how to set state for logged in users

var mongoose = require('mongoose'),
    Schema = mongoose.Schema,
    bcrypt = require('bcrypt'),
    SALT_WORK_FACTOR = 10;

var UserSchema = new Schema({
    username: { type: String, required: true, index: { unique: true } },
    password: { type: String, required: true }
});

module.exports = mongoose.model('User', UserSchema);