For updated count likes you should send updated count of likes from server
app.post("/index/:id", function(req,res){
TestData.findById(req.params.id, function(err, theUser){
if(err){
console.log(err);
return res.status(500).send('Something went wrong!'); // You should notify user about any error
} else {
theUser.likes += 1;
theUser.save(function(err){
if(err) return res.status(500).send('Something went wrong!');
return res.send({likes_count: theUser.likes});
});
}
});
});