How export and import classes in Javascript?

Question

Why isn't working?

index html:

<script src="app.js" type="module"></script>
<script src="cat.js" type="module"></script>

cat.js

export class Cat  {
    getCat() {
 console.log('cat')
 }
}

app.js

import {Cat} from "./cat";

let cat = new Cat();
cat.getCat();

I get this error:

Failed to load resource: the server responded with a status of 404 (Not Found)

What can I do?