How to use the dom-lib.activeElement function in dom-lib

focus() {
        let autoFocus = this.props.autoFocus;
        let modalContent = this.getDialogElement();
        let current = activeElement(ownerDocument(this));
        let focusInModal = current && contains(modalContent, current);

        if (modalContent && autoFocus && !focusInModal) {
            this.lastFocus = current;

            if (!modalContent.hasAttribute('tabIndex')) {
                modalContent.setAttribute('tabIndex', -1);
                new Error(false, 'The modal content node does not accept focus. ' + 'For the benefit of assistive technologies, the tabIndex of the node is being set to "-1".');
            }

            modalContent.focus();
        }
    },

checkForFocus() {
        if (canUseDom) {
            this.lastFocus = activeElement();
        }
    },

enforceFocus() {
        let {enforceFocus} = this.props;

        if (!enforceFocus || !this.isMounted() || !this.isTopModal()) {
            return;
        }

        let active = activeElement(ownerDocument(this));
        let modal = this.getDialogElement();

        if (modal && modal !== active && !contains(modal, active)) {
            modal.focus();
        }
    },