index.js

/**
 * Constants.
 */

var EXTENSION_RE = /\((--[\w-]+)\)/;

/**
 * Module export.
 */

module.exports = function customMedia(options) {
  return function (ast) {
    options = options || {};

    var map = options.map || {};
    var indices = [];

    // define custom media query aliases
    ast.rules.forEach(function (rule, i) {
      if (rule.type !== 'custom-media') return;
      map[rule.name] = rule.media;
      indices.push(i);
    });

    // substitute custom media query aliases
    ast.rules.forEach(function (rule, i) {
      if (rule.type !== 'media') return;
      rule.media = rule.media.replace(EXTENSION_RE, function(_, name) {
        var replacement = map[name];
        var column = rule.position.start.column;
        var line = rule.position.start.line;
        var source = rule.position.source;

        if (replacement) {
          return replacement;
        } else {
          console.warn(
            'WARNING: undefined CSS custom media alias "' + name + '" at ' +
            line + ':' + column + (source ? ' in ' + source : '') + '.\n' +
            'The rule has been removed from the output. Please check your ' +
            '@custom-media definitions.'
          );
          indices.push(i);
        }
      });
    });

    // remove @custom-media blocks from css in reverse order to avoid affecting
    // indices before they are removed
    for (var i = indices.length - 1; i >= 0; i -= 1) {
      ast.rules.splice(indices[i], 1);
    }
  };
};