Javascript

essentials

History of Javascript

1995 - Javascript in Netscape browser
2005 - AJAX based applications (Gmail)
2008 - Google Chrome with V8 engine
2009 - nodejs

What is node.js?

environment for running Javascript
doesn't need browser
using V8

Install node

https://nodejs.org/en/

➡️ Hello world

create file index.js with Javascript code
execute file using: node index.js

console.log('Hello world');

Javascript

Variables

~~var~~
let, const

let name = 'Martin';
name = 'Lucas';
const surname = 'Nuc';

Basic types

number
string
boolean
null, undefined
objects

Strings

// 1) double quotes
let name = "Martin";

// 2) single quotes
let name = 'Martin';

// 3) backticks
let name = `Martin`;

let fullName = `${name} Nuc`;

Strings

let name = 'Martin';
console.log(name.toUpperCase());

let characters = name.split('');
console.log(characters); // ['M', 'a', 'r'...]

have methods

null, undefined

undefined = value was not set
null = "nothing"
- value set but it means "nothing" on purpose

Objects

later...

Loops

for
while

Conditions

let a = 5;
if (a === 5) {
    console.log('is five');
}

for (let i = 0; i < 10; i++) {
    // ...
}

let i = 0;
while(i < 10) {
    // ...
    i++;
}

Use === for comparison

Functions

sequence of commands
inputs
output
- no return = undefined

function add(a, b) {
    return a + b;
}

How to create a function?

// 1) named function
function hello() {
    console.log('hello');
}

// 2) anonymous function
const hello = function() {
    console.log('hello');
}

// 3) anonymous using arrow function
const hello = () => console.log('hello');

named

anonymous

Arrow function

// with body
const one = () => {
    return 1;
};

// single returned statement
const two = () => 2;

Clojure

function initialize() {
  let cnt = 1;
  return () => cnt++;
}

const fn = initialize();
console.log(fn()) // 1
console.log(fn()) // 2
console.log(fn()) // 3

function see surrounding variables
= lexical context

Objects

not a primitive type
used for structured data
key: value
key = property

let teacher = {
    firstName: 'Martin',
    surname: 'Nuc',
    age: 32
}

Modify object

let teacher = {
    firstName: 'Martin',
    surname: 'Nuc',
    age: 32
}

// using dot notation
teacher.age = 33;

// using key name
teacher['age'] = 33;

let keyName = 'age';
teacher[keyName] = 33;

Remove property

let teacher = {
    firstName: 'Martin',
    surname: 'Nuc',
    age: 32
}

delete teacher.age;
console.log(teacher);

Shorthand when creating object

let firstName = 'Martin';
let age = 32;

// create object
let teacher = {
    firstName: firstName,
    age: age
}

// shorthand:
let teacher = {
    firstName,
    age
}

Spread operator

const martin = {
  firstName: 'Martin',
  age: 36
}

const richMartin = {
  ...martin,
  money: 1000000
}

used to create a new object from another object

Stored as a reference

let teacher = {
    firstName: 'Martin',
    age: 32
}
let teacherTwo = teacher;

teacherTwo.age = 55;
console.log(teacher.age); // ????

not like primitive types
use Object.assign() to create a copy (not in IE11)

Methods in objects

const obj = {
    one: function() { return 1; },
    two: () => 2,
    three() {
        return 3;
    }
};
obj.four = () => 4;

obj.one();
obj.two();
obj.three();
obj.four();

Destructuring object

const obj = {
    firstName: 'Martin',
    surname: 'Nuc',
    age: 32
};

const { firstName, surname } = obj;
console.log(name, surname);

function printAge({age}) {
    console.log(age);
}

printAge({
    name: 'Martin',
    age: 32
});

Rest operator

const obj = {
    name: 'Martin',
    surname: 'Nuc',
    age: 32
};

const { name, surname, ...other } = obj;
console.log(other); // { age: 32 }

"rest" after destructuring

typeof

typeof 5 // 'number'
typeof 'hi' // 'string'
typeof {} // 'object'
typeof [] // 'object'
typeof undefined // 'undefined'

typeof null // 'object' !!

