src/js/theme/sidebar.js
function init() {
    // Init last state if not mobile
    if (!platform.isMobile()) {
        toggleSidebar(gitbook.storage.get('sidebar', true), false);
    }

    // Close sidebar after clicking a link on mobile
    $(document).on('click', '.book-summary li.chapter a', function(e) {
        if (platform.isMobile()) toggleSidebar(false, false);
    });
}
function init() {
    // Init last state if not mobile
    if (!platform.isMobile()) {
        toggleSidebar(gitbook.storage.get('sidebar', true), false);
    }

    // Close sidebar after clicking a link on mobile
    $(document).on('click', '.book-summary li.chapter a', function(e) {
        if (platform.isMobile()) toggleSidebar(false, false);
    });
}
Prepare sidebar: state and toggle button