Option 1
Name your export instead of using default. It should look like this
// add.js
export const add = (a, b) => a + b;
// OR
// export const add = function(a, b) { return a+b };
// app.js
import { add } from './add';
Option 2
Use the export default syntax. It looks like this
// add.js
export default function add(a, b) {
return a + b;
}
// app.js
import add from './add';