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How to analyze Next.js app bundles

Posted on: Jul 25, 2023

12 mins read

Originally published at blog.logrocket.com

How to analyze your Next.js app bundles

One crucial aspect of any web application is the size and performance of its codebase. Next.js is a popular JavaScript framework for building server-rendered and statically-exported React applications.

Next.js provides the ability to automatically optimize an application's code and minimize its bundle size. This can reduce the amount of data that needs to be transferred to the client, thereby improving performance. However, regularly analyzing your Next.js app's bundle can help ensure that it is as optimized as possible and can also help identify potential areas for improvement.

In this article, we'll discuss what a Next.js app bundle is and how bundle analyzer tools work. We'll demonstrate how to use the @next/bundle analyzer package to inspect your Next.js app bundles and talk about a few alternatives. Lastly, we'll discuss how to use bundle analysis to improve Next.js app performance.

What is a Next.js app bundle?

When you build a Next.js app, the code gets organized into small units called modules. These modules are bundled together by webpack, a popular JavaScript module bundler, to form a single file called a bundle. This bundle is then served to the client's browser, which is executed to render the application.

As web applications become more complex and feature-rich, the size of their codebase can grow significantly. This can lead to slower loading times, which can frustrate users and negatively impact your app's performance.

Additionally, larger codebases are often more challenging to maintain, making it harder to identify and fix bugs. By analyzing your app bundles, you can identify areas where you can optimize your code to reduce its size and improve its performance.

Several bundlers exist, but this article will focus on using one of the most popular bundling tools, the webpack-based @next/bundle-analyzer package.

What is @next/bundle-analyzer?

The @next/bundle-analyzer package is a plugin for the Next.js framework that allows you to analyze the size and composition of your app's bundle. By identifying large or unnecessary code blocks, you can reduce overall bundle size, which can help optimize the performance of your Next.js app.

The bundle-analyzer plugin generates a visual report that provides information about the size and contents of your app bundle, making it easier to identify areas for optimization. @next/bundle-analyzer generates a static HTML file that contains a detailed report on your app's codebase, including information on the size of each component and its dependencies.

Installing the @next/bundle-analyzer package

To use the @next/bundle-analyzer package, you must install it into an existing Next.js project. For this tutorial, I'll use my Open source project, Tech Career Roadmap, as the Next.js example project.

To start, install the package with either npm or Yarn:

bash
# npm
npm install --save-dev @next/bundle-analyzer cross-env
# Yarn
yarn add -D @next/bundle-analyzer cross-env

You'll notice that I also installed cross-env; we'll use this package later in this article to set an environment variable. The cross-env package will allow us to set environment variables when running scripts in our package.json file without any errors, regardless of our platform-specific syntax or our OS.

Configuring the Next.js app

After you've installed the packages, you can enable the bundle analyzer by adding the following code to your next.config.js file:

js
const withBundleAnalyzer = require("@next/bundle-analyzer")({
enabled: process.env.ANALYZE === "true",
});
module.exports = withBundleAnalyzer({
// your Next.js configuration
});

Analyzing the Next.js app bundles

To run an analysis for your app bundles, you'll need to start your Next.js app in development mode with the ANALYZE environment variable set to true. Then, run the following command to view the analysis:

bash
ANALYZE=true npm run build

This command will automatically:

  1. Create an analyze folder in your .next folder inside your root directory and create three files: client.html, nodejs.html, and edge.html
  2. Open the three files in your browser; the client.html file represents the client side, and the nodejs.html file represents the server side. At this time of writing, there is no data supplied for the Edge.html page. It will presumably represent edge APIs, but we have been unable to get confirmation from the package's maintainers
  3. Provide bundle analysis information in your terminal

After the bundle analyzer has run, you can start inspecting your bundles:

Next.js Bundle Analyzer Example Terminal

Next.js Bundle Analyzer Example Terminal

Inspecting the Next.js app bundles

The @next/bundle-analyzer package provides a few ways to inspect your bundles: via the terminal, via the browser, and by using the sidebar filter. Let's take a look.

Via the terminal

In the above image showing the terminal output, there's a table displaying Page, Size, and First Load JS. Under the Page header is a list of all the pages in your Next.js app and chunks from the First Load JS bundle.

The First Load JS bundle is all the code shared or used by the app pages at initial load. Under the Size header, the package displays the file size of each page, bundle, and the chunks that make up the bundle.

