CSS-in-JS 101

• What is CSS-in-JS?
• Inline styles
  • Pros
  • Cons
  • Inline styles vs CSS-in-JS
• Style tag
  • Pros
  • Cons
• CSS Modules
  • Pros
  • Cons
  • Dead Code Elimination, Critical CSS
• Global Name Space, Globally Unique Identifier
• Dependencies
• Minification
  • Traditional CSS minification
  • Minification of class name
  • Dead Code Elimination
  • Critical CSS
  • Automatic Atomic CSS
• Sharing Constants, variables in CSS
  • Sharing variables inside CSS
  • Sharing variables in CSS and exposing it to JS
  • Passing variable from JS to CSS
• Non-deterministic Resolution
• Isolation
• Theming
  • Overriding styles
  • Overriding theme variables
• SSR, Server-Side Rendering
  • HTML SSR
  • CSS SSR
• Zero runtime dependency
• CSS-in-JS implementation specific features
  • Non-DOM targets
  • CSS as object (object literal)
  • CSS as template literal
  • Framework agnostic
  • Build step
  • Dynamic
  • Generate components based on CSS
  • Developer tools integration
• Progressive enhancement, graceful degradation
• Uncovered subjects
  • Security
  • Async components
  • CSS-in-JS and Async components
  • Atomic CSS
  • Animations

What is CSS-in-JS?

CSS-in-JS is an umbrella term for technologies which help you define styles in JS in more component-approach-way. The idea was introduced by @vjeux in 2014. Initially, it was described as inline styles, but since then battlefield changed a lot. There are about 50 different solutions in this area.

References

• @vjeux, 2014
• Big list of CSS-in-JS solutions
• Comparison of CSS-in-JS by styled-components
• Yet another comparison

Inline styles

This is built in feature of React. You can pass styles as an object to the component and it will be converted to the string and attached as style attribute to the element.

Pros

• No gloabl namespace
• Full isolation
• No non-deterministic resolution
• Clear dependencies
• Dead code elimination
• Variables, Passing variable from JS to CSS

Cons

• Code duplication in case of SSR.
• Additional costs in JS payload. Remember that styles which are embedded in JS are not for free. It is not only about download time, it is also about parsing and compiling. See this detailed explanation by Addy Osmani, why JS is expensive
• No media queries (@media)
• No CSS animations (@keyframes)
• No pseudo classes (:hover)
• No web fonts (@font)
• No autoprefixer (well there is inline-style-prefixer)

Example

TODO: verify

JSX:

hundred_length_array
  .map(x => <div key={x} style={{color: "#000"}}></div>)

Generated HTML:

<div style="color:#000"></div>
...(98 times)
<div style="color:#000"></div>

Inline styles vs CSS-in-JS

@mxstbr differentiate Inline styles and CSS-in-JS. By Inline styles he means React built-in support for style attribute and by CSS-in-JS he means a solution which generates CSS and injects it via style tag.

On the other hand, CSS-in-JS is the term coined by @vjeux in 2014 and he meant Inline styles. Inline styles is not React-only feature. There is, for example, Radium which also uses inline styles.

So I would suggest to use CSS-in-JS as an umbrella term and specify implementation:

• inline styles
• style tag. Also can be referred as "style element" or "real CSS"
• mixed (like Radium)

References

• @mxstbr, 2016

Style tag

This approach is alternative to Inline styles. Instead of attaching styles as property to the element you are inserting real CSS in style tag and append style tag to the document.

Pros and cons can vary from implementation to implementation. But basically, it looks like this:

Pros

• (Almost) No global namespace
• (Almost) Full isolation
• (Almost) No non-deterministic resolution
• Clear dependencies
• Dead code elimination
• Variables (depends on implementation)
• No code duplication in case of SSR
• Additional costs in JS payload (depends on implementation)
• Media queries (@media)
• CSS animations (@keyframes)
• Pseudo-classes (:hover)
• Web fonts (@font)

Cons

Cons depend on implementation.

Example

from this blog post:

const MyStyledComponent = props =>
  <div className="styled">
    Hover for red
    <style dangerouslySetInnerHTML={{__html: `
      .styled { color: blue }
    `}} />
  </div>

Generated HTML:

<div class="styled">
  Hover for red
  <style>      .styled { color: blue }    </style>
</div>

CSS Modules

A CSS Module is a CSS file in which all class names and animation names are scoped locally by default. All URLs (url(...)) and @imports are in module request format (./xxx and ../xxx means relative, xxx and xxx/yyy means in modules folder, i. e. in node_modules).

Pros

• (Almost) No global namespace
• (Almost) Full isolation
• (Almost) No non-deterministic resolution
• Clear dependencies
• (Almost) Dead code elimination
• Variables, Sharing variables in CSS and exposing it to JS
• No Code duplication in case of SSR
• No Additional costs in JS payload.
• Media queries (@media)
• CSS animations (@keyframes)
• Pseudo-classes (:hover)
• Web fonts (@font)
• Autoprefixer

Cons

See all points with "(Almost)"

Dead Code Elimination, Critical CSS

Strictly speaking, there are no official solutions to those problems in CSS Modules, but there is some work in this direction. Correct me if I'm wrong if there is one, why isn't it promoted?

