@emotion/babel-plugin

What is @emotion/babel-plugin?

The @emotion/babel-plugin package is a Babel plugin for Emotion, a popular library for writing CSS styles with JavaScript. This plugin enhances the developer experience and optimizes the runtime performance of styles defined in JavaScript by providing features such as source maps for better debugging, auto-labeling for easier identification of styles, and more efficient style insertion.

What are @emotion/babel-plugin's main functionalities?

Source Maps

Generates source maps for the styles, allowing developers to trace back the generated styles to the original source code, which is helpful for debugging.

/* No specific code for this feature as it works behind the scenes to provide source maps for styles */

Auto Labeling

Automatically adds labels to the class names generated by Emotion, making it easier to identify which component a class name refers to in the DOM.

`css
css	ext-align: center;

css
color: hotpink;
`

Optimized Style Insertion

Optimizes the way styles are inserted into the DOM, improving performance by reducing the number of reflows and repaints.

/* No specific code for this feature as it optimizes the style insertion during the build process */

Other packages similar to @emotion/babel-plugin

babel-plugin-styled-components

This plugin is similar to @emotion/babel-plugin but is tailored for styled-components, another popular CSS-in-JS library. It provides features like SSR support, better debugging with readable class names, and minification of styles.

README

@emotion/babel-plugin

Babel plugin for the minification and optimization of emotion styles.

@emotion/babel-plugin is highly recommended, but not required in version 8 and above of Emotion.

Features

Feature/Syntax	Native	Babel Plugin Required	Notes
css``	✅
css(...)	✅		Generally used for object styles.
components as selectors		✅	Allows an emotion component to be used as a CSS selector.
Minification		✅	Any leading/trailing space between properties in your css and styled blocks is removed. This can reduce the size of your final bundle.
Dead Code Elimination		✅	Uglifyjs will use the injected /*#__PURE__*/ flag comments to mark your css and styled blocks as candidates for dead code elimination.
Source Maps		✅	When enabled, navigate directly to the style declaration in your javascript file.
Contextual Class Names		✅	Generated class names include the name of the variable or component they were defined in.

Example

In

const myStyles = css`
  font-size: 20px;
  @media (min-width: 420px) {
    color: blue;
    ${css`
      width: 96px;
      height: 96px;
    `};
    line-height: 26px;
  }
  background: green;
  ${{ backgroundColor: 'hotpink' }};
`

Out

const myStyles = /* #__PURE__ */ css(
  'font-size:20px;@media(min-width:420px){color:blue;',
  /* #__PURE__ */ css('width:96px;height:96px;'),
  ';line-height:26px;}background:green;',
  { backgroundColor: 'hotpink' },
  ';'
)

Installation

yarn add --dev @emotion/babel-plugin

or if you prefer npm

npm install --save-dev @emotion/babel-plugin

Usage

Via .babelrc (Recommended)

.babelrc

Without options:

{
  "plugins": ["@emotion"]
}

With options:

Defaults Shown

{
  "plugins": [
    [
      "@emotion",
      {
        // sourceMap is on by default but source maps are dead code eliminated in production
        "sourceMap": true,
        "autoLabel": "dev-only",
        "labelFormat": "[local]",
        "cssPropOptimization": true
      }
    ]
  ]
}

Recommended Setup

.babelrc

{
  "plugins": ["@emotion"]
}

Via CLI

babel --plugins @emotion/babel-plugin script.js

Via Node API

require('@babel/core').transform('code', {
  plugins: ['@emotion/babel-plugin']
})

Options

sourceMap

boolean, defaults to true.

This option enables the following:

• Injected source maps for use in browser dev tools

Documentation

Note:

Source maps are on by default in @emotion/babel-plugin but they will be removed in production builds

autoLabel

'dev-only' | 'always' | 'never', defaults to dev-only.

This option enables the following:

• Automatically adds the label property to styles so that class names generated by css or styled include the name of the variable the result is assigned to.
• Please note that non word characters in the variable will be removed (Eg. iconStyles$1 will become iconStyles1) because $ is not valid CSS ClassName Selector

Each possible value for this option produces different output code:

• with dev-only we optimize the production code, so there are no labels added there, but at the same time we keep labels for development environments,
• with always we always add labels when possible,
• with never we disable this entirely and no labels are added.

css

In

const brownStyles = css({ color: 'brown' })

Out

const brownStyles = /*#__PURE__*/ css({ color: 'brown' }, 'label:brownStyles;')

brownStyles's value would be css-1q8eu9e-brownStyles

labelFormat

string, defaults to "[local]".

This option only works when autoLabel is set to 'dev-only' or 'always'. It allows you to define the format of the resulting label. The format is defined via string where variable parts are enclosed in square brackets []. For example labelFormat: "my-classname--[local]", where [local] will be replaced with the name of the variable the result is assigned to.

Allowed values:

• [local] - the name of the variable the result of the css or styled expression is assigned to.
• [filename] - name of the file (without extension) where css or styled expression is located.
• [dirname] - name of the directory containing the file where css or styled expression is located.

This format only affects the label property of the expression, meaning that the css prefix and hash will be prepended automatically.

css

In

// BrownView.js
// autoLabel: 'dev-only'
// labelFormat: '[filename]--[local]'
const brownStyles = css({ color: 'brown' })

Out

const brownStyles = /*#__PURE__*/ css(
  { color: 'brown' },
  'label:BrownView--brownStyles;'
)

BrownView--brownStyles's value would be css-hash-BrownView--brownStyles

styled

In

const H1 = styled.h1({
  borderRadius: '50%',
  transition: 'transform 400ms ease-in-out',
  boxSizing: 'border-box',
  display: 'flex',
  ':hover': {
    transform: 'scale(1.2)'
  }
})

Out

const H1 = /*#__PURE__*/ styled('h1', {
  label: 'H1'
})({
  borderRadius: '50%',
  transition: 'transform 400ms ease-in-out',
  boxSizing: 'border-box',
  display: 'flex',
  ':hover': {
    transform: 'scale(1.2)'
  }
})

H1's class name attribute would be css-hash-H1

cssPropOptimization

boolean, defaults to true.

This option assumes that you are using something to make @emotion/react's jsx function work for all jsx. If you are not doing so and you do not want such optimizations to occur, disable this option.

importMap

This option allows you to tell @emotion/babel-plugin what imports it should look at to determine what it should transform so if you re-export Emotion's exports, you can still use the Babel transforms

An example file:

import { anotherExport } from 'my-package'
import { someExport, thisIsTheJsxExport } from 'some-package'

An example config:

{
  "my-package": {
    "anotherExport": {
      "canonicalImport": ["@emotion/styled", "default"],
      "styledBaseImport": ["my-package/base", "anotherExport"]
    }
  },
  "some-package": {
    "someExport": {
      "canonicalImport": ["@emotion/react", "css"]
    },
    "thisIsTheJsxExport": {
      "canonicalImport": ["@emotion/react", "jsx"]
    }
  }
}

Babel Macros

Instead of using @emotion/babel-plugin, you can use emotion with babel-plugin-macros. Add babel-plugin-macros to your babel config (which is included in Create React App 2.0) and use the imports/packages shown below.

import {
  css,
  keyframes,
  injectGlobal,
  flush,
  hydrate
} from '@emotion/css/macro'
import { jsx, css, Global, keyframes } from '@emotion/react/macro'
import styled from '@emotion/styled/macro'