Under the First Load JS, you can see the final size of the page or after the First Load JS has been added. The First Load JS size is also the initial size of the pages on initial load.

Next.js also color codes these sizes. Green tells us that the size is small and ok, while red indicates that the size is too large. If your First Load JS bundle size is shown in red, try to reduce it until it is displayed in green because the size of the bundle also affects the size and state of all your pages.

Via the browser

To inspect your app bundles in your browser, simply go to your browser to see two pages opened for you by the analyzer.

The first page, client.html, displays all the bundles for the client side:

Next.js App Bundles Client Side

Next.js App Bundles Client Side

The second page, nodejs.html, displays all the bundles for the server side:

Next.js App Bundles Server Side

Next.js App Bundles Server Side

If your app's pages are not automatically loaded, or if you have already closed the browser, just go into the analyze folder and right-click on any file names in order to open them in your browser.

Next, simply hover over any of the bundles to see its stats. You'll be able to see the Stat size, Parsed size, and Gzipped size of each bundle, as well as the bundles' file path.

Here's a little more detail on the size types:

  • Stat size: The size of the JavaScript bundle when served to the client; this is the size of the JavaScript code that the client's web browser has to download and execute to run the app
  • Parsed size: The size of the bundle after the web browser has parsed it; this is the amount of memory that the JavaScript code takes up in the web browser after it has executed the code
  • Gzipped size: The size of the JavaScript bundle when compressed using the Gzip algorithm; this is the amount of data the client's web browser must download to run the app. Gzip is a common compression algorithm used to reduce the size of web assets like JavaScript files. It can significantly reduce the amount of data transferred over the network

With the analyzer pages in the browser, you'll be able to see which bundles are taking up the most space based on their size type. Also, the bundles are arranged based on their file sizes:

Next.js Bundle Analyzer Example

Next.js Bundle Analyzer Example

You can also right-click on the root bundle to view options related to that chunk. You can click Hide chunks to hide that particular chunk from view and view the rest, click Hide all other chunks to hide all other chunks except the one you clicked on, or click Show all chunks to show all the chunks that are currently hidden:

Viewing Next.js Bundle Analyzer Hide Chunk Option

Using the sidebar filter

Another way to interact with and inspect your bundles is to use the sidebar filter. In the sidebar, you can specify a particular bundle or set of bundles that you'd like to view.

This approach can be useful for visualizing and inspecting smaller bundles quickly. You can also opt to have them ordered based on the size type: Stat, Parsed, or Gzipped:

Viewing Next.js Bundle Analyzer Sidebar

Viewing Next.js Bundle Analyzer Sidebar

You can search for modules using a particular file, folder, or phrase from your app; the analyzer will display results based on your search input:

Searching Next.js Analyzer Modules File Folder Phrase

Searching Next.js Analyzer Modules File Folder Phrase

Or, you can select an entry point for initial chunks (i.e., any of your app's pages):

Filter Next.js Bundle Analyzer Initial Chunks

Filter Next.js Bundle Analyzer Initial Chunks

Creating an analysis command/script

To save time, you can also create a quick command/script to run in your package.json file. By doing so, you can avoid setting the environment variable on every call.

In your package.json file, add the following script:

js
"scripts": {
...
"analyze": "cross-env ANALYZE=true next build",
},

As I mentioned previously, cross-env allows us to set environment variables in our package.json file. In this case, we are setting ANALYZE to true. If you were using a Windows operating system and didn't install cross-env, you'd get an error like the one shown below when running the above script:

Error Did Not Install Cross-Env

Error Did Not Install Cross-Env

Now, let's test our code by running one of the following commands:

bash
# npm
npm run analyze
# Yarn
yarn analyze

Running either of the above command results in the same action as running the following:

bash
ANALYZE=true npm run build

Optimizing app performance with bundle analysis

Bundle analysis is an essential tool for optimizing app performance. It helps identify potential bottlenecks and areas for improving an app's load speed and reducing its bundle size.

