Reducing Next.js Bundle Size with Next.js Bundle Analyzer
Bundle size is one of the most important factors affecting the performance of a Next.js application. Large bundles can lead to slow load times, increased Time-to-Interactive (TTI), and ultimately, poor user experiences. Fortunately, Next.js provides a tool known as the Next.js Bundle Analyzer, which helps developers identify and eliminate unnecessary code, making it easier to optimize the bundle size.
In this article, we'll explore how to use the Next.js Bundle Analyzer to reduce your bundle size and discuss additional optimization strategies to ensure your Next.js app runs as efficiently as possible.
Setting Up Next.js Bundle Analyzer
To start optimizing your Next.js bundle size, you first need to install and configure the Next.js Bundle Analyzer.
Installation
You can install the analyzer by running the following command in your Next.js project:
npm install @next/bundle-analyzer
After installing, you'll need to modify your next.config.js file to enable the bundle analyzer. Here's how you can configure it:
// next.config.js
const withBundleAnalyzer = require('@next/bundle-analyzer')({
enabled: process.env.ANALYZE === 'true',
});
module.exports = withBundleAnalyzer({
// Your Next.js configuration goes here
});
By setting ANALYZE to true, you can conditionally enable the bundle analyzer. This is useful for running the analyzer only during specific builds, such as development or production.
To generate the bundle analysis, run the following command:
ANALYZE=true npm run build
Once the build completes, you’ll have access to a visual report that shows the size of each module in your bundles.
Understanding the Output
The Next.js Bundle Analyzer provides a graphical representation of your JavaScript bundles. Each module is represented as a rectangle, where the size of the rectangle corresponds to the module’s size in your bundle. Larger rectangles represent heavier modules, which take up more space and time to load.
When reviewing the output, focus on the following key areas:
- Vendor Code: Check if third-party libraries, like react or lodash, are excessively large.
- Common Code: Ensure that commonly used code is being shared effectively between pages.
- Duplicate Code: Look out for duplicate code that appears in multiple bundles.
By identifying and targeting the largest chunks of your bundle, you can reduce overall size and improve load times.
Reducing Bundle Size
Once you’ve identified large or redundant modules in your bundle, there are several strategies you can employ to reduce the size of your Next.js bundle.
1. Tree Shaking
Tree shaking is a technique that removes unused code from your bundle, ensuring only the necessary parts of libraries are included. Next.js supports tree shaking out of the box, but there are additional ways to ensure optimal results:
Avoid full library imports: For example, importing the entire lodash library can unnecessarily increase your bundle size. Instead, import only the functions you need:
Bad Example:
import _ from 'lodash';
const filtered = _.filter(array, predicate);
Optimized Example:
import filter from 'lodash/filter';
const filtered = filter(array, predicate);
Use ES modules: When using third-party libraries, always prefer ES module imports (when available), as they are more tree-shakeable.
2. Code Splitting
Code splitting ensures that only the necessary JavaScript is loaded when a user visits a page. Next.js automatically splits your code by page, but you can manually split code further using dynamic imports:
import dynamic from 'next/dynamic';
const DynamicComponent = dynamic(() => import('../components/HeavyComponent'), {
ssr: false, // Disable server-side rendering for this component
});
function Home() {
return <DynamicComponent />;
}
export default Home;
By dynamically importing heavier components only when needed, you can reduce the initial load time and improve performance.
3. Bundle External Dependencies
Another useful optimization is moving heavy third-party libraries, such as moment.js or chart.js, to a CDN. This reduces the size of your JavaScript bundles since the library will be loaded from the CDN instead.
For example, you can include chart.js from a CDN by adding this to your Next.js _document.js file:
import { Html, Head, Main, NextScript } from 'next/document';
function MyDocument() {
return (
<Html>
<Head>
<script
src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.5.0/chart.min.js"
integrity="sha384-9SFbYBjntbXQ34S5hc4FymRB7D3w4v6r3SxWxbLGCmvnBYy+Z5hFdYG/h9jLMczT"
crossOrigin="anonymous"
></script>
</Head>
<body>
<Main />
<NextScript />
</body>
</Html>
);
}
export default MyDocument;
By loading libraries via a CDN, you free up space in your bundle, further improving performance.
Optimizing Images and Fonts
Beyond JavaScript, large images and fonts can significantly inflate your bundle size. Next.js has built-in support for optimizing images and fonts, which can be further leveraged to reduce bundle size.
Image Optimization
Make sure you’re using the next/image component, which automatically optimizes images on-demand.
import Image from 'next/image';
function HomePage() {
return (
<div>
<Image
src="/static/large-image.jpg"
alt="Optimized Image"
width={800}
height={600}
/>
</div>
);
}
export default HomePage;
By doing this, Next.js will optimize the image to deliver the best format (e.g., WebP), size, and quality based on the user’s device and screen size.
Font Optimization
Using custom fonts can also increase your bundle size. To minimize this, Next.js automatically optimizes Google Fonts. If you’re using other fonts, make sure to load them asynchronously or preload them for better performance.
// next.config.js
module.exports = {
experimental: {
optimizeFonts: true,
},
};
This will ensure your fonts are served in an optimized format, reducing their impact on performance.
Server-Side Optimizations: Minimizing Server Response Time
Besides focusing on the client-side bundle size, you also need to optimize the server-side response time. If your server is slow to deliver the necessary JavaScript files, it will degrade performance regardless of bundle size.
Leveraging HTTP/2 and Compression
Use HTTP/2 and Brotli compression to minimize the size of files sent from the server to the client. This reduces load times significantly, especially for larger JavaScript bundles.
Here’s how to enable Brotli compression in your Next.js app (same as before but mentioned in context):
const express = require('express');
const compression = require('compression');
const next = require('next');
const app = next({ dev: false });
const handle = app.getRequestHandler();
const server = express();
server.use(compression());
server.all('*', (req, res) => handle(req, res));
server.listen(3000, () => {
console.log('> Server running on http://localhost:3000');
});
By enabling compression, you ensure that even large assets are delivered as efficiently as possible, improving load times for your users.
Conclusion
Reducing the bundle size in a Next.js app is a crucial step in improving website performance, load times, and user experience. With the help of tools like Next.js Bundle Analyzer, tree shaking, code splitting, and other techniques discussed, you can dramatically optimize your application.
By leveraging Next.js’s built-in features and adopting external optimizations, your site will not only perform better but will also provide a smoother experience for users, leading to better engagement and, ultimately, more revenue.
Catch Metrics is the leading solution to help solve ad stack performance problems.Get in touchto learn more from our experts