source: node_modules/react-syntax-highlighter/scripts/build-stylesheets-highlightjs.js

'use strict';
/*
 * Quick and dirty script to build javascript stylesheets from highlight.js css
 */
const path = require('path');
const fs = require('fs');
const css = require('css');
const camel = require('to-camel-case');
const autogenMessage =
  '//\n// This file has been auto-generated by the `npm run build-styles-hljs` task\n//\n\n';

fs.readdir(
  path.join(__dirname, '../node_modules/highlight.js/styles'),
  (err, files) => {
    if (err) {
      throw err;
    }

    files.forEach(file => {
      if (file.includes('.css')) {
        createJavascriptStyleSheet(file);
      }
    });
    const onlyCSSFiles = files.filter(file => file.includes('.css'));
    const availableStyleNames = onlyCSSFiles.map(file =>
      file.split('.css')[0] === 'default'
        ? 'default-style'
        : file.split('.css')[0]
    );
    const styles = availableStyleNames.map(name => `\n* ${camel(name)}`);
    const defaultExports = availableStyleNames.map(
      name => `export { default as ${camel(name)} } from './${name}';\n`
    );
    const styleMD = `## Available \`stylesheet\` props ${styles.join('')}`;
    fs.writeFile(
      path.join(__dirname, '../AVAILABLE_STYLES_HLJS.MD'),
      styleMD,
      err => {
        if (err) {
          throw err;
        }
      }
    );
    fs.writeFile(
      path.join(__dirname, '../src/styles/hljs/index.js'),
      defaultExports.join(''),
      err => {
        if (err) {
          throw err;
        }
      }
    );

    const demoStylesArray = `${autogenMessage}export default [${availableStyleNames
      .sort()
      .map(style => `\n  '${style}'`)}\n];`;
    fs.writeFile(
      path.join(__dirname, '../demo/styles/hljs.js'),
      demoStylesArray,
      err => {
        if (err) {
          throw err;
        }
      }
    );
  }
);

function createJavascriptStyleSheet(file) {
  const ignoreStyleWithThis = '.hljs a';
  const fileWithoutCSS =
    file.split('.css')[0] === 'default'
      ? 'default-style'
      : file.split('.css')[0];
  fs.readFile(
    path.join(__dirname, `../node_modules/highlight.js/styles/${file}`),
    'utf-8',
    (err, data) => {
      if (err) {
        throw err;
      }
      const javacriptStylesheet = css
        .parse(data)
        .stylesheet.rules.reduce((sheet, rule) => {
          if (rule.type === 'rule') {
            const style = rule.selectors.reduce((selectors, selector) => {
              if (!selector.includes(ignoreStyleWithThis)) {
                const selectorObject = rule.declarations.reduce(
                  (declarations, declaration) => {
                    if (
                      declaration.type === 'declaration' &&
                      declaration.property
                    ) {
                      declarations[camel(declaration.property)] =
                        declaration.value;
                    }
                    return declarations;
                  },
                  {}
                );
                selectors[selector.substring(1)] = selectorObject;
              }
              return selectors;
            }, {});
            sheet = Object.keys(style).reduce((stylesheet, selector) => {
              if (stylesheet[selector]) {
                stylesheet[selector] = Object.assign(
                  {},
                  stylesheet[selector],
                  style[selector]
                );
              } else {
                stylesheet[selector] = style[selector];
              }
              return stylesheet;
            }, sheet);
          }
          return sheet;
        }, {});
      fs.writeFile(
        path.join(__dirname, `../src/styles/hljs/${fileWithoutCSS}.js`),
        `export default ${JSON.stringify(javacriptStylesheet, null, 4)}`,
        err => {
          if (err) {
            throw err;
          }
        }
      );
    }
  );
}