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Alex Merced
Alex Merced

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Ultimate Express & Mongo Reference

This is not a guide on how to use express and mongo, but a useful reference especially for those starting to learn these technologies. This guide will serve as documentation of the basics of all the main functions and patterns in these libraries.


  • Installation Command: npm install express
  • Importing: const express = require("express")


The express function returns a new application object. The application object has several different functions.

  • can register middleware and routes
  • can initiate the http listener on a particular port

Typical use

// import express
const express = require("express")

// create an application object
const app = express()

// create new router
const router = express.Router()

// register middleware with the application, (each request will run through each of these one by one)
app.set("view engine", "ejs") // specify the view engine for view rendering
app.use(express.static("public")) // serve a folder called "public" as static
app.use(express.json()) // parse json bodies based on Content-Type header
app.use(express.urlencoded({extended: false})) // parse urlencoded form data
app.use("/", router) // register the router with the application

// register routes with the router
router.get("/path", getHandlerFunction) // invokes function for matching get request"/path", postHandlerFunction) // invokes function for matching post request
router.put("/path", putHandlerFunction) // invokes function for matching put request
router.delete("/path", deleteHandlerFunction) // invokes function for matching delete request

// Initiate the server listening on a port with a function that runs when server starts listening
app.listen(3000, () => console.log("Listening on Port 3000"))
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The Request and Response Objects

When the listener receives a request two objects are generated, Request and Response (req, res) which is passed to each middleware function & route one by one until a response is sent. These request and response objects have several useful properties.


  • req.params: an objects with an defined url parameters (defined in router paths using colons... "/:these/:are/:params")

  • req.query: an object of any url queries in the request url

  • req.headers: an object of all the headers in the request

  • req.method: a string detailing the method of the request ("get", "post", etc.)

  • req.body: object containing the data from the request, must be parsed by middleware that can parse the type of data sent. (express.json for json data, express.urlencoded for form data)


  • res.send("string"): function for sending a text response, will auto-send arrays and objects as json responses. Will send html strings as html responses.

  • res.json({data}): send a json response

  • res.render("template", {data}): tells view engine to render the specified template in the views folder with the data in the second argument. The resulting HTML file is sent as a response.

  • res.set("header-name","header-value"): function for setting a response header

  • res.redirect("/path"): redirect the browser to a different path

Popular 3rd Party Middleware

  • npm install cors : middleware for setting cors headers
  • npm install method-override: override request methods based on a url query
  • npm install morgan: request logging
  • npm install express-session: use session cookies with express
  • npm install connect-mongo: session cookie store using mongodb

Popular View Engines

  • EJS
  • Handlebars
  • Mustache
  • Nunjucks
  • Pug
  • Marko
  • Express-React-Views
  • Liquid


ODM (Object Document Mapper/Manager) for Mongo databases. Allows you to connect to mongo databases and create model objects.

  • npm install mongoose

Connecting to Mongo

const mongoose = require("mongoose")

// connect to the database, 1st argument the connection string, 2nd argument a configuration object
mongoose.connect(MongoURI, ConfigObject)

//set responses to database events
.on("open", () => console.log("connected to mongo"))
.on("close", () => console.log("connected to mongo"))
.on("error", (error) => console.log(error))
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Creating a Model Object

const {Schema, model} = require("mongoose")

// create schema
const ModelSchema = new Schema({
    property1: String,
    property2: Number,
    property3: Boolean
}, {timestamps: true})

// create model specifying model/collection name and schema
const Model = model("Model", ModelSchema)
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3 Ways to Write Queries with your model with error handling

// Callback Syntax
Model.find({}, (error, data) => {
    if (error){
    } else {

// .then syntax
.then((data) => console.log(data))
.catch((error) => console.log(error))

// using async/await
const queryFunction = async() => {

        const data = await Model.find({})

    } catch(error){




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Model Functions

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Further Reading

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