gistfile1.txt

 /** Force number in input field of Organisator phone number */
    let textarea = document.getElementById('event_form_author_phone');
    textarea.addEventListener('keydown', (e) => {
        console.log(e.key);
        if (['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'Backspace'].indexOf(e.key) === -1) {
            e.preventDefault();
        }
    });