Concept

const express = require("express");
const app = express();
const mongoose = require("mongoose");
const cors = require("cors");
const bodyParser = require("body-parser");

// This will allow our presentation layer to retrieve data from this API without
// running into cross-origin issues (CORS)
app.use(cors());
app.use(bodyParser.json());

// ============================================
// ==========  DATABASE CONNECTION  ===========
// ============================================
// Connect to running database
mongoose.connect(
  `mongodb://${process.env.DB_USER}:${process.env.DB_PW}@127.0.0.1:27017/monolithic_app_db`,
  { useNewUrlParser: true }
);

// User schema for mongodb
const UserSchema = mongoose.Schema(
  {
    name: { type: String },
    email: { type: String },
  },
  { collection: "users" }
);

// Define the mongoose model for use below in method
const User = mongoose.model("User", UserSchema);

function getUserByEmail(email, callback) {
  try {
    User.findOne({ email: email }, callback);
  } catch (err) {
    callback(err);
  }
}

// set the view engine to ejs
app.set("view engine", "ejs");

// index page
app.get("/", function (req, res) {
  res.render("home");
});

// ============================================
// ============  API ENDPOINT  ================
// ============================================
app.post("/register", function (req, res) {
  const newUser = new User({
    name: req.body.name,
    email: req.body.email,
  });

  newUser.save((err, user) => {
    res.status(200).json(user);
  });
});

// ============================================
// ==============  SERVER =====================
// ============================================
app.listen(8080);
console.log("Visit app at http://localhost:8080");

<!-- File: home.ejs -->

<!-- On form submit, home.ejs executes the getDataFromBusinessLayer() function -->

<form id="emailform" onsubmit="getDataFromBusinessLayer()">
  <input name="email" id="email" placeholder="Enter email..." />
  <button type="submit">Load Profile</button>
</form>

// File: presentation-layer-user.js

function getDataFromBusinessLayer() {
  event.preventDefault();
  const email = $("#email").val();

  // Perform the GET request to the business layer
  // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  $.ajax({
    url: `http://localhost:8081/get-user/${email}`,
    type: "GET",
    success: function (user) {
      // Render the user object on the page
      // Ommitted for brevity
    },
    error: function (jqXHR, textStatus, ex) {
      console.log(textStatus + "," + ex + "," + jqXHR.responseText);
    },
  });
}

// File: business-layer-user.js

app.get("/get-user/:useremail", function (req, res) {
  // Makes a call to the data access layer
  // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  const user = User.getUserByEmail(req.params.useremail, (error, user) => {
    res.status(200).json({
      name: user.name,
      email: user.email,
      profileUrl: user.profileUrl,
    });
  });
});

// File: data-layer-user.js

module.exports.getUserByEmail = (email, callback) => {
  try {
    // Makes a call to the database
    // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    User.findOne({ email: email }, callback);
  } catch (err) {
    callback(err);
  }
};