Arrays

store multiple items
in order
.length = number of items
push to add

let items = [1, 2, 3]; // creation
items.push(4);

console.log(items); // [1, 2, 3, 4]
console.log(items.length) // 4

Access the index

index starts with 0

let arr = ['one', 'two', 'three'];

console.log(arr[0]); // 'one'
console.log(arr[1]); // 'two'

console.log(arr[5]); // undefined

Spread operator

expands array into individual items
used to create a new array from existing

const arr = ['one', 'two', 'three'];

const newArray = [...arr, 'four'];

➡️ Palindrome

create a function which determines whether input is palindrome
- palindrome = word that reads the same backwards as forwards

Array methods

.join()

let arr = [1, 2, 3, 4];
let output = arr.join('-'); 
console.log(output); // '1-2-3-4'

creates string by joining items in the array

.reverse()

let arr = [1, 2, 3, 4];
arr.reverse();
console.log(arr); // [4, 3, 2, 1]

.forEach()

loops over the array

[1, 2, 3, 4].forEach(item =>
    console.log(item)
);

.every(), .some()

checks condition for every item

let result = [1, 2, 3, 4].every(item =>
    item > 0
);
console.log(result); // true

.map()

creates a new array
every item modified by a function

let result = [1, 2, 3, 4].map(item =>
    item + 1;
);
console.log(result); // [2, 3, 4, 5]

.filter()

creates a new array
includes only items that pass the condition

let result = [1, 2, 3, 4].filter(x => x > 2);
console.log(result); // [3,4]

.reduce()

accumulates intermediate result

[1,2,3,4].reduce((accumulator, current) =>
    accumulator + current
, 0); // 10

accumulator	current	result
0	1	1
1	2	3
3	3	6
6	4	10

➡️ Sum of positives

you have an array of numbers
(1) count how many positive numbers are there
- example: [1,-4,7,12] => 3
(2) you would like to calculate sum of positive values only
- example: [1,-4,7,12] => 1 + 7 + 12 = 20

String methods

.split()

creates an array
split by character

'Good morning'.split(' '); // ['Good', 'morning']

.replace()

replaces only first match and returns a new string

let result = 'Good morning'
    .replace('morning', 'afternoon');

.match()

search using RegEx

let result = 'aaa,aab,aac,abc,acc'.match(/aa.?/g);
console.log(result) // [ 'aaa', 'aab', 'aac' ]

➡️ Shortest word

given a string of words, return the length of the shortest word(s)
example: "Hi my name is Martin" => 2

Classes

What is a class?

template for future objects
needs to be instantiated using new keyword
like a "recipe" for a chocolate cake. Using recipe you make a cake (instance of a cake)

class Dog {
    bark() {
        console.log('woof-woof');
    }
}

let rex = new Dog();
rex.bark();

Might have properties

class Dog {
    setName(newName) {
        this.name = newName;
    }
    bark() {
        console.log('woof-woof, I am ' + this.name);
    }
}

let rex = new Dog();
rex.setName('Rex');
let lassie = new Dog();
lassie.setName('Lassie');

rex.bark();  // woof-woof, I am Rex
lassie.bark();  // woof-woof, I am Lassie

use this keyword to access properties

Constructor

class Dog {
    constructor(name) {
        this.name = name;
    }
    bark() {
        console.log('woof-woof, I am ' + this.name);
    }
}

let rex = new Dog('Rex');
rex.bark();  // woof-woof, I am Rex

method which is executed when the class is instantiated (when used with new)

Inheritance

class Animal {
    eat() {
        console.log('yum yum');
    }
}

class Dog extends Animal {
    constructor(name) {
        this.name = name;
    }
    bark() {
        console.log('woof-woof, I am ' + this.name);
    }
}

let rex = new Dog('Rex');
rex.bark();  // woof-woof, I am Rex
rex.eat(); // yum yum

class can extend another class
parent class should be more generic

setTimeout

setTimeout()

do something later
asynchronous

console.log(1);
setTimeout(() => console.log(2), 1000);
setTimeout(() => console.log(3), 2000);