• Comment by @sokra on critical CSS
• isomorphic-style-loader
• Support Tree shaking of CSS
• Atomic CSS a la styletron
• CSSO scopes

References

• css-modules
• @markdalgleish, 2015

Global Name Space, Globally Unique Identifier

All declarations in CSS are global, which is bad because you never know what part of application change in global scope will affect.

Possible solutions

• Attach styles to each element (Inline styles)
• Use Globally Unique Identifiers for classes (CSS modules)
• Use naming conventions (BEM and others)

References

• @vjeux, 2014

Dependencies

Ability to programmatically resolve dependency between component (JS and HTML) and styles, to decrease error of forgetting to provide appropriate styles, to decrease fear of renaming CSS classes or moving them between files.

Possible solutions

• bundle styles with-in component (CSS-in-JS)
• import styles from "styles.css" (CSS modules)

Related

• Dead Code Elimination

Minification

There is more than one aspect of minification. Let's explore:

Traditional CSS minification

This is the simplest approach - remove whitespace, minify color name, remove unnecessary quotes, collapse CSS rules etc. See big list of minifiers here

Minification of class name

In CSS modules and CSS-in-JS you do not use class names directly, instead, you use JS variables, so class names can be easily mangled.

Note: This type of minification is not possible for traditional CSS.

Example

import styles from "styles.css";

<div className={styles.example} />

styles compiles to { example: "hASh"}

Dead Code Elimination

Because there is no connection between JS/HTML and CSS, you cannot be sure if it is safe to remove some parts of CSS or not. If it is stale or not? If it is used somewhere or not?

CSS-in-JS solves this problem because of a link between JS/HTML and CSS is known, so it is easy to track if this CSS rule required or not.

Related

• Dependencies
• Critical CSS

References

• @vjeux, 2014

Critical CSS

The ability of a system to extract and inline styles in head required for current page viewed by the user not more nor less.

Note: this is slightly different from the definition by @addyosmani, which defines critical as above-the-fold.

Example

aphrodite:

import { StyleSheet, css } from 'aphrodite'

const styles = StyleSheet.create({
  heading: { color: 'blue' }
})

const Heading = ({ children }) => (
  <h1 className={css(styles.heading)}>
    { children }
  </h1>
)

import { StyleSheetServer } from 'aphrodite'

const { html, css } = StyleSheetServer.renderStatic(
  () => ReactDOMServer.renderToString(<App />)
)

Related

• Dependencies
• Dead Code Elimination
• SSR

References

• @markdalgleish, 2017

Automatic Atomic CSS

In CSS modules and CSS-in-JS you do not use class names directly, instead, you use JS variables, so class names can be easily mangled. The same as in "Minification of class name". But we can go further - generate smaller classes and reuse them to achieve smaller CSS

Note: This type of minification is not possible for traditional CSS.

Example

styletron

import {injectStyle} from 'styletron-utils';
injectStyle(styletron, {
  color: 'red',
  display: 'inline-block'
});
// → 'a d'
injectStyle(styletron, {
  color: 'red',
  fontSize: '1.6em'
});
// → 'a e'

Related

• Atomic CSS

References

• @markdalgleish, 2017

Sharing Constants, variables in CSS

There are different approaches.

Sharing variables inside CSS

• CSS variables
• Can I use css variables
• CSS variables in preprocessors, like PostCSS, SASS etc.

Sharing variables in CSS and exposing it to JS

This is mainly a feature of CSS modules with variables.

Example

postcss-icss-values

/* colors.css */
@value primary: #BF4040;

Sharing variables in CSS:

@value primary from './colors.css';

.panel {
  background: primary;
}

Exposing it to JS:

import { primary } from './colors.css';
// will have similar effect
console.log(primary); // -> #BF4040

Passing variable from JS to CSS

This is only possible with CSS-in-JS. This approach gives maximum flexibility and dynamics.

Example

styling

import styling from 'styling'
import {baseColor} from './theme'

export let button = styling({
  backgroundColor: baseColor
})

Related

• Overriding theme variables

Non-deterministic Resolution

Resolution depends on the order of declarations in stylesheets (if declarations have the same specificity).

References

• @vjeux, 2014

Isolation

Because of CSS cascading nature and Global Name Space, there is no way to isolate things. Any other part code can use more specificity or use !important to override your "local" styles and it is hard to prevent this situation

Strictly speaking, only inline styles gives full isolation. Every other solution gives just a bit more isolation over pure CSS, because of solving Global Name Space problem.

References

• @vjeux, 2014

Two CSS properties walk into a bar.

A barstool in a completely different bar falls over.

— Thomas "Kick Nazis out, @jack" Fuchs (@thomasfuchs) July 28, 2014

Theming

The idea is to be able to change the look of existing components without the need to change actual code.

