[ruslan_mart]

.class2 {
   background: #222;
   height: 100px;
   width: 100px;
}

<div id="myDiv" class="class1"></div>

function divClass(a) {
   divClass.elem.classList[a ? 'add' : 'remove']('class2');
}

divClass.elem = document.getElementById('myDiv');

window.onresize = function() {
   divClass(this.innerWidth < 1000);
}

window.onresize();