nodejs-repl can't evaluate the following JavaScript code in Emacs?

I am using nodejs-repl.el with Emacs and nodejs to evaluate JavaScript code. Everything has been fine until I tested the following code:

//define a Mammal psuedo-class
var Mammal = function (name) {  this.name = name; };
Mammal.prototype.get_name = function ( ) {    return this.name; };
Mammal.prototype.says = function ( ) {  return this.saying || ''; }; //!problem line

The above code is from the book "JavaScript, the good parts". It runs in every other environment I tested: node-repl on the command line, the online https://babeljs.io/repl . But it fails in Emacs with nodejs-repl.el.

The problem is with the last line, from which I get the following error:

> Mammal.prototype.says = function ( ) {  return this.saying   ''; };
SyntaxError: repl: Unexpected token, expected ; (1:61)
> 1 | Mammal.prototype.says = function ( ) {  return this.saying   ''; };
    |                                                              ^

Can anyone please help explain what went wrong and how to fix my Emacs environment (nodejs-repl.el)?

My Emacs (24.5 Ubuntu 16.04) configuration is below:

(require 'comint)
(add-to-list 'comint-preoutput-filter-functions
             (lambda (output)
               (let ((case-fold-search nil))
         (replace-regexp-in-string "\033\\[[0-9]+[A-Z]" "" output))))

(setq babel-repl-cli-arguments '("--presets"  "es2015" ))

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