Use React's [classSet addon](https://facebook.github.io/react/docs/class-name-manipulation.html) when building a list of classes like this. Marathon is using a build of React with addons, so the function is available:

```javascript
var instancesClassSet = React.addons.classSet({
    "instances": true,
    "text-right": true,
    "has-error": !model.allInstancesBooted()
});
```