So I have a box shadow that i want to add to a bar as shown in this example :

<https://jsfiddle.net/eddietal2/qgLvsx2v/>

this is the javascript that I have:

```
var topBar = document.getElementById('top-bar');
var wrapper = document.getElementById('wrapper');

wrapper.onscroll = function () {
  topBar.style.boxShadow = "10px 20px 30px blue"
}

```

The effect I am going for is, when the user scrolls, I want the box shadow to appear, but when they stop scrolling, I want the box shadow to disappear. How can I achieve this effect?