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Senior Software Engineer
Last updated onJun 3, 2024
Last updated onMay 10, 2024
Next.js has emerged as a leading framework for building server-rendered React applications. Its server-side rendering and static site generation make it a versatile choice for developers aiming to deliver high-performance web applications. With Next.js, you can create scalable projects that cater to a global audience, thanks to features like automatic code splitting, optimized prefetching, and the support for edge server bundles.
When optimizing your Next.js app, understanding and refining your bundle size is crucial. A bundle is a collection of js files, including your app's code and its dependencies, packaged together by a module bundler like Webpack. Large bundles can lead to slow load times, which negatively impact user experience and SEO rankings. By using a bundle analyzer, you can visualize the contents of your js bundle, identify large dependencies, and detect unused code that contributes to bloat.
In this post, we will guide you through the process of analyzing your Next.js bundles using the next bundle analyzer and other tools. We'll cover how to set up and interpret the results from the next bundle analyzer, how to manage environment variables to create different analysis scenarios, and how to optimize your Next.js bundle for better performance.
By the end of this post, you'll be equipped with the knowledge to effectively analyze and optimize your Next.js app bundles, ensuring your app remains fast and efficient.
Next.js uses Webpack under the hood to create bundles for your application. Bundling is the process of merging and optimizing multiple JS files and modules into single or several JS bundle files. This is done to reduce the number of HTTP requests needed to load all the necessary code for your app, which can significantly improve load times and performance.
When you build your Next.js app using the next build command, Next.js performs several steps to prepare your app for production. It compiles your React components and any imported modules into JavaScript code, optimizes the code for performance, and splits it into various bundles. These bundles include:
Main bundle: Contains Webpack runtime and the code for bootstrapping your app.
Pages bundles: Each page in your Next.js project gets its own js bundle, ensuring that only the necessary code is loaded for each page.
Commons chunk: Includes common modules shared between multiple pages to avoid duplication.
Vendor bundle: Contains third-party npm packages that are used across your app.
Webpack is a powerful module bundler that Next.js leverages to manage and optimize the assets and modules of your project. It takes care of tasks such as:
Resolving module dependencies.
Transforming assets like CSS and images.
Minifying and uglifying the code to reduce js size.
Implementing code splitting and lazy loading.
Webpack's role is central to the performance of a Next.js app, as it determines how the final js bundle or bundles will be structured and how efficiently they can be served to the client.
Next.js creates several types of bundles to optimize the delivery and execution of your app:
Main bundle: This core JS bundle includes Webpack's runtime and the logic to load other bundles dynamically.
Chunks: These are smaller js bundle files created by splitting the main bundle. They can be loaded on demand, which is useful for code splitting.
Modules: Individual code pieces that represent your app's functionality or components. They are included in bundles based on the pages or features that require them.
Browser bundle: Specifically optimized to run in the user's browser, containing only the client-side code.
Server bundle: Contains the server-side code for server-rendered pages, including the logic for initial rendering and API routes.
Edge server bundle: A specialized server bundle designed to run on edge servers for improved performance and lower latency.
One of the primary reasons to analyze your Next.js bundles is to identify large dependencies that may be bloating your app's size. Large npm packages can significantly increase the js size of your bundles, leading to longer download times and slower app performance. Using a next bundle analyzer, you can get a clear view of which packages or modules take up the most space and consider alternatives or optimizations to reduce their impact.
Duplicate code across different bundles can lead to unnecessary bloat and increased js bundle sizes. This often happens when multiple pages or components import the same module independently. A bundle analyzer can help you detect these duplications by providing a visual report of your app's bundle composition. Once identified, you can refactor your code or adjust your Webpack config to ensure that common code is shared efficiently between bundles.
The size of your JS bundle directly affects the load times of your Next.js app. Large bundles take longer to download and parse, which can lead to a poor user experience, especially on mobile devices or slow network connections. By analyzing and optimizing your bundles, you can improve load times, resulting in a snappier, more responsive app. This not only delights users but also contributes positively to SEO, as search engines favor fast-loading sites.
