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https://github.com/thekashey/used-styles

📝All the critical styles you've used to render a page.
https://github.com/thekashey/used-styles

codesplitting critical-css css inlines-critical-styles server ssr

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📝All the critical styles you've used to render a page.

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README 

        


  
used-styles


  


  Get all the styles, you have used to render a page.


  (without any puppeteer involved)

  


  



[![Build Status](https://travis-ci.org/theKashey/used-styles.svg?branch=master)](https://travis-ci.org/theKashey/used-styles)

[![NPM version](https://img.shields.io/npm/v/used-styles.svg)](https://www.npmjs.com/package/used-styles)






> 👋**Version 3** migration notice: `import { discoverProjectStyles } from 'used-styles/node'`. That's it



---



> Bundler and framework independent CSS part of SSR-friendly code splitting



Detects used `css` files from the given HTML, and/or **inlines critical styles**. Supports sync or **stream** rendering.



Read more about critical style extraction and this library: https://dev.to/thekashey/optimising-css-delivery-57eh



- 🚀 Super Fast - no browser, no jsdom, no runtime transformations

- 💪 API - it's no more than an API - integrates with everything

- 🤝 Works with `strings` and `streams`

- ⏳ Helps preloading for the "real" style files



Works in two modes:



- 🚙 inlines style **rules** required to render given HTML - ideal for the first time visitor

- 🏋️‍♀️inlines style **files** required to render given HTML - ideal for the second time visitor (and code splitting)



Critical style extraction:



- 🧱 will _load_ all used styles at the beginning of your page in a **string** mode

- 💉 will _interleave_ HTML and CSS in a **stream** mode. This is the best experience possible



## How it works



1. Scans all `.css` files, in your `build` directory, extracting all style rules names.

2. Scans a given `html`, finding all the `classes` used.

3. Here there are two options:

   3a. Calculate all **style rules** you need to render a given HTML. 3b. Calculate all the style **files** you have

   send to a client.

4. Injects `` or ``

5. After the page load, hoist or removes critical styles replacing them by the "real" ones.



## Limitation



For the performance sake `used-styles` inlines a bit more styles than it should - it inlines everything it would be "not

fast" to remove.



- inlines all `@keyframe` animations

- inlines all `html, body` and other tag-based selectors (hello css-reset)

- undefined behavior if `@layer a,b,c` is used multiple times



### Speed



> Speed, I am speed!



For the 516kb page, which needs **80ms** to `renderToString`(React) resulting time for the `getCriticalRules`(very

expensive operation) would be around **4ms**.



# API



## Discovery API



Use it to scan your `dist`/`build` folder to create a look up table between classNames and files they are described in.



1. `discoverProjectStyles(buildDirrectory, [filter]): StyleDef` - generates class lookup table

   > you may use the second argument to control which files should be scanned



`filter` is very important function here. It takes `fileName` as input, and returns

`false`, `true`, or a `number` as result. `False` value would exclude this file from the set, `true` - add it,

and `number`

would change **the order** of the chunk. Keeping chunk ordered "as expected" is required to preserve style declaration

order, which is important for many existing styles.



```js

// with chunk format [chunkhash]_[id] lower ids are potentialy should be defined before higher

const styleData = discoverProjectStyles(resolve('build'), (name) => {

  // get ID of a chunk and use it as order hint

  const match = name.match(/(\d)_c.css/);

  return match && +match[1];

});

```



> ⚠️ generally speaking - this approach working only unless there are no order-sensive styles from different chunks applied to a single DOM Element.