Event loop

JS is single-threaded
things get pull from event loop

console.log(1);
setTimeout(() => console.log(2), 0);
console.log(3);

Context

this

What is context?

the value of this keyword
related not only to classes, it's everywhere
typically it's set to object which the function belongs to BUT you never know

class Dog {
    constructor(name) {
        this.name = name;
    }
    bark() {
        console.log('woof-woof, I am ' + this.name);
    }
}

let rex = new Dog('Rex');
rex.bark();

Context

depends on how the function is called

let obj = {
    name: 'Martin',
    sayHi() {
        console.log(this.name);
    }
}

obj.sayHi();
let fn = obj.sayHi;
fn();

Enforce context

using call+apply, bind

function fn(num) {
    console.log(this, num);
}

// classic invocation
fn(1); // this = undefined

// call + apply
fn.call('test', 2); // this = 'test'
fn.apply('test', [3]); // this = 'test'

// bind
let bound = fn.bind('test');
bound(4); // this = 'test'

Example

jQuery uses context:

$('a').each(function() {
  console.log(this.href);
});

Problem

What context does a callback function have?

let obj = {
  name: 'Martin',
  hi() {
    console.log(this.name);
    setTimeout(function () {
      console.log(this.name);
    }, 1000);
  }
}

obj.hi();

Arrow functions

always pass the current context

let obj = {
  name: 'Martin',
  hi() {
    console.log(this.name);
    setTimeout(() => {
      console.log(this.name);
    }, 1000);
  }
}

obj.hi();

➡️ Test your JS skills

Write a function to find the first not repeated (unique) character in a string

obj = {a: 2, b: 3};
for(key in obj) {
  console.log(key)
}

Promises

What are they for

to avoid "callback hell"
flat and readable code
mostly used for asynchronous operations
- any asynchronous operation can be wrapped to a Promise

Problem: callback hell

setTimeout(() => {
  console.log('after 1s');
  setTimeout(() => {
    console.log('after another 1s');
    setTimeout(() => {
      console.log('after another 1s');
    }, 1000);
  }, 1000);
}, 1000);

Flat code with Promises

promise
  .then(fn1)
  .then(fn2)
  .then(fn3)

Promise state

pending
fulfilled (=resolved)
rejected

pending

rejected

fullfilled

.then(...)

.catch(...)

How to create a promise

= instantiate a Promise object

function wait5seconds() {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(), 5000);
    });
};

wait5seconds().then(() => console.log('after 5s'));

How to create a promise

shorthand for resolved / rejected promise

Promise.resolve()
Promise.reject()

Pass data

argument of resolve
obtained in the first parameter of then

function wait5seconds() {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve([1, 2, 3]), 5000);
    });
};

wait5seconds().then((data) => console.log(data));

1. Error handling - catch

function fetchData() {
    return new Promise((resolve, reject) => {
        setTimeout(() => reject('This is error'), 5000);
    });
};

fetchData()
  .then((data) => console.log('success', data))
  .catch((error) => console.log(error));

Promise chaining

what you return in then you get in the next then
when it returns a Promise it will wait for that promise to resolve before going to next then

Promise.resolve('hey')
    .then(data => console.log(data)) // "hey"
    .then(() => anotherPromise()) // async
    .then(() => {
        return 'hello'; // sync
    })
    .then(param => console.log(param)); // "hello"

Waiting for multiple promises

Promise.all([promise1, promise 2])
- returns a single promise - resolved when all of them are resolved
Promise.race([promise1, promise 2])
- resolved when the fastest one is resolved

➡️ Wrap setTimeout in Promise

create function wait
- parameter: how long it should wait
use promise chain (.then) to count down:
- console.log: 3,2,1, go!

async / await

async/await

syntactic sugar around promises
works only in async function

async function countDown() {
    console.log(3);
    await wait(1000);
    console.log(2);
    await wait(1000);
    console.log(1);
    await wait(1000);
    console.log('go!');
}

countDown().then(() => console.log('done'));

async/await

async function countDown() {
    console.log(3);
    await wait(1000);
    console.log(2);
    await wait(1000);
    console.log(1);
    await wait(1000);
    console.log('go!');
}

countDown().then(() =>
   console.log('done'));

function countDown() {
    console.log(3);
    return wait(1000)
      .then(() => console.log(2))
      .then(() => wait(1000))
      .then(() => console.log(1)
      .then(() => wait(1000))
      .then(() => console.log('go!'))
}

countDown().then(() =>
   console.log('done'));

