Make this a single function (i.e., always send in an object with 1 or more properties), and move it to the prototype:

```javascript
BoundingBox.prototype.applyStyles(styles) {
  var div = this.div;
  Object.keys(styles).forEach(function(style) {
    div.style[ style ] = styles[ style ];
  });
};
```
Now convert all your callers of `applyStyle` to pass an object with a single prop/value.