postcss-pixels-to-rem/index.js

Converts pixel measurements into rems or ems. Works with the same pixels to rem or pixels to em syntax as Bourbon, so no rewrite of CSS source code is required.

var postcss = require('postcss');

module.exports = postcss.plugin('postcss-pixels-to-rem', function pixelstorem(options) {
  return function (css) {
      options = options || {};
      var base = options.base ? options.base : 16,
          mediaQueries = options.mediaQueries === undefined ? true: options.mediaQueries,
          exclude = options.exclude || [];

      function findMatches (el) {
        return el.match(/(em\(\d+\)|rem\(\d+\)|\d+px)/ig, "");
      };

      function convertValues(matches) {
        var revised = matches.map(function(el, i) {
          var regExVal = new RegExp(/\d+/, 'g'),
              regExType = new RegExp(/(^em|rem|px)/, 'ig'),
              unit = regExType.exec(el)[0].toString();
          var measureType = options.unit ? options.unit : unit !== 'px' ? unit: 'rem';
          return convertedVal = regExVal.exec(el) / base + measureType;
        });
        return revised
      }

    function replaceItem(item, convertedValues, matches) {
      var revisedParam = item;
      convertedValues.map(function(el, i) {
        revisedParam = revisedParam.replace(matches[i], el);
      });
      return revisedParam
    }

    css.walkRules(function (rule) {
      var ruleParent = rule.parent;

      if (ruleParent.type === "atrule" && ruleParent.name === "media" && mediaQueries) {
        var matches = findMatches(ruleParent.params) || false,
            convertedVal = matches? convertedVal = convertValues(matches): false;

        if (convertedVal) {
          rule.parent.params = replaceItem(rule.parent.params, convertedVal, matches);
        }
      }
      rule.walkDecls(function (decl, i) {
        var excludedTest = exclude.some(function(el, i){
            return decl.prop === el;
        });
        var matches = findMatches(decl.value);
        if (matches && !excludedTest) {
          var convertedValues = convertValues(matches);
          decl.value = replaceItem(decl.value, convertedValues, matches);
        }
      });
    });
  }
});