Extract resolved data

async function fn() {
  const data = await Promise.resolve('hello');
}

Error handling in async fn

async function() {
    try {
        await Promise.reject('this is reason');
    } catch (err) {
        console.error(err);
    }
}

➡️ Rewrite using async/await

function fetchData(url) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      Math.random() < 0.5 ? resolve(Math.random()) : reject('Fetching failed')
    }, 1000);
  })
}

function execute() {
  const results = [];
  fetchData('seznam.cz')
    .then((seznamData) => results.push(seznamData))
    .then(() => fetchData('google.com'))
    .then((googleData) => results.push(googleData))
    .catch(error => console.log(error))
    .then(() => console.log('done'))
}

fetchData ramdomly fails

NPM

Dependency management

Node package manager

packaging system
large repository 3rd party libs
https://www.npmjs.com

{
  "name": "my-package",
  "version": "1.0.0",
  "description": "This is just description of my awesome package",
  "main": "index.js",
  "scripts": {
    "dev": "nodemon --exec npm run start",
    "start": "tsc && node dist/index.js",
    "test": "mocha --opts mocha.opts"
  },
  "author": "Martin Nuc",
  "license": "ISC",
  "dependencies": {
    "@types/chai": "4.0.4",
    "@types/mocha": "2.2.43",
    "@types/node": "8.0.28",
    "@types/sinon": "2.3.4",
    "chai": "4.1.2",
    "mocha": "3.5.3",
    "nodemon": "1.12.1",
    "sinon": "3.2.1",
    "ts-node": "3.3.0",
    "typescript": "2.5.2"
  }
}

package.json

Scripts


"scripts": {
  "start": "react-scripts start",
  "build": "react-scripts build",
  "test": "react-scripts test",
  "eject": "react-scripts eject"
},

used to execute commands
npm dependencies executables resolution (from node_modules/.bin/*)

npm run <name>

Shortcut for start and test scripts only. For others you have to use npm run

Runs any script from npm.

npm start

npm test

👉

➡️ Create NPM project

create a folder and run npm init
create index.js
create start script to run it

Dependencies

npm install lodash

installs lodash library:

Use library in your code

import lodash from 'lodash';
lodash.difference([1, 2, 3], [2, 3]);

Dependencies and GIT

we don't commit dependencies
put node_modules folder in .gitignore
npm install to install dependencies

{
  "name": "my-package",
  "version": "1.0.0",
  "description": "This is just description of my awesome package",
  "main": "index.js",
  "scripts": {
    "dev": "nodemon --exec npm run start",
    "start": "tsc && node dist/index.js",
    "test": "mocha --opts mocha.opts"
  },
  "author": "Martin Nuc",
  "license": "ISC",
  "dependencies": {
    "@types/chai": "4.0.4",
    "@types/mocha": "2.2.43",
    "@types/node": "8.0.28",
    "@types/sinon": "2.3.4",
    "lodash": "4.17.5",
    "chai": "4.1.2",
    "mocha": "3.5.3",
    "nodemon": "1.12.1",
    "sinon": "3.2.1",
    "ts-node": "3.3.0",
    "typescript": "2.5.2"
  }
}

package.json

Semantic versioning

6.11.2

patch

minor version

major version

Semantic versioning

6.11.2

patch

minor version

major version

- major changes, breaks API

Semantic versioning

6.11.2

patch

minor version

- new features

- doesn't break API

major version

- major changes, breaks API

Semantic versioning

6.11.2

patch

- only bugfixes

minor version

- new features

- doesn't break API

major version

- major changes, breaks API

Tieto Javascript essentials 2023-04-03

By Martin Nuc

Tieto Javascript essentials 2023-04-03

Martin Nuc

martin_nuc