When working with complex JavaScript projects, it’s common to use tools like Rollup.js to bundle our code and its dependencies into a single file. However, as our codebase grows, it’s important to ensure that we’re only including the necessary dependencies and artifacts in our build output. Including unused dependencies or artifacts can bloat our bundle size, adversely affecting performance and user experience.

In this blog post, we will explore techniques to analyze the Rollup.js build output and identify any unused dependencies or artifacts. By performing this analysis, we can optimize our bundle and remove any unnecessary code, resulting in a leaner and more efficient application.

Step 1: Generate the Rollup.js Build Output

To begin the analysis, we first need to generate the Rollup.js build output. Assuming you already have a Rollup.js configuration file rollup.config.js, you can run the following command to generate the build output:

npx rollup -c

This command will execute Rollup.js based on your configuration file and generate the bundle file according to your specified output options.

Step 2: Static Analysis of Build Output

Once we have the generated bundle file, we can perform a static analysis to identify any unused dependencies and artifacts. There are several tools available that can help us achieve this. One such tool is Webpack Bundle Analyzer. Even though it’s primarily designed for Webpack, it can also be used with Rollup.js.

Install the Webpack Bundle Analyzer by running the following command:

npm install --save-dev webpack-bundle-analyzer

Now, we need to update the rollup.config.js file to generate Webpack-compatible JSON output. Add the following plugin to your Rollup plugins array:

import { asJson } from 'rollup-plugin-node-convert';

// ...

plugins: [
  // ...
  asJson({
    include: '**/*.js',
    exclude: 'node_modules/**',
    preferConst: true,
    plugins: [analyze()],
  })
]

With these changes, rerun the Rollup build command, and it will generate a stats.json file containing the plugin output.

Step 3: Visualizing the Bundle Analysis

After generating the stats.json file, we can use the Webpack Bundle Analyzer tool to visualize the bundle analysis. Create a new file called bundle-analysis.js and add the following code:

const { create } = require('webpack-bundle-analyzer').BundleAnalyzerPlugin;

module.exports = {
  plugins: [
    create({ analyzerMode: 'static', reportFilename: 'bundle-analysis.html' }),
  ],
};

Save the file and run the following command:

npx rollup -c bundle-analysis.js

This will generate a bundle-analysis.html file containing the visualization of the bundle analysis. Open it in your browser to see a detailed breakdown of your bundle, including the sizes of individual modules and dependencies.

Step 4: Identifying Unused Dependencies or Artifacts

Within the bundle analysis report, you can examine the various modules and dependencies that make up your bundle. Look for any modules or dependencies that are not being used by your application. These are potential candidates for removal.

You can manually review the analysis report and cross-reference it with your codebase to identify any unused dependencies or artifacts. Unused dependencies can be safely removed from your project, reducing the bundle size and improving performance. Unused artifacts, such as images or CSS files, can be optimized or removed if they are no longer required.

Conclusion

Analyzing the Rollup.js build output is a crucial step in optimizing our JavaScript applications. By identifying and removing unused dependencies and artifacts, we can improve the performance and efficiency of our bundle. The Webpack Bundle Analyzer tool provides a powerful visualization that can assist in this analysis process, enabling us to make informed decisions about code and resource removal.

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