JavaScript implementaion of ruby's gsub

JS gsub

gsub = function(source, pattern, replacement) {
  var match, result;
  if (!((pattern != null) && (replacement != null))) {
    return source;
  }
  result = '';
  while (source.length > 0) {
    if ((match = source.match(pattern))) {
      result += source.slice(0, match.index);
      result += (typeof replacement === 'function') ? replacement(match[0]) : replacement;
      source = source.slice(match.index + match[0].length);
    } else {
      result += source;
      source = '';
    }
  }
  return result;
};