Keep Your Javascript Bundle Size in Check

August 16, 2021

Are you a developer who is concerned about the size of newly added libraries? Or do you want to find a culprit in a rather big Javascript bundle?

If you’re like me, then you answered yes to both questions.

In this post I’ll cover a few tools that come in handy for a quick analysis of bundle sizes without changing or ejecting your build architecture.

VS Code extension: Import Cost

Immediately see the weight of what you import in VS Code.

Immediately see the weight of what you import in VS Code.

Understand the cost of an import early.

This extension will display inline in the editor the size of the imported package. It supports tree shaking, so the size should be displayed correctly for a few exported functions.

With this you may spot mistakes like these:

import moment from 'moment'; // 289.7KB;

import { now } from 'moment'; // 0.082KB

It’s also available for JetBrains IDE, Atom and Vim.


Website: Bundlephobia

Analysing the size of Moment.js with Bundlephobia.

This website lets you search for libraries and display their sizes without the need to install. It shows the size of each version and even suggests alternatives to similar libraries that might be lighterโ€”talking about a new framework or library every week.

You could also drop your package.json file and order it by size to see your biggest libraries. Personally I find this quite fun, but usually I use this tool to check bundle sizes of not-yet-installed libraries.


NPM: source-map-explorer

Visualise your imported packages in an interactive top down view.

Like the name suggests you need to build source maps. With modern framework CLIs it’s enabled by default in prod builds.

Useful tool for imported package visualisation in relation to their size. By clicking on the packages, you can further inspect their sizes and children.

๐Ÿ‘‰ npx source-map-explorer ./dist *.js

Website: PageSpeed Insight / Lighthouse

If your site is already public you can use Google’s PageSpeed Insight to detect big Javascript bundles.

Bonus: It also includes Javascript files, that are downloaded on runtime from your ad networks, Google Tag Manager and other tools.


Check out this tweet to see the treemap in action:

Simon Wicki

Simon Wicki is a Freelance Frontend Developer in Berlin. Passionate and fluent in Vue, Angular, React and Ionic. Interested in Tech, frontend, web perf & non-fiction books

Twitter Follow @zwacky