@stylexjs/stylex

@stylexjs/stylex

StyleX is a JavaScript library for defining styles for optimized user interfaces.

Installation

To start playing with StyleX without having to set up any build settings you can install just two packages:

npm install --save @stylexjs/stylex

Compiler

StyleX is designed to extract styles to a static CSS style sheet during an app's build process. StyleX provides a Babel plugin along with plugin integrations for Webpack, Rollup and NextJS.

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

For more information on working with the compiler, please see the documentation for @stylexjs/babel-plugin.

Runtime compiler

The runtime compiler should only be used for development and testing purposes.

npm install --save-dev @stylexjs/dev-runtime

Import @stylexjs/dev-runtime in your JS entry-point to set everything up.

import inject from '@stylexjs/dev-runtime';

if (process.env.NODE_ENV !== 'production') {
  inject({
    // configuration options
    classNamePrefix: 'x-',
    dev: true,
    test: false,
  });
}

For more information on working with the compiler, please see the documentation for @stylexjs/dev-runtime.

API

stylex.create()

Styles are defined as a map of CSS rules using stylex.create(). In the example below, there are 2 different CSS rules. The names "root" and "highlighted" are arbitrary names given to the rules.

import stylex from '@stylexjs/stylex';

const styles = stylex.create({
  root: {
    width: '100%',
    color: 'rgb(60,60,60)',
  },
  highlighted: {
    color: 'yellow',
  }
});

Pseudo-classes and Media Queries can be nested within style definitions:

import stylex from '@stylexjs/stylex';

const styles = stylex.create({
  root: {
    width: '100%',
    color: 'rgb(60,60,60)',
    '@media (min-width: 800px)': {
      maxWidth: '800px',
    },
  },
  highlighted: {
    color: 'yellow',
    ':hover': {
      opacity: '0.9',
    }
  }
});

The compiler will extract the rules to CSS and replace the rules in the source code with a "compiled style" object.

stylex.spread()

Applying style rules to specific elements is done using stylex.spread. Each argument to this function must be a reference to a compiled style object, or an array of compiled style objects. The function merges styles from left to right.

<div {...stylex.spread(styles.root, styles.highlighted)} />

The stylex.spread function returns React props as required to render an element. StyleX styles can still be passed to other components via props, but only the components rendering host platform elements will use stylex.spread(). For example:

const styles = stylex.create({
  internalRoot: {
    padding: 10
  },
  exportedRoot: {
    position: 'relative'
  }
});

function InternalComponent(props) {
  return <div {...props} {...stylex.spread([ styles.internalRoot, props.style ])} />
}

export function ExportedComponent(props) {
  return <InternalComponent style={[ styles.exportedRoot, props.style ]} />
}

Styles can be conditionally included using standard JavaScript.

<div {...stylex.spread(styles.root, isHighlighted && styles.highlighted)} />

And the local merging of styles can be used to control the relative priority of rules. For example, to allow a component's local styles to take priority over style property values passed in via props.

<div {...stylex.spread(props.style, styles.root)} />

You can even mix compiled styles with inline styles

<div {...stylex.spread(styles.root, { opacity })} />

stylex.firstThatWorks()

Defining fallback styles is done with stylex.firstThatWorks(). This is useful for engines that may not support a specific style property.

import stylex from '@stylexjs/stylex';

const styles = stylex.create({
  header: {
    position: stylex.firstThatWorks('sticky', '-webkit-sticky', 'fixed'),
  },
});

This is equivalent to defining CSS as follows:

.header {
  position: fixed;
  position: -webkit-sticky;
  position: sticky;
}

Types

StyleX comes with full support for Static Types.

XStyle<>

The most common type you might need to use is XStyle<>. This lets you accept an object of arbitrary StyleX styles.

type Props = {
  ...
  style?: XStyle<>,
};

function MyComponent({style, ...}: Props) {
  return (
    <div {...stylex.spread(localStyles.foo, localStyles.bar, style)} />
  );
}

XStyleWithout<>

To disallow specific style properties, use the XStyleWithout<> type.

type Props = {
  // ...
  style?: XStyleWithout<{
    postion: unknown,
    display: unknown
  }>
};

XStyleValue<>

To accept specific style properties only, use the XStyle<{...}> and XStyleValue types. For example, to allow only color-related style props:

type Props = {
  // ...
  style?: XStyle<{
    color?: StyleXValue,
    backgroundColor?: StyleXValue,
    borderColor?: StyleXValue,
    borderTopColor?: StyleXValue,
    borderEndColor?: StyleXValue,
    borderBottomColor?: StyleXValue,
    borderStartColor?: StyleXValue,
  }>,
};

XStyleValueFor<>

To limit the possible values for style properties, use the XStyleValueFor<> type. Pass in a type argument with a union of literal types that provide the set of possible values that the style property can have. For example, if a component should accept marginTop but only accept one of 0, 4, or 8 pixels as values.

type Props = {
  ...
  style?: XStyle<{
    marginTop: XStyleValueFor<0 | 4 | 8>
  }>,
};

How StyleX works

StyleX produces atomic styles, which means that each CSS rule contains only a single declaration and uses a unique class name. For example:

import stylex from '@stylexjs/stylex';

const styles = stylex.create({
  root: {
    width: '100%',
    color: 'red',
  }
}

From this code, StyleX will generate 2 classes. One for the width: '100%' declaration, and one for the color: 'red' declaration. If you use the declaration width: '100%' anywhere else in your application, it will reuse the same CSS class rather than creating a new one.

One of the benefits of this approach is that the generated CSS file grows logarithmically as you add new styled components to your app. As more style declarations are added to components, they are more likely to already be in use elsehwere in the app. As a result of this CSS optimization, the generated CSS style sheet for an app is usually small enough to be contained in a single file and used across routes, avoiding style recalculation and layout thrashing as users navigate through your app.