Node.js System: The Landing

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

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

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)

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

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

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();

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();

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

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

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

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)
}

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();

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/

Node.js System: The Landing

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