toDisplayString = (val) => {
      return isString(val)
          ? val
          : val == null
              ? ''
              : isArray(val) ||
                  (isObject(val) &&
                      (val.toString === objectToString || !isFunction(val.toString)))
                  ? JSON.stringify(val, replacer, 2)
                  : String(val);
  }
For converting {{ interpolation }} values to displayed strings.
@private