React.cloneElement -> Add className element

The structure of React elements is stable, see Nodes and Elements, so your approach is completely safe (and recommended).

If you do a lot of className manipulation, I recommend using the classnames module.

Your previous example would then become:

var cx = require('classnames');

React.cloneElement(hello2, {
  className: cx(hello2.props.className, "test1"),
});