> Quite often it never happen, but if you are looking for a better way - follow to [#26](https://github.com/theKashey/used-styles/issues/26) ☣️



1. `loadStyleDefinitions` is a "full control API", and can used to feed `used-styles` with any custom data, for example

   providing correct critical css extraction in dev mode (no files written on disk)



```ts

return loadStyleDefinitions(

  /*list of files*/ async () => cssFiles,

  /*data loader*/ (file) => fetchTxt(`http://localhost:${process.env.DEV_SERVER_PORT}/${file}`)

  /*filter and order */ // (file) => order.indexOf(cssToChunk[file])

);

```



## Scanners



Use to get used styled from render result or a stream



2. `getUsedStyles(html, StyleDef): string[]` - returns all used **files**, you will need to import them

3. `getCriticalStyles(html, StyleDef) : string` - returns all used selectors and other applicable rules, wrapped

   with `style`

4. `getCriticalRules(html, StyleDef): string` - **the same**, but without `` tag, letting you handle in a way you

   want



5. `createStyleStream(lookupTable, callback(fileName):void): TransformStream` - creates Transform stream - will

   inject `<links`

6. `createCriticalStyleStream(lookupTable, callback(fileName):void): TransformStream` - creates Transform stream - will

   inject `<styles`.



### React



There are only two things about react:



1. to inline critical styles use another helper - `getCriticalRules` which does not wrap result with `style` letting you

   do it



```js

import { getCriticalRules } from 'used-styles';



const Header = () => (

  <style data-used-styles dangerouslySetInnerHTML={{ __html: getCriticalRules(markup, styleData) }} />

);

```



2. React produces more _valid_ code, and you might enable optimistic optimization, making used-styles a bit faster.



```js

import { enableReactOptimization } from 'used-styles';



enableReactOptimization(); // just makes it a but faster

```



## Serialize API



Use it to separate generation of styles lookup from your runtime.



It is useful in cases, where you can't directly use Discovery APIs on your client CSS bundles during app's runtime, e.g. various serverless runtimes.

Also it may be useful for you, if you want to save on the size of your container for the server app, since it allows you to only load styles lookup into it, without CSS bundles.



1. `serializeStylesLookup(def: StyleDef): SerializedStyleDef` - creates a serializable object from original styles lookup. Result can be then stringified with `JSON.stringify`

2. `loadSerializedLookup(def: SerializedStyleDef): StyleDef` - transforms serialized style definition back to normal `StyleDef`, which can be used with any Scanner API



### Example



#### During your build



1. Add separate script to generate style lookup and store it as you like.



```js

// project/scripts/generate_styles_lookup.mjs

import { serializeStylesLookup, discoverProjectStyles } from 'used-styles';

import { writeFileSync } from 'fs';



const stylesLookup = discoverProjectStyles('./path/to/dist/client');



await stylesLookup;



writeFileSync('./path/to/dist/server/styles-lookup.json', JSON.stringify(serializeStylesLookup(lookup)));

```



2. Run this code after your build



```sh

yarn build

node ./scripts/generate_styles_lookup.mjs

```



Notice, that you can store serialized lookup in any way, that suits you and your case, example above is not the only valid option.



#### During your runtime



1. Access previously created and stored styles lookup, convert it to `StyleDef` with `loadSerializedLookup` and use it normally



```js

import { loadSerializedLookup } from 'used-styles';



const stylesLookup = loadSerializedLookup(require('./dist/server/styles-lookup.json'));



// ...



getCriticalStyles(markup, stylesLookup);

```



# Example



## Demo



- [React SSR](/example/ssr-react/README.md)

- [React SSR + TS](/example/ssr-react-ts/README.md)

- [React Streaming SSR](/example/ssr-react-streaming/README.md)

- [React Streaming SSR + TS](/example/ssr-react-streaming-ts/README.md)



## Static rendering



There is nothing interesting here - just render, just `getUsedStyles`.



```js

import {getUsedStyles} from 'used-styles';

import {discoverProjectStyles} from 'used-styles/node';



// generate lookup table on server start

const stylesLookup = discoverProjectStyles('./build');



async function MyRender() {

  await stylesLookup;// it is "thenable"

  // render App

  const markup = ReactDOM.renderToString(<App/>)

  const usedStyles = getUsedStyles(markup, stylesLookup);



  usedStyles.forEach(style => {

    const link = `<link  rel="stylesheet" href="build/${style}">\n`;

    // or

    const link = `<link rel="prefetch" as="style" href="build/${style}">\n`;

    // append this link to the header output or to the body

  });



// or



  const criticalCSS = getCriticalStyles(markup, stylesLookup);



// append this link to the header output

```



Any _bulk_ CSS operations, both `getCriticalStyles` and `getUsedStyles` **are safe** and preserve the selector rule

order. You **may combine** both methods, to prefetch full styles, and inline critical CSS.



