The document provides sample code examples for key Node.js concepts including prototype-based object-oriented programming, asynchronous programming with callbacks, promises, and async/await, automated testing with Mocha and Chai, and using TypeScript with Node.js. The examples cover topics such as object prototypes, classes, timers, promises, generator functions, generics, and writing automated tests. Useful links are also provided for further learning Node.js, asynchronous programming, testing, and TypeScript.

1. Node.js System:
The Landing Haci M. Yaman
29/4/2021

2. Content
1. Prototype-based OOP
• Sample Code
2. Asynchronous Programming
• Sample Code (a)
• Sample Code (b)
3. Automated Tests
• Sample Code
4. Sample Code (TypeScript)
5. Sample Code (Generators Functions)
6. Sample Code (Generics)
7. The End

3. 1. Prototype-based OOP
• It is like working with templates and copying those templates
• Object prototypes are dynamic; they can be changed at run-time
• Objects based on those prototypes are also dynamic
• Prototypes can copy behaviour of other prototypes (Inheritance)
• Multiple prototypes can implement same behaviour (Polymorphism)

4. 1. Sample Code (a)
// 1.a. generic object
const person0 = { name: 'Adam', age: 101, friends: [] };
person0.follow = friend => { this.friends.push(friend); };
const person1 = Object.create(person0);
// 1.b. we can also put it inside a 'factory' function and return person object
const makePerson = (name = '', age = 0, friends = []) => { /* create person0 */ return person0; };
// 2. function definition
function Person(name = '', age = 0, friends = []) {
this.name = name;
this.age = age;
this.friends = friends;
this.follow = friend => { this.friends.push(friend); };
}
const person2 = new Person('Becky', 102);
// 3. class definition
class PersonModel {
constructor(name = '', age = 0, friends = []) {
this.name = name;
this.age = age;
this.friends = friends;
}
follow(friend) { this.friends.push(friend); }
}
PersonModel.prototype.talk = sentence => { console.log(sentence); /* TODO use TTS engine */ }
const person3 = new PersonModel('Conor', 103);
Ref: https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Objects/Object-oriented_JS

5. 2. Asynchronous Programming
• Callback functions: Fire and forget; we will call you back!
• Functions as arguments to other functions
• Timers: setTimeout(), setInterval()
• Promises: special object that “mimic” threads
• Special methods: then(), catch()
• Callback hell:
• Async/Await heaven

6. 2. Sample Code (a)
// Timers
function showTime() { console.log(new Date()); }
const id = setInterval(showTime, 1000); // run every second
function stopShowingTime() { clearInterval(id); console.log('stopped'); process.exit(0); }
setTimeout(stopShowingTime, 10 * 1000); // run after ten seconds once
console.log('started');
// Promises
function promiseHandler(resolve, reject) {
const r = Math.random();
0.9 <= r // ternary expression used as statement
? resolve({ success: 'we got: ' + r }) // captured by then() callback
: reject({ error: 'sorry, we got: ' + r }); // captured by catch() callback
}
const randomPromise = new Promise(promiseHandler);
const startRandomPromise = () => {
randomPromise.then(console.info).catch(console.error);
}
startRandomPromise();

7. 2. Sample Code (b)
// Async/Await
const puppeteer = require('puppeteer’);
const sleep = async (ms = 1000) => new Promise(resolve => setTimeout(resolve, ms));
async function countTheStars() {
let i = 0, limit = 10, duration = 60 * 1000, url = 'https://github.com/nodejs/node’;
let linkSelector = 'a.social-count', linkElement, linkText;
const browser = await puppeteer.launch();
const page = await browser.newPage();
while(i < limit) { // count the stars every minute ;)
await page.goto(url);
linkElement = await page.$(linkSelector);
if (linkElement) {
linkText = await page.evaluate(el => el.textContent, linkElement);
console.info(new Date(), 'stars', linkText);
}
await sleep(duration);
i++;
}
await browser.close();
}
countTheStars();

8. 3. Automated Tests
• “Mocha is a feature-rich JavaScript test framework running on
Node.js and in the browser”
• Define test suites/cases by using simple function calls and callback functions
• Use async/await within callback functions, if preferred,
• “Chai is a BDD / TDD assertion library for node and the browser”
• Verify the expectations using chainable interfaces: expect, should, assert

