Bundle Analysis & Code Splitting

Q: What is code splitting in Next.js and why does it matter for performance?

Code splitting breaks your JavaScript into smaller chunks that load only when they are needed, instead of sending the entire application code to the browser on the first page load. This makes your app start faster because users only download the code for the page they are actually viewing.

Q: How do you manually split a large component using dynamic imports in Next.js?

The next/dynamic function lets you import a component only when it is actually needed, such as when a modal opens, keeping that component's code out of the main bundle until the moment it is required, which reduces the initial page weight.

Q: How do you analyze your Next.js application's bundle size to find what is making it large?

You can use the official @next/bundle-analyzer package, which generates a visual report showing exactly which packages and files take up the most space in your JavaScript bundles, making it easy to spot unexpectedly large dependencies that could be replaced or lazy loaded.

Q: What strategies can reduce bundle size beyond basic code splitting?

You can remove unused dependencies, replace large libraries with smaller focused alternatives, use tree shaking friendly imports that only pull in the specific functions you need, and avoid importing entire icon or utility libraries when only a few items are actually used.

Q: How does route based code splitting interact with shared layouts in the App Router?

Layouts that wrap multiple routes are loaded once and reused across navigations within that layout, while each individual page's specific code is fetched separately when the user navigates to it, so shared navigation or sidebar code does not need to reload on every page change.

5 questions found

What is code splitting in Next.js and why does it matter for performance?

Beginner

Code splitting breaks your JavaScript into smaller chunks that load only when they are needed, instead of sending the entire application code to the browser on the first page load. This makes your app start faster because users only download the code for the page they are actually viewing.

// Next.js automatically splits code by route
// pages/about.js only loads when a user visits /about

Real-world example A large e-commerce site loads only the code needed for the homepage first, and loads the checkout page code separately only when a user actually starts the checkout process, keeping the initial load fast.

Common follow-ups: Does Next.js do code splitting automatically or do you need to configure it?;What is the difference between route based splitting and component based splitting?

Performance & Lazy Loading;Turbopack & Build Performance

How do you manually split a large component using dynamic imports in Next.js?

Intermediate

The next/dynamic function lets you import a component only when it is actually needed, such as when a modal opens, keeping that component's code out of the main bundle until the moment it is required, which reduces the initial page weight.

import dynamic from 'next/dynamic';

const HeavyChart = dynamic(() => import('../components/HeavyChart'), {
  loading: () => <p>Loading chart...</p>
});

Real-world example A dashboard page loads a heavy charting library only when the user clicks a button to view analytics, keeping the initial dashboard load fast for users who never open the charts.

Common follow-ups: Can you disable server side rendering for a dynamically imported component?;How do you show a custom loading state while a dynamic component loads?

Rendering (SSR/SSG/ISR);Performance & Lazy Loading

How do you analyze your Next.js application's bundle size to find what is making it large?

Intermediate

You can use the official @next/bundle-analyzer package, which generates a visual report showing exactly which packages and files take up the most space in your JavaScript bundles, making it easy to spot unexpectedly large dependencies that could be replaced or lazy loaded.

// next.config.js
const withBundleAnalyzer = require('@next/bundle-analyzer')({
  enabled: process.env.ANALYZE === 'true'
});

module.exports = withBundleAnalyzer({});

Real-world example A team runs the bundle analyzer and discovers that a date formatting library is adding a large amount of unused code, so they switch to a smaller alternative and shrink their bundle significantly.

Common follow-ups: What command do you run to generate the bundle analysis report?;How often should a team check their bundle size during development?

Turbopack & Build Performance;Web Vitals & Performance Monitoring

What strategies can reduce bundle size beyond basic code splitting?

Advanced

You can remove unused dependencies, replace large libraries with smaller focused alternatives, use tree shaking friendly imports that only pull in the specific functions you need, and avoid importing entire icon or utility libraries when only a few items are actually used.

// Instead of importing the whole library
import _ from 'lodash';

// Import only what you need
import debounce from 'lodash/debounce';

Real-world example A team switches from importing an entire icon library to importing only the specific icons they use, cutting several hundred kilobytes from their production bundle.

Common follow-ups: How do you know if a library supports tree shaking?;What tools help automatically detect unused code in a project?

Turbopack & Build Performance;Third-Party Script Optimization

How does route based code splitting interact with shared layouts in the App Router?

Advanced

Layouts that wrap multiple routes are loaded once and reused across navigations within that layout, while each individual page's specific code is fetched separately when the user navigates to it, so shared navigation or sidebar code does not need to reload on every page change.

// app/dashboard/layout.js loads once
// app/dashboard/page.js and app/dashboard/settings/page.js
// load separately as the user navigates between them

Real-world example A dashboard with a persistent sidebar keeps that sidebar loaded across every dashboard page the user visits, while only fetching the new page specific content, making navigation feel instant.

Common follow-ups: Does the layout re-render every time you navigate to a new nested page?;How does this affect the amount of data fetched on each navigation?

Layouts & Templates;Routing (App/Pages Router)