Remove unused code

npm makes adding code to your project a breeze. But are you really using all those extra bytes?

Registries like npm have transformed the JavaScript world for the better by allowing anyone to easily download and use over half a million public packages. But we often include libraries we're not fully utilizing. To fix this issue, analyze your bundle to detect unused code. Then remove unused and unnecessary libraries.

Impact on Core Web Vitals

By removing unused code, you can improve your website's Core Web Vitals. Largest Contentful Paint, for example, can be affected by unused code by increased bandwidth contention caused by larger than necessary assets. LCP can also be affected if large JavaScript assets that render markup solely on the client contain references to LCP candidates by delaying when these resources can load.

Other metrics such as First Input Delay (FID) and Interaction to Next Paint (INP) can also be affected by unused code, because even unused JavaScript must be downloaded, parsed, compiled, and then executed. The unused code can introduce unnecessary delays in resource load time, memory usage, and main thread activity that contribute to poor page responsiveness.

This guide will help you get a handle on your project's unused code by showing you how to analyze your project's codebase, and offer strategies for pruning unused code from the JavaScript assets you ship to your users in production.

Analyze your bundle

DevTools makes it easy to see the size of all network requests:

  1. Press `Control+Shift+J` (or `Command+Option+J` on Mac) to open DevTools.
  2. Click the Network tab.
  3. Select the Disable cache checkbox.
  4. Reload the page.

Network panel with bundle request

The Coverage tab in DevTools will also tell you how much CSS and JS code in your application is unused.

Code Coverage in DevTools

By specifying a full Lighthouse configuration through its Node CLI, an "Unused JavaScript" audit can also be used to trace how much unused code is being shipped with your application.

Lighthouse Unused JS Audit

If you happen to be using webpack as your bundler, Webpack Bundle Analyzer will help you investigate what makes up the bundle. Include the plugin in your webpack configurations file like any other plugin:

module.exports = {
  //...
  plugins: [
    //...
    new BundleAnalyzerPlugin()
  ]
}

Although webpack is commonly used to build single-page applications, other bundlers, such as Parcel and Rollup, also have visualization tools that you can use to analyze your bundle.

Reloading the application with this plugin included shows a zoomable treemap of your entire bundle.

Webpack Bundle Analyzer

Using this visualization allows you to inspect which parts of your bundle are larger than others, as well as get a better idea of all the libraries that you're importing. This can help identify if you are using any unused or unnecessary libraries.

Remove unused libraries

In the previous treemap image, there are quite a few packages within a single @firebase domain. If your website only needs the firebase database component, update the imports to fetch that library:

import firebase from 'firebase';
import firebase from 'firebase/app';
import 'firebase/database';

It is important to emphasize that this process is significantly more complex for larger applications.

For the mysterious looking package that you're quite sure is not being used anywhere, take a step back and see which of your top-level dependencies are using it. Try to find a way to only import the components that you need from it. If you aren't using a library, remove it. If the library isn't required for the initial page load, consider if it can be lazy loaded.

And in case you're using webpack, check out the list of plugins that automatically remove unused code from popular libraries.

Remove unnecessary libraries

Not all libraries can be easily broken down into parts and selectively imported. In these scenarios, consider if the library could be removed entirely. Building a custom solution or leveraging a lighter alternative should always be options worth considering. However, it is important to weigh the complexity and effort required for either of these efforts before removing a library entirely from an application.