BookStack/resources/assets/js/services/dom-polyfills.js

/**
 * Polyfills for DOM API's
 */

// https://developer.mozilla.org/en-US/docs/Web/API/Element/matches
if (!Element.prototype.matches) {
    Element.prototype.matches = Element.prototype.msMatchesSelector || Element.prototype.webkitMatchesSelector;
}

// https://developer.mozilla.org/en-US/docs/Web/API/Element/closest#Browser_compatibility
if (!Element.prototype.closest) {
    Element.prototype.closest = function (s) {
        var el = this;
        var ancestor = this;
        if (!document.documentElement.contains(el)) return null;
        do {
            if (ancestor.matches(s)) return ancestor;
            ancestor = ancestor.parentElement;
        } while (ancestor !== null);
        return null;
    };
}