react-native-less-transformer

Use Less to style your React Native apps.

Behind the scenes the .less files are transformed to react native style objects (look at the examples).

This transformer can be used together with React Native CSS modules.

How does it work?

Your App.less file might look like this:

@nice-blue: #5b83ad;
@light-blue: @nice-blue + #111;

.myClass {
  color: @light-blue;
}
.myOtherClass {
  color: red;
}
.my-dashed-class {
  color: green;
}

When you import your stylesheet:

import styles from "./App.less";

Your imported styles will look like this:

var styles = {
  myClass: {
    color: "#6c94be"
  },
  myOtherClass: {
    color: "red"
  },
  "my-dashed-class": {
    color: "green"
  }
};

You can then use that style object with an element:

Plain React Native:

<MyElement style={styles.myClass} />

<MyElement style={styles["my-dashed-class"]} />

React Native CSS modules using className property:

<MyElement className={styles.myClass} />

<MyElement className={styles["my-dashed-class"]} />

React Native CSS modules using styleName property:

<MyElement styleName="myClass my-dashed-class" />

Installation and configuration

Minimum React Native version for this transformer is 0.52. If you are using an older version, please update to a newer React Native version before trying to use this transformer.

Step 1: Install

npm install --save-dev react-native-less-transformer less

yarn add --dev react-native-less-transformer less

Step 2: Configure the react native packager

For Expo SDK v41.0.0 or newer

Merge the contents from your project's metro.config.js file with this config (create the file if it does not exist already).

metro.config.js:

const { getDefaultConfig } = require("expo/metro-config");

module.exports = (() => {
  const config = getDefaultConfig(__dirname);

  const { transformer, resolver } = config;

  config.transformer = {
    ...transformer,
    babelTransformerPath: require.resolve("react-native-less-transformer")
  };
  config.resolver = {
    ...resolver,
    sourceExts: [...sourceExts, "less"]
  };

  return config;
})();

For React Native v0.72.1 or newer

Merge the contents from your project's metro.config.js file with this config (create the file if it does not exist already).

metro.config.js:

const { getDefaultConfig, mergeConfig } = require("@react-native/metro-config");

const defaultConfig = getDefaultConfig(__dirname);
const { assetExts, sourceExts } = defaultConfig.resolver;

/**
 * Metro configuration
 * https://reactnative.dev/docs/metro
 *
 * @type {import('metro-config').MetroConfig}
 */
const config = {
  transformer: {
    babelTransformerPath: require.resolve("react-native-less-transformer")
  },
  resolver: {
    sourceExts: [...sourceExts, "less"]
  }
};

module.exports = mergeConfig(defaultConfig, config);

LESS options

If you need to pass options (e.g. plugins) to less, you can do so by creating a transformer.js file and doing the following:

const upstreamTransformer = require("@react-native/metro-babel-transformer");
const lessTransformer = require("react-native-less-transformer");

module.exports.transform = function ({ src, filename, options, ...rest }) {
  if (filename.endsWith(".less")) {
    var opts = Object.assign(options, {
      lessOptions: {
        plugins: [require("less-plugin-glob")]
      }
    });
    return lessTransformer.transform({ src, filename, options: opts, ...rest });
  } else {
    return upstreamTransformer.transform({ src, filename, options, ...rest });
  }
};

After that in metro.config.js point the babelTransformerPath to that file:

-require.resolve("react-native-less-transformer")
+require.resolve("./transformer.js")

CSS Custom Properties (CSS variables)

You need version 1.2.1 or newer

:root {
  --text-color: blue;
}

.blue {
  color: var(--text-color);
}

CSS variables are not supported by default, but you can add support for them by using PostCSS and postcss-css-variables plugin.

Start by installing dependencies:

npm install postcss postcss-css-variables react-native-postcss-transformer --save-dev

yarn add postcss postcss-css-variables react-native-postcss-transformer --dev

Add postcss-css-variables to your PostCSS configuration with one of the supported config formats, e.g. package.json, .postcssrc, postcss.config.js, etc.

After that create a transformer.js file and do the following:

const upstreamTransformer = require("@react-native/metro-babel-transformer");
const lessTransformer = require("react-native-less-transformer");
const postCSSTransformer = require("react-native-postcss-transformer");

module.exports.transform = function ({ src, filename, ...rest }) {
  if (filename.endsWith(".less")) {
    return lessTransformer
      .renderToCSS({ src, filename, options })
      .then((css) =>
        postCSSTransformer.transform({ src: css, filename, ...rest })
      );
  } else {
    return upstreamTransformer.transform({ src, filename, ...rest });
  }
};