index.js

const { convert } = require('startijenn-rem');

const defaults = {
  name: 'rem-convert',
  fallback: false,
  convert: 'rem',
  baseline: 16,
  precision: 5
};

module.exports = (options = {}) => {
  const { name, fallback, convert: to, ...convertOptions } = {...defaults, ...options};
  const regexp = new RegExp('(?!\\W+)' + name + '\\(([^\(\)]+)\\)', 'g');

  return {
    postcssPlugin: 'postcss-rem',
    Once (root) {
      if (fallback && to !== 'px') {
        root.walkDecls((decl) => {
          if (decl.value && decl.value.includes(name + '(')) {
            let values = decl.value.replace(regexp, '$1');
            decl.cloneBefore({
              value: convert(values, 'px', convertOptions)
            });
            decl.value = convert(values, 'rem', convertOptions);
          }
        });
      } else {
        root.replaceValues(regexp, { fast: name + '(' }, (_, values) => convert(values, to, convertOptions));
      }
    }
  }
};

module.exports.postcss = true;