function calc(str, rem = DEFAULT_REM) {
  let koefStr = str.substr(0, str.length - SUFFIX.length);
  let koef = koefStr === '' ? 1 : parseFloat(koefStr);
  if (isNaN(koef)) {
    throw new Error('Invalid rem value: ' + str);
  }
  return rem * koef;
}
Calculate rem to pixels: '1.2rem' => 1.2 * rem
@param {String} str
@param {Number} rem
@returns {number}