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Size Limit

Size Limit is a performance budget tool for JavaScript. It checks every commit on CI, calculates the real cost of your JS for end-users and throws an error if the cost exceeds the limit.

• ES modules and tree-shaking support.
• Add Size Limit to Travis CI, Circle CI, GitHub Actions or another CI system to know if a pull request adds a massive dependency.
• Modular to fit different use cases: big JS applications that use their own bundler or small npm libraries with many files.
• Can calculate the time it would take a browser to download and execute your JS. Time is a much more accurate and understandable metric compared to the size in bytes.
• Calculations include all dependencies and polyfills used in your JS.

With GitHub action Size Limit will post bundle size changes as a comment in pull request discussion.

With --why, Size Limit can tell you why your library is of this size and show the real cost of all your internal dependencies.

Who Uses Size Limit

• MobX
• Material-UI
• Autoprefixer
• PostCSS reduced 25% of the size.
• Browserslist reduced 25% of the size.
• EmojiMart reduced 20% of the size
• nanoid reduced 33% of the size.
• React Focus Lock reduced 32% of the size.
• Logux reduced 90% of the size.

How It Works

1. Size Limit contains a CLI tool, 3 plugins (file, webpack, time) and 3 plugin presets for popular use cases (app, big-lib, small-lib). A CLI tool finds plugins in package.json and loads the config.
2. If you use the webpack plugin, Size Limit will bundle your JS files into a single file. It is important to track dependencies and webpack polyfills. It is also useful for small libraries with many small files and without a bundler.
3. The webpack plugin creates an empty webpack project, adds your library and looks for the bundle size difference.
4. The time plugin compares the current machine performance with that of a low-priced Android devices to calculate the CPU throttling rate.
5. Then the time plugin runs headless Chrome (or desktop Chrome if it’s available) to track the time a browser takes to compile and execute your JS. Note that these measurements depend on available resources and might be unstable. See here for more details.

Usage

JS Applications

Suitable for applications that have their own bundler and send the JS bundle directly to a client (without publishing it to npm). Think of a user-facing app or website, like an email client, a CRM, a landing page or a blog with interactive elements, using React/Vue/Svelte lib or vanilla JS.

Show instructions

1. Install the preset:

   $ npm install --save-dev size-limit @size-limit/preset-app
   

2. Add the size-limit section and the size script to your package.json:

   + "size-limit": [
   +   {
   +     "path": "dist/app-*.js"
   +   }
   + ],
     "scripts": {
       "build": "webpack ./webpack.config.js",
   +   "size": "npm run build && size-limit",
       "test": "jest && eslint ."
     }
   

3. Here’s how you can get the size for your current project:

   $ npm run size
   
     Package size: 30.08 KB with all dependencies, minified and gzipped
     Loading time: 602 ms   on slow 3G
     Running time: 214 ms   on Snapdragon 410
     Total time:   815 ms
   

4. Now, let’s set the limit. Add 25% to the current total time and use that as the limit in your package.json:

     "size-limit": [
       {
   +     "limit": "1 s",
         "path": "dist/app-*.js"
       }
     ],
   

5. Add the size script to your test suite:

     "scripts": {
       "build": "webpack ./webpack.config.js",
       "size": "npm run build && size-limit",
   -   "test": "jest && eslint ."
   +   "test": "jest && eslint . && npm run size"
     }
   

6. If you don’t have a continuous integration service running, don’t forget to add one — start with [Travis CI].

Reports

Size Limit has a GitHub action that comments and rejects pull requests based on Size Limit output.

1. Install and configure Size Limit as shown above.
2. Add the following action inside .github/workflows/size-limit.yml

name: "size"
on:
  pull_request:
    branches:
      - master
jobs:
  size:
    runs-on: ubuntu-latest
    env:
      CI_JOB_NUMBER: 1
    steps:
      - uses: actions/checkout@v1
      - uses: andresz1/size-limit-action@v1.0.0
        with:
          github_token: ${{ secrets.GITHUB_TOKEN }}

JS API

const sizeLimit = require('size-limit')
const filePlugin = require('@size-limit/file')
const webpackPlugin = require('@size-limit/webpack')

sizeLimit([filePlugin, webpackPlugin], [filePath]).then(result => {
  result //=> { size: 12480 }
})