How do I fix "Expected to return a value at the end of arrow function" warning?

The easiest way only if you don't need return something it'ts just return null

The warning indicates that you're not returning something at the end of your map arrow function in every case.

A better approach to what you're trying to accomplish is first using a .filter and then a .map, like this:

this.props.comments
  .filter(commentReply => commentReply.replyTo === comment.id)
  .map((commentReply, idx) => <CommentItem key={idx} className="SubComment"/>);

A map() creates an array, so a return is expected for all code paths (if/elses).

If you don't want an array or to return data, use forEach instead.