Express

Overview

Express Server Mechanics: Routing, Middleware, and Approaches to Testing

Class Outline

• Code Review
• Warmup question
• Lecture

Learning Objectives

Students will be able to

Describe and Define

• Node Modules
• Code Modularization
• Express Middleware
• HTTP Status Codes
• TDD and Testing

Execute

• Write an express API server
• Incorporate application level middleware
• Properly test an express server

Notes

Express Routing

• Event driven system
  • app.get('/thing', (req,res) => {})
  • This is the same pattern we see in Vanilla JS, jQuery
  • ‘When a get event happens in our server, on “/thing”, run this function…’
• The Request Object
  • (req,..)
  • /:parameters
    • app.get('/api/:thing',...) = req.params.thing
  • Query Strings
    • http://server/route?ball=round = req.query.ball
• The Response Object
  • (..., res)
  • Responsible for sending data back to the browser
  • Has methods like send() and status() that Express uses to format the output to the browser properly
    • Sends Headers
      • Cookies
      • Status Codes

Express Middleware

• What does it do?
  • A series of functions that the request “goes through”
  • Each function receives request, response and next as parameters
  • Application Middleware run on every route/request
    • Error Handling, Logging, BODY Parsing
  • Route Middleware runs on specific routes
    • Are you logged in?

Middleware runs your code, and then runs the next() middleware in the series.

function myLogger(req,res,next) {
  console.log(req.method);
  next(); // runs the next middleware in line
}

If you call next() with an argument, it’ll skip all remaining middleware and run your error handler, with that argument as the error

function loggedIn(req,res,next) {
  if( validUser ) { next(); } // Run the next middleware
  else { next("you need to login"); } // Run the error handler, skipping all other middleware
}

Your route handler (your normal (req,res) function) is always the last middleware in the series!

Server Testing

• Generally, avoid starting the actual server for testing
• Instead, export your server as a module in a library
• Then, you can a “mocking library” such as supertest to run your tests
  • This will hit your routes as though your server was running, without actually starting it

server.js

const express = require('express');
const app = express();
app.get('/data', (req,res) => res.json({}));
// Export an object with the "app" in it.
module.exports = {
  start: () => app.listen(3000),
  server: app
}

server.test.js

const supertest = require('supertest');
const myServer = require('server.js');
const client = supertest(myServer.server);
describe('my server', () => {
  test('can send data', () => {
    return client.get('/data')
      .then( response => {
        expect(response.body).toBeDefined();
      })
  })
})

Setup for Testing

1. Install “Jest” so that you can run your tests npm install jest

2. Add the proper “testing scripts” to your package.json file

   "scripts": {
     "test": "jest --coverage",
     "watch": "jest --coverage --watchAll"
   }
   

3. Run your tests on demand
   • npm test
4. Run your tests automatically as you save your files1
   • npm run watch