Let's consider the scenario of a Next.js app that was analyzed with the @next/bundle-analyzer tool; the following issues were identified:

  • Large image file sizes: this can slow down an app's load speed
  • Duplicate npmpackages: this will increase the app's bundle size unnecessarily
  • Unused npm packages: this will contribute to the app's bundle size and slow down its load speed
  • Heavy JavaScript files: Unoptimized JavaScript files will add to an app's bundle size

To optimize the app's performance, here are some possible ways to address the above issues:

  • Compress or resize images with large file sizes
  • Remove duplicate npm packages
  • Remove unused npm packages
  • Use minification, tree shaking, and code splitting to optimize JavaScript files

Alternatives to @next/bundle-analyzer

While @next/bundle-analyzer is a popular choice for analyzing the bundle size of a Next.js app, several other options are available. Here are some alternatives to consider:

webpack-bundle-analyzer: This widely-used tool analyzes the bundle size of webpack-based applications, including Next.js. It generates interactive visualizations of the bundle size and composition, making it easy to identify which modules take up the most space

next-bundle-analyzer: This community-driven alternative to the official @next/bundle-analyzer package provides similar functionality but with customized reports specifically for Next.js users

source-map-explorer: This popular tool analyzes the bundle size of an app by examining the source maps generated during the build process. It provides a treemap visualization of the bundle size and identifies where each byte of code is coming from

Statoscope: This web-based tool analyzes the size and composition of webpack bundles. It provides a simple interface for users to upload webpack stats files and generate visualizations that help identify which modules and dependencies contribute most to the app's bundle size

These are just a few of the many options available for analyzing the bundle size of a Next.js app. Consider experimenting with different tools to find the one that best meets your needs.

Conclusion

In this article, we discussed what a Next.js bundle is and investigated how bundle size can impact website performance. We also demonstrated how to use the @next/bundle-analyzer package to analyze and inspect the bundles in various ways, such as in the terminal, the browser, and the tool's sidebar filter.

Through this analysis, you'll identify potential bottlenecks, such as duplicated or unused npm packages and large image sizes. You'll be able to optimize your app's performance by reducing bundle size and improving load speed. Bundle analysis is a critical tool for any developer looking to optimize their Next.js application and enhance user experience.

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Frequently Asked Questions

1. What is a Next.js app bundle?

When you build a Next.js app, the code gets organized into small units called modules. These modules are bundled together by webpack, a popular JavaScript module bundler, to form a single file called a bundle. This bundle is then served to the client's browser, which is executed to render the application.

2. Why should I analyze my Next.js app bundle?

Analyzing the app bundle is essential because it helps identify the size and composition of the codebase. By understanding the bundle's contents, you can optimize its size, improve app performance, and identify potential areas for improvement.

3. What is @next/bundle-analyzer and how does it help in analyzing Next.js app bundles?

The @next/bundle-analyzer package is a plugin for the Next.js framework that allows you to analyze the size and composition of your app's bundle. It allows you to analyze the size and composition of your app's bundle. It generates a visual report showing information about each component's size, dependencies, and more. This makes it easier to identify large or unnecessary code blocks, enabling you to optimize the bundle and enhance your Next.js app's performance.

4. How do I optimize my Next.js app performance?

Bundle analysis is an essential tool for optimizing app performance. It helps identify potential bottlenecks and areas for improving an app's load speed and reducing its bundle size.

To optimize the app's performance, here are some possible ways to address the above issues:

  • Compress or resize images with large file sizes
  • Remove duplicate npm packages
  • Remove unused npm packages
  • Use minification, tree shaking, and code splitting to optimize JavaScript files
5. Are there alternative tools for analyzing Next.js app bundles?

Yes, there are several alternative tools for bundle analysis, such as webpack-bundle-analyzer, next-bundle-analyzer, source-map-explorer, and Statoscope. Each tool offers unique features and visualizations to help you inspect and optimize your app bundles effectively.

6. How can bundle analysis help me optimize my Next.js app's load speed?

By inspecting the size and composition of your app's bundles, you can identify which modules contribute most to the bundle's size. By optimizing these modules, removing unused dependencies, duplicate npm packages, and reducing unnecessary code, you can reduce the overall bundle size. A smaller bundle size leads to faster load times, improving your app's performance and user experience.

7. What should I do after analyzing my Next.js app bundles?

After analyzing your app bundles, you can start addressing the issues identified during the analysis. This may involve compressing or resizing large images, removing duplicate or unused npm packages, and optimizing JavaScript files using techniques like minification, tree shaking, and code splitting. By taking these steps, you can optimize your app's performance and provide a faster and more efficient user experience.

8. How can I create a script to easily run the bundle analysis for my Next.js app?

To create a script for running the bundle analysis, you can add a custom script in your package.json file. Use the cross-env package to set the ANALYZE environment variable to true, which enables the bundle analyzer during the build process. This way, you can easily run the analysis by executing the script in your terminal or command prompt.

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