docs: function toNumberOrUndefined(n) {
      if (n === null || n === undefined) { return undefined; }
      if (validation.isObject(n) || validation.isArray(n)) { return undefined; }
      if (typeof n === 'boolean' || (typeof n === 'string' && n.length < 1)) { return undefined; }
      return isNaN(+n) ? undefined : +n;
  }