Use Cases for the _app.js File
| Use Case | Description |
|---|---|
| Global Styles | To add a global stylesheet that will be applied to the entire application. |
| Layouts | To create a layout that will be shared across all the pages in the application. |
| State Management | To wrap the application in a state management library, such as Redux or MobX. |
| Error Handling | To add a global error boundary to the application. |
Step 3: Solve the Problem
Here’s how we can use the _app.js file to add a global stylesheet to our application:
// pages/_app.js
import '../styles.css'
export default function MyApp({ Component, pageProps }) {
return <Component {...pageProps} />
}In this example, we import the global stylesheet in the _app.js file. This will cause the stylesheet to be applied to every page in the application.
Practice Question
You want to create a layout that will be shared across all the pages in your application. Which of the following would you use?