Reactjs: Key undefined when accessed as a prop

key and ref aren't really 'props'. They're used internally by react and not passed to components as props. Consider passing it as a prop such as 'id'.

It is best to use id. Then in the eventHandler you can have event.target.id.

function getMessageListItem(message) {

  return (
    <MessageListItem key={message.id} id={message.id} message={message}/>
  );
}

As we already established in other answers that you can't use key since it isn't passed as a prop and instead used internally by react. Here is my 2 cents as an alternative:
Since you're passing the entire array of messages as a prop while creating the <MessageListItem> component, you don't necessarily need to pass another prop with id. You can simply use {this.props.message.id} whenever you need to use id.