isFunctionString(fn) {
    if (typeof fn === 'string') {
      return (fn
        .slice(0, 'function'.length)
        .toLowerCase() === 'function');
    }
    return false;
  }
@desc Return TRUE, on a valid JS function string
Note: This does just a VERY simply sanity check. And may give false positives.

@param {String} fn - String of JS function to validate
@returns {Boolean} TRUE if the string passes basic validation