Regular bundle analysis should be an integral part of your development process. It enables you to maintain a high-performance app over time by catching size regressions and performance bottlenecks early. Strategies such as setting performance budgets, monitoring the impact of adding new dependencies, and refactoring code to remove unused code or split large bundles can all be informed by the insights gained from bundle analysis.
Next.js offers a built-in bundle analyzer that integrates seamlessly with your Next.js project. This tool, powered by the popular Webpack Bundle Analyzer, provides a visual representation of the size of your webpack output files with an interactive treemap.
The Webpack Bundle Analyzer is a standalone plugin that can be used with any Webpack project, including Next.js apps. It gives you a detailed breakdown of what's inside your webpack bundles, making it easier to understand where code bloat is coming from.
Source-map-explorer analyzes space usage through source maps, which allows you to understand exactly what code is being used and how it is being bundled. It can be particularly useful for pinpointing unexpected code or third-party code that has been included in your bundles.
Next.js Built-in Bundle Analyzer: Best for Next.js developers who want a quick and easy setup without leaving the Next.js ecosystem. It provides a straightforward visualization of your bundles, helping you to identify large modules and chunks.
Webpack Bundle Analyzer: Offers more detailed insights and customization options. It's a good choice for developers who want to dive deeper into their webpack bundles and may already be familiar with Webpack's configuration.
Source-map-explorer: Unlike the other two, which visualize the webpack output, source-map-explorer works with source maps to show you a more accurate representation of your source code before Webpack transforms it. This can be useful for analyzing the original size of your code and how it contributes to the overall bundle size.
1npm install @next/bundle-analyzer --save-dev
1// next.config.js 2const withBundleAnalyzer = require('@next/bundle-analyzer')({ 3 enabled: process.env.ANALYZE === 'true', 4}); 5module.exports = withBundleAnalyzer({});
1"scripts": { 2 "dev": "next dev", 3 "build": "next build", 4 "start": "next start", 5 "analyze": "ANALYZE=true next build" 6}
1npm run analyze
This command will produce a visual report that opens automatically in your default browser.
When the Next.js bundle analyzer completes its process, it will generate a visual report that displays each bundle in a treemap format. Here's how to interpret the results:
Large Rectangles: Each rectangle represents a module or package, with its size proportional to the overall bundle size. Larger rectangles indicate modules that take up more space.
Colors: The colors often group similar types or related files together, making it easier to see which parts of your app are contributing the most to the bundle size.
Hovering: You can hover over individual rectangles to see more information about each module, such as its name and size.
Clicking: Clicking on a rectangle will zoom in for a more detailed view, which can be helpful for examining nested modules.
Look for Unexpectedly Large Modules: If a module is larger than you expect, it might be including unnecessary code or dependencies. Investigate whether you can import only the parts you need or replace them with a lighter alternative.
Check for Duplicates: If you see the same module repeated across different bundles, it may indicate an issue with your code-splitting strategy. Consider refactoring to ensure that common dependencies are shared effectively.
Analyze Third-party Packages: Third-party npm packages can sometimes be larger than anticipated. Look for any packages that dominate the treemap and research if smaller alternatives exist.
Review Dynamic Imports: Ensure that dynamic imports are being used effectively to split your code into manageable chunks that are loaded on demand.
Monitor Over Time: Regularly run the next bundle analyzer as part of your development process to monitor changes in bundle size and catch any regressions.
1npm install --save-dev webpack-bundle-analyzer
1// next.config.js 2const { BundleAnalyzerPlugin } = require('webpack-bundle-analyzer'); 3 4module.exports = { 5 webpack(config, { isServer }) { 6 if (process.env.ANALYZE) { 7 config.plugins.push(new BundleAnalyzerPlugin({ 8 analyzerMode: 'static', 9 reportFilename: isServer ? '../analyze/server.html' : './analyze/client.html', 10 openAnalyzer: false 11 })); 12 } 13 return config; 14 } 15};
In this configuration, the analyzerMode is set to 'static', which will generate static report files. The reportFilename option specifies where to output the report files, and openAnalyzer is set to false to prevent the report from automatically opening in the browser.
