<a href="http://yandex.ru" id="menu-link"> Меню</a>
    <div id='menu' class="menu slide-in-left">
        <nav>
                <a href="">menu 1</a>
               <a href="">menu 2</a>
        </nav>
    </div> 
  <script src="script.js"></script>  

.menu {
    display: flex;
    width: 200px;
    height: 500px;
    border: solid red;
    transition: 2s;
}

#menu {
/*     display: none; */
  left: -250px;
  position: absolute;
}

.slide-in-left {
  
}

var menuLink = document.getElementById('menu-link');
menuLink.addEventListener('click', openMenu, false);

function openMenu(e) {
  e.preventDefault();
    let menu = document.getElementById('menu');
    menu.style.left = 0;
}