! Keep in mind - calling two functions is as fast, as calling a single one !



### Stream rendering



Please keep in mind - stream rendering in **NOT SAFE** in terms of CSS, as long as **it might affect the ordering of

selectors**. Only pure BEM and Atomic CSS are "safe", _just some random CSS_ might be not compatible. Please **test**

results before releasing into production.



> If you do not understand why and how selector order is important - please **do not use** stream transformer.



Stream rendering is much harder, and much more efficient, giving you the best Time-To-First-Byte. And the second byte.



Stream rendering could be interleaved(more efficient) or block(more predictable).



### Interleaved Stream rendering



In case or React rendering you may use **interleaved streaming**, which would not delay TimeToFirstByte. It's quite

similar how StyledComponents works



```js

import express from 'express';

import { discoverProjectStyles } from 'used-styles/node';

import { loadStyleDefinitions, createCriticalStyleStream, createStyleStream, createLink } from 'used-styles';



const app = express();



// generate lookup table on server start

const stylesLookup = isProduction

  ? discoverProjectStyles('./dist/client')

  : // load styles for development

    loadStyleDefinitions(async () => []);



app.use('*', async (req, res) => {

  await stylesLookup;



  try {

    const renderApp = (await import('./dist/server/entry-server.js')).default;



    // create a style steam

    const styledStream = createStyleStream(stylesLookup, (style) => {

      // _return_ link tag, and it will be appended to the stream output

      return createLink(`dist/${style}`); // <link href="dist/mystyle.css />

    });



    // or create critical CSS stream - it will inline all styles

    const styledStream = createCriticalStyleStream(stylesLookup); // <style>.myClass {...



    await renderApp({ res, styledStream });

  } catch (err) {

    res.sendStatus(500);

  }

});

```



```js

// entry-server.js

import React from 'react';

import { renderToPipeableStream } from 'react-dom/server';

import App from './App';



const ABORT_DELAY = 10000;



async function renderApp({ res, styledStream }) {

  let didError = false;



  const { pipe, abort } = renderToPipeableStream(

    <React.StrictMode>

      <App />

    </React.StrictMode>,

    {

      onShellError() {

        res.sendStatus(500);

      },

      onAllReady() {

        res.status(didError ? 500 : 200);

        res.set({ 'Content-Type': 'text/html' });



        // allow client to start loading js bundle

        res.write(`<!DOCTYPE html><html><head><script defer src="client.js"></script></head><body><div id="root">`);



        styledStream.pipe(res, { end: false });



        // start by piping react and styled transform stream

        pipe(styledStream);



        styledStream.on('end', () => {

          res.end('</div></body></html>');

        });

      },

      onError(error) {

        didError = true;

        console.error(error);

      },

    }

  );



  setTimeout(() => {

    abort();

  }, ABORT_DELAY);

}



export default renderApp;

```



**!! THIS IS NOT THE END !!** Interleaving links and react output would break a client side rehydration, as long as _

injected_ links were not rendered by React, and not expected to present in the "result" HTML code.