1"scripts": { 2 "dev": "next dev", 3 "build": "next build", 4 "start": "next start", 5 "analyze": "ANALYZE=true next build" 6}
1npm run analyze
After the build completes, you'll find the generated reports in the specified locations within the analyze folder.
Customize Report Filenames: You can change the reportFilename option to organize your reports differently or to avoid overwriting previous reports.
Change Analyzer Mode: The analyzerMode option can be set to 'server', 'static', or 'disabled'. Choose 'server' if you want to start a local web server to inspect the report immediately after the build.
Filter Modules: You can use the excludeAssets option to filter out certain assets from the report, which can be useful if you're only interested in analyzing specific parts of your bundle.
When you run the Webpack Bundle Analyzer, it generates a visual, interactive treemap that represents the contents of your webpack bundles. Here's how to navigate and understand the visualization:
Modules and Sizes: Each block in the treemap represents a module or package. The size of the block correlates with the size of the module in your bundle.
Grouping: Modules are grouped by their path or chunk, making it easier to see which parts of your app or third-party packages are contributing the most to the bundle size.
Hover Details: Hovering over a block will show you the module's name and size, giving you immediate insight into its impact on your bundle.
Zoom In/Out: Click on a block to zoom in for a more detailed view of that module's contents, or use the provided controls to zoom out and get an overview of your entire bundle.
Source maps are files that map the transformed, bundled, or minified code back to its original source. They are crucial for debugging because they allow developers to see their original code instead of the transformed code that the browser executes. In the context of performance optimization, source maps are equally important because they provide a way to analyze the size and contents of the code as it was written, rather than the potentially obfuscated or minified production code.
Ensure Source Maps Are Generated First, you need to make sure that your Next.js build process is generating source maps. By default, Next.js should produce source maps for production builds.
Install Source-map-explorer Install the source-map-explorer package as a development dependency in your project.
1npm install --save-dev source-map-explorer
1npx source-map-explorer .next/static/chunks/pages/*.js
This command will analyze the source maps for your page bundles. You can adjust the path to target different sets of bundles as needed.
When you run source-map-explorer, it generates a visualization that displays your code in a treemap format, similar to the Webpack Bundle Analyzer. Here's how to interpret the visualization:
Colored Areas: Each colored area in the treemap represents a different file or module from your source code. The size of the area is proportional to the file's size in the bundle.
Labels: The visualization includes labels that indicate the names of files or modules, along with their sizes. This helps you identify specific items in your codebase.
Hover for Details: Hovering over an area will give you more detailed information, such as the full path of the file and its exact size.
Zoom Functionality: Click on an area to zoom in and see more granular details about that part of your code. This can be particularly helpful for large projects with many nested modules.
In this post, we've explored the importance of analyzing and optimizing your Next.js app bundles. We've covered a range of tools, from the Next.js built-in bundle analyzer to the Webpack Bundle Analyzer and Source-map-explorer, each offering unique insights into your app's performance. By understanding how to configure and interpret these tools, you can identify large dependencies, detect duplicate code, and uncover opportunities to improve load times and user experience.
We've also discussed practical strategies for reducing bundle size, such as implementing code splitting with dynamic imports, removing or replacing bulky packages, and leveraging tree shaking to eliminate unused code. Additionally, we provided tips for using lighter alternatives for heavy libraries and lazy loading components and images.
Optimizing your Next.js bundles is an ongoing process that requires regular attention and maintenance. As your app grows and evolves, new challenges in performance optimization may arise. By incorporating bundle analysis into your development workflow and staying informed about best practices, you can ensure that your app remains fast, efficient, and enjoyable for your users.
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