Uncaught SyntaxError: Unexpected identifier ? + Uncaught ReferenceError: ElementPart is not defined at HTMLButtonElement

I have 2 js files:
utils.js:

class ElementPart
{
    constructor(text)
    {
        this.text = text;
    }
}

elem.js

const btn = document.querySelector('.elem_btn');

btn.addEventListener("click", () =>
{
    const p = new ElementPart("aaaa");
    console.log(p);
})

When I load the page I get the following error:

Uncaught SyntaxError: Unexpected identifier utils.js:1

And when I click on the button:

Uncaught ReferenceError: ElementPart is not defined
    at HTMLButtonElement.<anonymous> (elem.js:5)
(anonymous) @ elem.js:5

I load the files on the html file like this:

 <script src="/utils.js"></script>
 <script src="/createelem.js"></script>