9. 3. Sample Code
// Automated tests
// myMathLib.js ======================================================
function euclideanDistance(pt1, pt2) {
const xDiff = Math.pow(pt1.x - pt2.x, 2);
const yDiff = Math.pow(pt1.y - pt2.y, 2);
return Math.sqrt(xDiff + yDiff);
}
module.exports = { euclideanDistance };
// myMathLib.test.js =================================================
const { expect } = require('chai');
const { euclideanDistance } = require('./myMathLib');
describe('euclideanDistance', () => {
it('should return 2 when points are [2,0] and [0,0]', () => {
const out = euclideanDistance({ x: 2, y: 0 }, { x: 0, y: 0 });
expect(out).to.equal(2);
});
});
// package.json ======================================================
// scripts: { "test": "mocha *.test.js" }
// command line: npm run test

10. 4. Sample Code (TypeScript)
// TypeScript
// npm i typescript ts-node @types/node
// npm i -g typescript ts-node
// to generate tsconfig.json: tsc --init
// myMathLib.ts ====================================================
export interface Point {
x: number;
y: number;
}
export function euclideanDistance(pt1: Point, pt2: Point): number {
const xDiff: number = Math.pow(pt1.x - pt2.x, 2);
const yDiff: number = Math.pow(pt1.y - pt2.y, 2);
return Math.sqrt(xDiff + yDiff);
}
export default euclideanDistance;
// sample.ts ========================================================
import euclideanDistance, { Point } from './myMathLib';
const pt1a: Point = { x: 2, y: 0 }, pt1b: Point = { x: 0, y: 0 };
const pt2a: Point = { x: 2, y: 2 }, pt2b: Point = { x: -1, y: -1 };
const dist1: number = euclideanDistance(pt1a, pt1b);
const dist2: number = euclideanDistance(pt2a, pt2b);
console.log('the distance between', pt1a, 'and', pt1b, 'is', dist1);
console.log('the distance between', pt2a, 'and', pt2b, 'is', dist2);
//------------------------------
// console: ts-node ./sample.ts

11. 5. Sample Code (Generator Functions)
// Generator Functions
function* fibonacciIterator(limit: number = Infinity) {
let old: number[] = [0, 1];
for (let i = 0; i < limit; i++) {
if (i === 0) yield old[0];
if (i === 1) yield old[1];
old[2] = old[1] + old[0];
yield old[2];
old[0] = old[1];
old[1] = old[2];
}
return -1; // special flag, we are done
}
// tsconfig.json compilerOptions.downlevelIteration = true
for (const n of fibonacciIterator(10)) {
console.log(n)
}

12. 6. Sample Code (Generics)
import EventEmitter from 'events';
import { rword } from 'rword';
interface IRacer { name: string; age: number; dist: number; }
class Race<T extends IRacer = IRacer> extends EventEmitter {
private timer?: NodeJS.Timeout;
constructor(private racers: T[] = [], private distance = 1000) { super(); }
run() {
this.racers.forEach(h => { h.dist += 1 + Math.round(Math.random() * 10); });
this.racers = this.racers.sort((a, b) => b.dist - a.dist); // order: DESC
console.log(this.racers.map(h => `${h.name} at ${h.dist}`).join(' | ')); // TODO animate
if (this.distance <= this.racers[0].dist) this.stop(); // we have a winner
}
start() { this.timer = setInterval(() => this.run(), 1000); this.emit('start'); }
stop() { if (this.timer) clearInterval(this.timer); this.emit('stop', this.racers[0]); }
}
interface Horse extends IRacer {}
const makeHorse = (): Horse => ({
name: String(rword.generate(1)), age: 2 + Math.round(Math.random() * 10), dist: 0,
});
const horses: Horse[] = Array.from({ length: 5 }).map(makeHorse);
const race = new Race<Horse>(horses);
race.on('start', () => console.log('START'));
race.on('stop', winner => console.log('FINISH: the winner is', winner));
race.start();

13. The End
Thank you
Useful links:
https://nodejs.dev/learn
https://pptr.dev/
https://mochajs.org/
https://www.chaijs.com/
https://istanbul.js.org/
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Generator
https://www.typescriptlang.org/