add-plus-one-button-to-toster-comments.js

(function(){
    const links = document.querySelectorAll('a.comment__reply[href^="#mention"]');

    for (let i = 0; i < links.length; i++) {
        let link = links[i];
        let login = link.dataset.username;
        let parent = link.parentElement;
        let form = parent.parentElement.parentElement.parentElement.nextElementSibling;

        let textarea = form.querySelector('textarea.form__input');

        let submit = form.querySelector('button[type="submit"]');

        let plusButton = document.createElement('a');
        plusButton.href = "#";
        plusButton.className = "toster-plus-one";
        plusButton.innerHTML = "+1";
        plusButton.onclick = function(e) {
            e.preventDefault();
            let val = textarea.value;
            textarea.value = `@${login}: +1 ${val}`;
            submit.disabled = "";
        }
        parent.appendChild(plusButton);
    }
})();