This is a very simple and lightweight helper function that makes it easier to set up a focus trap in a modal window. This implementation is not library specific and can therefore be used with VanillaJS, React or any other library/framework.

First import the helper function. You don't necessarily need to name it focusTrap, you can use any name you want.

import focusTrap from 'modal-focus-trap';

Set up the focus trap by passing in an HTMLElement which is your modal element. The function returns a callback you can use later on to disengage the focus trap. Cache this callback somewhere in your code, you will need it later.

const modalElement = document.querySelector('.modal');
let restoreFocus = focusTrap(modalElement);

After closing the modal, you need to disengage the focus trap and restore the focus to the previously focused element by invoking the callback:

if (restoreFocus) {
    restoreFocus();
    restoreFocus = null;
}

You can also create a hook to use this helper with React:

import { useRef, useEffect } from 'react';
import focusTrap from 'modal-focus-trap';

export default function useFocusTrap() {
    const ref = useRef(null);

    // Create on didMount.
    useEffect(() => {
        const destroy = focusTrap(ref.current);

        // Destroy on willUnmount.
        return destroy;
    }, []);

    return ref;
}

See this sample project for more info.