disconnectBeforeIndex.js

var mongoose = require('mongoose');
mongoose.connect('localhost', 'testing_disconnectBeforeIndex');

var user = new mongoose.Schema({
    username: { type: String, index: { unique: true } },
    password: String
});

var model = mongoose.model('user', user);
model.on('index', function () {
  console.error('indexed!');
  mongoose.disconnect();
});

var instance = new model();
instance.username = 'abc'+Math.random();
instance.password = '123';

instance.save(function(err, doc) {
    if (err) {
        console.log('err', err);
    } else {
        console.log('doc', doc);
    }
});