Size Limit is a tool to prevent JavaScript libraries bloat. With it, you know exactly for how many kilobytes your JS library increases the user bundle.

You can add Size Limit to your continuous integration service (such as Travis CI) and set the limit. If you accidentally add a massive dependency, Size Limit will throw an error.

Size Limit could tell you not only library size. With --why argument it can tell you why your library has this size and show real cost of all your internal dependencies.

• 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.
• Logux reduced 90% of the size.

You can find more examples in Size Limit: Make the Web lighter article.

To be really specific, Size Limit creates an empty webpack project in memory. Then, it adds your library as a dependency to the project and calculates the real cost of your libraries, including all dependencies, webpack’s polyfills for process, etc.

First, install size-limit:

$ npm install --save-dev size-limit

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

+ "size-limit": [
+   {
+     "path": "index.js"
+   }
+ ],
  "scripts": {
+   "size": "size-limit",
    "test": "jest && eslint ."
  }

The path option:

• For an open source library, specify compiled sources, which will be published to npm (usually the same value as the main field in the package.json);
• For an application, specify a bundle file and use webpack: false (see the Applications section).

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

$ npm run size

  Package size: 8.46 KB
  With all dependencies, minified and gzipped

If your project size starts to look bloated, run Webpack Bundle Analyzer for analysis:

$ npm run size -- --why

Now, let’s set the limit. Determine the current size of your library, add just a little bit (a kilobyte, maybe) and use that as a limit in your package.json:

 "size-limit": [
    {
+     "limit": "9 KB",
      "path": "index.js"
    }
 ],

Add the size script to your test suite:

  "scripts": {
    "size": "size-limit",
-   "test": "jest && eslint ."
+   "test": "jest && eslint . && npm run size"
  }

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

Size Limits supports 3 ways to define config.

1. size-limit section to package.json:

     "size-limit": [
       {
         "path": "index.js",
         "limit": "9 KB"
       }
     ]

2. or separated .size-limit config file:

   [
     {
       path: "index.js",
       limit: "9 KB"
     }
   ]

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

   module.exports = [
     {
       path: "index.js",
       limit: "9 KB"
     }
   ]

Each section in config could have 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"].
• limit: size limit for files from path option. It should be a string with a number and unit (100 B, 10 KB, etc).
• name: the name of this section. It will be useful only if you have multiple sections.
• webpack: with false will disable webpack.
• gzip: with false will disable gzip compression.
• config: a path to custom webpack config.

Webpack inside Size Limit is very useful for small open source library. But if you want to use Size Limit for application, not open source library, you could already have webpack to make bundle.

In this case you can disable internal webpack:

 "size-limit": [
    {
      "limit": "300 KB",
+     "webpack": false,
      "path": "public/app-*.js"
    }
 ],

const getSize = require('size-limit')

const index = path.join(__dirname, 'index.js')
const extra = path.join(__dirname, 'extra.js')

getSize([index, extra]).then(size => {
  if (size.gzip > 1 * 1024 * 1024) {
    console.error('Project is now larger than 1MB!')
  }
})

Main difference between Size Limit and bundlesize, that Size Limit uses webpack to build bundle. It has more accurate result and can show you what and why causes the bloat.

1. Size Limit has the --why mode to run Webpack Bundle Analyzer — this way, you can see what went wrong in a nice graphical representation.
2. Instead of bundlesize, Size Limit prevents the most popular source of libraries bloat — unexpected huge dependency.
3. Also Size Limit prevents increasing library size because of wrong process or path usage, when webpack will add big unnecessary polyfill.
4. Size Limit runs only on first CI job, so it is more respectful to CI resources.