[рони]

dmk,

var btnMenu = document.querySelectorAll('.btn-menu'),
    subMenu = document.querySelectorAll('.sub-menu'),
    i, btn, sub;



for (i = 0; i < btnMenu.length; i++) {
    btn = btnMenu[i];
    btn.addEventListener('click', menuOpen(i));
};

function menuOpen(i) {
    return function (event) {
        event.preventDefault();
        [].forEach.call( btnMenu , function(el,a) {
               a == i ?  (el.classList.toggle('btn-menu--active'), subMenu[a].classList.toggle('sub-menu--show')) : (el.classList.remove('btn-menu--active'), subMenu[a].classList.remove('sub-menu--show'));
        });

    };
};