Overriding styles

This way you can override styles based on "class names" (keys of objects in case of inline styles).

Example

react-themeable

With CSS modules:

import theme from './MyComponentTheme.css';
<MyComponent theme={theme} />

Same with inline styles:

const theme = {
  foo: {
    'color': 'red'
  },
  bar: {
    'color': 'blue'
  }
};
<MyComponent theme={theme} />

Overriding theme variables

This way you can override styles based on variables passed to the theme. The theme basically works like a function - accepts variables as input and produce styles as a result.

Related

• Variables, Passing variable from JS to CSS

SSR, Server-Side Rendering

HTML SSR

Make sure that CSS-in-JS solution doesn't brake default React isomorphism e.g. you are able to generate HTML on the server, but not necessary CSS.

CSS SSR

Be able to prerender CSS on the server the same way as HTML can be prerendered for React.

Example

aphrodite:

import { StyleSheet, css } from 'aphrodite'

const styles = StyleSheet.create({
  heading: { color: 'blue' }
})

const Heading = ({ children }) => (
  <h1 className={css(styles.heading)}>
    { children }
  </h1>
)

import { StyleSheetServer } from 'aphrodite'

const { html, css } = StyleSheetServer.renderStatic(
  () => ReactDOMServer.renderToString(<App />)
)

TODO: add example with Inline Styles

Related

• Critical CSS

Zero runtime dependency

Almost all CSS-in-JS solutions have runtime dependency e.g. library required to generate styles at runtime and CSS encoded as JS.

Some solutions do not have this issue, they basically vanished after build step. Examples: CSS modules, linaria.

Example

linaria

import React from 'react';
import { css, styles } from 'linaria';

const title = css`
  text-transform: uppercase;
`;

export function App() {
  return <Header {...styles(title)} />;
}

Transpiled to:

.title__jt5ry4 {
  text-transform: uppercase;
}

import React from 'react';
import { styles } from 'linaria/build/index.runtime';

const title = 'title__jt5ry4';

export function App() {
  return <Header {...styles(title)} />;
}

CSS-in-JS implementation specific features

Non-DOM targets

React can target different platforms, not just DOM. It would be nice to have CSS-in-JS solution which supports different platforms too. For example: React Native, Sketch.

CSS as object (object literal)

const color = "red"
const style = {
  color: 'red',
}

CSS as template literal

const color = "red"
const style = `
  color: ${color};
`

Framework agnostic

Does it depend on React or not?

Build step

If build step required or not?

Related

• SSR
• Progressive enhancement
• Dynamic

Dynamic

If you can pass values to CSS at runtime.

Note: it is not the same as Variables, Passing variable from JS to CSS, for example in linaria you can pass variables from JS to CSS, but only at build time.

Note 2: cannot stop myself from drawing analogy between static and dynamic type systems.

Related

• Build step
• Variables, Passing variable from JS to CSS

Generate components based on CSS

If your component has pretty simple structure and you care more about how it looks instead of markup (which most likely will be div anyway). You can go straight to write CSS and library will generate components for you.

Examples

decss

import React from 'react'
import { Button } from './style.css'

<Button>
  Panic
</Button>

styled-components

import React from 'react'
import styled from 'styled'

const Button = styled.button`
  background-color: red;
`;

<Button>
  Panic
</Button>

Developer tools integration

If there are special perks for developer tools?

Example

emotion supports source maps for styles authored in javascript

GIF of source maps in action

Progressive enhancement, graceful degradation

If you do not know what is it read this article.

In the context of CSS-in-JS it boils down to one question - will your website be styled with disabled JS.

The first requirement would be to have some HTML rendered on the server (SSR or snapshoting). After this you have two options:

• prebuild CSS e.g. Build step required
• rendered CSS e.g. CSS SSR required

Related

• SSR
• Build step

Uncovered subjects

Security

See this post

Async components

Also known as code splitting, dynamic import

Async component is a technique (typically implemented as a higher order component) for loading components with dynamic import. There are a lot of solutions in this field here are some examples:

• loadable-components
• react-loadable
• react-async-component
• react-code-splitting

References

• Dynamic import is the TC39 proposal.

Webpack has a feature to split your codebase into “chunks” which are loaded on demand. Some other bundlers call them “layers”, “rollups”, or “fragments”. This feature is called “code splitting”.

— Code splitting

CSS-in-JS and Async components

This works for most CSS-in-JS solutions because CSS is bundled inside JS. This is a more complicated task for CSS modules.

Atomic CSS

Also known as immutable, functional, utility-class.

Idea boils down to use one property per class, so you create required look by composing more than one class. Because each class contains only one property, you do not override those properties and this can be interpreted as immutability.

Do not confuse with Atomic CSS framework.

References

• Functional CSS - The Good, The Bad, and Some Protips for React.js Users
• immutable-css

Animations

Sequential

Basically CSS3 animations. Pros: can be GPU accelerated.

Interruptible

Also known as interactive.

Basically JS animations. Pros: can be interrupted.

References

• Building Interruptible and Responsive Interactions

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