Analyze JavaScript Bundles with Webpack Bundle Analyzer

Mar 4, 2020

The webpack-bundle-analyzer npm module generates an interactive treemap (not to be confused with Java's TreeMap class) of a given Webpack bundle. This map is useful for finding what are the npm packages that are adding the most to your bundle size, so you can see where to focus when trying to trim your bundle size.


First, you need to install Webpack, webpack-cli, and webpack-bundle-analyzer:

npm install webpack webpack-cli webpack-bundle-analyzer

Next, let's install Vue and Axios to put together a trivial Vue app.

npm install vue axios

Here's an src/index.js file creates a simple Vue app.

const Vue = require('vue');
const axios = require('axios');

const url = '';

const app = new Vue({
  data: () => ({ user: '' }),
  template: `
      Hello, {{user}}
  mounted: function() {
      then(res =>
      then(user => { this.user = user; }).
      catch(err => console.log(err));

Running the Bundle Analyzer

To run the bundle analyzer, first you need to run Webpack with the --profile and --json flags to export the raw data that the bundle analyzer needs:

$ ./node_modules/.bin/webpack --profile --json > stats.json

The stats.json file looks something like this:

$ head stats.json 
  "errors": [],
  "warnings": [
    "configuration\nThe 'mode' option has not been set, webpack will fallback to 'production' for this value. Set 'mode' option to 'development' or 'production' to enable defaults for each environment.\nYou can also set it to 'none' to disable any default behavior. Learn more:"
  "version": "4.42.0",
  "hash": "a4433cf21bc97d0be252",
  "time": 269,
  "builtAt": 1583167656248,
  "publicPath": "",

Next, run the Webpack bundle analyzer on the stats.json file:

$ ./node_modules/.bin/webpack-bundle-analyzer stats.json

Webpack bundle analyzer will open up a browser window with the tree map:

Here's a live example of the above image. The way to read the treemap is that the node_modules square contains everything underneath it. So the bundled node_modules contains vue/dist and axios. The size of the vue/dist square is proportional to the size of the bundle, so you can tell that vue/dist is much bigger than axios.

And underneath vue/dist and axios/lib/core are the individual files. Vue is bundled into one vue.runtime.esm.js file. And axios/lib/core has several smaller files, the largest one of which is utils.js.

Did you find this tutorial useful? Say thanks by starring our repo on GitHub!

More Webpack Tutorials