How to add multiple classes to a ReactJS Component?

I use classnames when there is a fair amount of logic required for deciding the classes to (not) use. An overly simple example:

...
    var liClasses = classNames({
      'main-class': true,
      'activeClass': self.state.focused === index
    });

    return (<li className={liClasses}>{data.name}</li>);
...

That said, if you don’t want to include a dependency then there are better answers below.