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 GitHub Actions, Circle CI 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.

• MobX
• Material-UI
• Ant Design
• 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.

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.

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.

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.

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. Libraries like React are good examples for this preset.

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.

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 }}

Plugins or plugin presets will be loaded automatically from package.json. For example, if you want to use @size-limit/webpack, you can just use npm install --save-dev @size-limit/webpack, or you can use our preset @size-limit/preset-big-lib.

Plugins:

• @size-limit/file checks the size of files with Brotli (default), Gzip 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/esbuild-why add reports for esbuild plugin about your library is of this size to show the cost of all your dependencies.
• @size-limit/time uses headless Chrome to track time to execute JS.

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.

Third-party plugins and presets named starting with size-limit- are also supported. For example:

• size-limit-node-esbuild is like @size-limit/esbuild but for Node libraries.
• size-limit-preset-node-lib is like @size-limit/preset-small-lib but for Node libraries which contains above node-esbuild and core file plugins.
• nx-size-limit is an NX build system community plugin.

Size Limits supports three ways to define limits 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"
     }
   ]

4. or types .size-limit.ts:

   import type { SizeLimitConfig } from '../../packages/size-limit'
   
   module.exports = [
     {
       path: "index.js",
       import: "{ createStore }",
       limit: "500 ms"
     }
   ] satisfies SizeLimitConfig

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', * to test all exports, 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.
• message: an optional custom message to display additional information, such as guidance for resolving errors, relevant links, or instructions for next steps when a limit is exceeded.
• 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 true it will use Gzip compression and disable Brotli compression.
• brotli: with false it will disable any 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.

You can run size-limit --why to analyze the bundle.

You will need to install @size-limit/esbuild-why or @size-limit/webpack-why depends on which bundler you are using (default is esbuild).

For @size-limit/esbuild-why, it will generate a esbuild-why.html at the current directory & open it in the browser.

If you also specify --save-bundle <DIR>, the report will be generated inside <DIR>.

If you have multiple sections in your config, the files will be named esbuild-why-{n}.html, or you can give it a custom name:

[
  {
    "name": "cjs",
    /* snap */
  },
  {
    "name": "esm",
    /* snap */
  }
]

This will produce esbuild-why-cjs.html and esbuild-why-esm.html respectively.

For @size-limit/webpack-why, it will generate the report and open it in the browser automatically.

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 }
})