You have to move injected styles out prior rehydration.



```js

import React from 'react';

import { moveStyles } from 'used-styles/moveStyles';

import ReactDOM from 'react-dom/client';

import App from './App';



// Call before `ReactDOM.hydrateRoot`

moveStyles();



ReactDOM.hydrateRoot(

  document.getElementById('root'),

  <React.StrictMode>

    <App />

  </React.StrictMode>

);

```



You might want to remove styles after rehydration to prevent duplication. Double check that corresponding _real_ CSS is

loaded.



```js

import { removeStyles } from 'used-styles/moveStyles';



removeStyles();

```



## Block rendering



> Not sure this is a good idea



Idea is to:



- push `initial line` to the browser, with `the-main-script` inside

- push all used `styles`

- push some `html` between `styles` and `content`

- push `content`

- push `closing` tags



That's all are streams, concatenated in a right order. It's possible to interleave them, but that's is not expected buy

a `hydrate`.



```js

import { createStyleStream, createLink } from 'used-styles';

import { discoverProjectStyles } from 'used-styles/node';

import MultiStream from 'multistream';



// .....

// generate lookup table on server start

const lookup = await discoverProjectStyles('./build'); // __dirname usually



// small utility for "readable" streams

const readableString = (string) => {

  const s = new Readable();

  s.push(string);

  s.push(null);

  s._read = () => true;

  return s;

};



// render App

const htmlStream = ReactDOM.renderToNodeStream(<App />);



// create a style steam

const styledStream = createStyleStream(lookup, (style) => {

  // emit a line to header Stream

  headerStream.push(createLink(`dist/${style}`));

  // or

  headerStream.push(`<link href="dist/${style}" rel="stylesheet">\n`);

});



// allow client to start loading js bundle

res.write(`<!DOCTYPE html><html><head><script defer src="client.js"></script>`);



const middleStream = readableString('</head><body><div id="root">');

const endStream = readableString('</head><body>');



// concatenate all steams together

const streams = [

  headerStream, // styles

  middleStream, // end of a header, and start of a body

  styledStream, // the main content

  endStream, // closing tags

];



MultiStream(streams).pipe(res);



// start by piping react and styled transform stream

htmlStream.pipe(styledStream, { end: false });

htmlStream.on('end', () => {

  // kill header stream on the main stream end

  headerStream.push(null);

  styledStream.end();

});

```



> This example is taken from [Parcel-SSR-example](https://github.com/theKashey/react-imported-component/tree/master/examples/SSR/parcel-react-ssr)

> from **react-imported-component**.



# Hybrid usage



The advanced pattern described in [Optimizing CSS Delivery](https://dev.to/thekashey/optimising-css-delivery-57eh)

article proposes to:



- inline critical CSS for a first time customers

- use cached `.css` files for recurring



This library does not provide a way to distinguish "one" cohort of customers from another, although, provides an API to

optimize the delivery.



- use `createCriticalStyleStream`/`getCriticalStyles` to **inline** critical CSS

- use `createStyleStream`/`getUsedStyles` to use `.css` files

- use `alterProjectStyles` with `filter` options to create two different sets of styles: not yet _cache_ set

  for `critical` styles, and the _cached_ ones for `used`.

- yes - you have to use or two transformers, or call two functions, one after another.



> Theoretically - all styles "critical" now, are "cached" ones next view.



# Performance



Almost unmeasurable. It's a simple and single RegExp, which is not comparable to the React Render itself.



# Comparison



> comparing with tools listed at [Google's Optimize CSS Delivery](https://developers.google.com/web/tools/lighthouse/audits/unused-css#inlining)



- [penthouse](https://github.com/pocketjoso/penthouse) - a super slow puppetter based solution. No integration with a

  real run time renderer is possible. Generates one big style block at the beginning of a file.

- [critical](https://github.com/addyosmani/critical) - a super slow puppetter based solution. Able to extract critical

  style "above the fold".

- [inline-critical](https://github.com/bezoerb/inline-critical) - slow jsdom based solution. Generates one big style

  block at the beginning of a file, and replaces all other `links` by async variants. However, it does not detect any

  critical or used styles in provided HTML - HTML is used only as a output target. 👎



- [critters-webpack-plugun](https://github.com/GoogleChromeLabs/critters) - is the nearest analog of used-styles, build

  on almost same principles.



`used-styles` is faster that libraries listed above, and optimized for multiple runs.



# License



MIT