ex.js

let div = document.createElement('div');

div.style.height = '100px';
div.style.width = '100px';
div.style.background = 'red';
div.style.display = 'inline-block';


document.body.appendChild(div);

let animator = animate(100, 500, 30);

let animationStep = function () {
    const next = animator.next();
    if (next.done) return;
    div.style.width = next.value + 'px';
    requestAnimationFrame(animationStep);
}

animationStep();