Error handling under Mongoose save()

add an else before res.redirect('/'); . Its probably sending a 400 and then redirecting to /

Alternatively, you could return res.redirect('/'); so code execution would stop.

Using Promises are better and It Fix my issue for me

I had a similar issue and fix it by using the Promise returned by the u.save() in your example.

var mongoose = require("mongoose");
var models = mongoose.models;

exports.user = function (req, res) {
var u = new models.Users(req.body);

u.save()
    .then((user) => {
        // If everything goes as planed
        //use the retured user document for something
         res.redirect("/");
    })
    .catch((error) => {
        //When there are errors We handle them here
        console.log(err);
        res.send(400, "Bad Request");

    });

};