focustrap.js

function trapFocus(element, namespace) {
    var focusableEls = element.querySelectorAll('a[href]:not([disabled]), button:not([disabled]), textarea:not([disabled]), input[type="text"]:not([disabled]), input[type="radio"]:not([disabled]), input[type="checkbox"]:not([disabled]), select:not([disabled])'),
        firstFocusableEl = focusableEls[0];  
        lastFocusableEl = focusableEls[focusableEls.length - 1];
        KEYCODE_TAB = 9;

    element.addEventListener('keydown', function(e) {
        var isTabPressed = (e.key === 'Tab' || e.keyCode === KEYCODE_TAB);

        if (!isTabPressed) { 
            return; 
        }

        if ( e.shiftKey ) /* shift + tab */ {
            if (document.activeElement === firstFocusableEl) {
                lastFocusableEl.focus();
                e.preventDefault();
            }
        } else /* tab */ {
            if (document.activeElement === lastFocusableEl) {
                firstFocusableEl.focus();
                e.preventDefault();
            }
        }

    });
}