Hi, I see three problems with your code. First of all React requires <tbody> tag to enclose the <tr>. I forgot if it refuses to render if you forget or just complains, but thats number one. Then you do your ajax request in the componentWillMount method. I think you can only do synchronous stuff in there. It should work fine when you use the componentDidMount.
The last thing is that your render method on the LeaderboardRows class isn’t returning a React component. So you should take a look at that. Maybe do the data.map() not in the return, but assign your desired output to a variable and return that. If I find time I’ll try to give it a shot.