loop.js

_.find(data.messages, function(message) {
  if (data.message_id == message.id) {
    showMessage(message);
    return;
  }
});
  
for (var i=0; i< data.messages.length; i++) {
  if (data.message_id == data.messages[i].id) {
    showMessage(data.messages[i]);
    break;
  }
}

What about:

showMessage(_.findWhere(data.messages, { id: data.message_id }));