Remove unused code

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

Nov 5, 2018

Houssein Djirdeh

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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 unneeded libraries.

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.

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

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.

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.

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.

Remove unneeded 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.