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.
We are using Statoscope for this analysis.

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.

JS Application and Time-based Limit

File size limit (in kB) is not the best way to describe your JS application
cost for developers. Developers will compare the size of the JS bundle
with the size of images. But browsers need much more time to parse 100 kB
of JS than 100 kB of an image since JS compilers are very complex.

This is why Size Limit support time-based limit. It runs headless Chrome
to track the time a browser takes to compile and execute your JS.

Big Libraries

JS libraries > 10 kB in size.

This preset includes headless Chrome, and will measure your lib’s execution
time. You likely don’t need this overhead for a small 2 kB lib, but for larger
ones the execution time is a more accurate and understandable metric that
the size in bytes. Library like React is a good example for this preset.

Small Libraries

JS libraries < 10 kB in size.

This preset will only measure the size, without the execution time, so it’s
suitable for small libraries. If your library is larger, you likely want
the Big Libraries preset above. Nano ID or Storeon are good examples
for this preset.

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
        with:
          github_token: ${{ secrets.GITHUB_TOKEN }}

Config

Size Limits supports three ways to define config.

1. size-limit section in package.json:

     "size-limit": [
       {
         "path": "index.js",
         "import": "{ createStore }",
         "limit": "500 ms"
       }
     ]

2. or a separate .size-limit.json config file:

   [
     {
       "path": "index.js",
       "import": "{ createStore }",
       "limit": "500 ms"
     }
   ]

3. or a more flexible .size-limit.js or .size-limit.cjs config file:

   module.exports = [
     {
       path: "index.js",
       import: "{ createStore }",
       limit: "500 ms"
     }
   ]

Each section in the config can have these options:

• path: relative paths to files. The only mandatory option.
  It could be a path "index.js", a pattern "dist/app-*.js"
  or an array ["index.js", "dist/app-*.js", "!dist/app-exclude.js"].
• import: partial import to test tree-shaking. It could be "{ lib }"
  to test import { lib } from 'lib' or { "a.js": "{ a }", "b.js": "{ b }" }
  to test multiple files.
• limit: size or time limit for files from the path option. It should be
  a string with a number and unit, separated by a space.
  Format: 100 B, 10 kB, 500 ms, 1 s.
• name: the name of the current section. It will only be useful
  if you have multiple sections.
• entry: when using a custom webpack config, a webpack entry could be given.
  It could be a string or an array of strings.
  By default, the total size of all entry points will be checked.
• webpack: with false it will disable webpack.
• running: with false it will disable calculating running time.
• gzip: with false it will disable gzip compression.
• brotli: with true it will use brotli compression and disable
  gzip compression.
• config: a path to a custom webpack config.
• ignore: an array of files and dependencies to exclude from
  the project size calculation.
• modifyWebpackConfig: (.size-limit.js only) function that can be used
  to do last-minute changes to the webpack config, like adding a plugin.
• compareWith: path to stats.json from another build to compare
  (when --why is using).
• uiReports: custom UI reports list (see Statoscope docs).

If you use Size Limit to track the size of CSS files, make sure to set
webpack: false. Otherwise, you will get wrong numbers, because webpack
inserts style-loader runtime (≈2 kB) into the bundle.

Plugins and Presets

Plugins:

• @size-limit/file checks the size of files with Gzip, Brotli
  or without compression.
• @size-limit/webpack adds your library to empty webpack project
  and prepares bundle file for file plugin.
• @size-limit/webpack-why adds reports for webpack plugin
  about your library is of this size to show the cost of all your
  dependencies.
• @size-limit/webpack-css adds css support for webpack plugin.
• @size-limit/esbuild is like webpack plugin, but uses esbuild
  to be faster and use less space in node_modules.
• @size-limit/time uses headless Chrome to track time to execute JS.
• @size-limit/dual-publish compiles files to ES modules with dual-publish
  to check size after tree-shaking.

Plugin presets:

• @size-limit/preset-app contains file and time plugins.
• @size-limit/preset-big-lib contains webpack, file, and time plugins.
• @size-limit/preset-small-lib contains esbuild and file plugins.

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 }
})