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Notes on JS learning

From codecademy I obtained the following webpage that contains documentation on the methods for strings:

js documentation

methods are actions we can perform. We call or use this methods by:

  • a period
  • the name of the method
  • opening and closing parethesis

Built-in objects

There are objects already define in JS that we can use. Each of this objects have methods. More on this, here

Example with math object:

Math.floor(Math.random() * 50)

This performs:

  • random generates number random between 0 and 1
  • Multiplyind by 50 makes it between 0 and 50
  • floor round the number to the lowest whole number

Notes on properties and methods

  • Objects can have properties (stored information). Properties are denoted with a . after the name of the object, for example: Hello.length
  • Objects including instances of data types can have methods which perform actions.

Notes on variables

  • var is pre E56 versions
  • Variables are stored in memory
  • let is preferred way to declare a variable when we want to be it reassigned.
  • Variables not initialized are assigned to undefined
  • + can concatenate strings
  • In E56 backtick and ${} are use to interpolate values into a string
  • typeof return the data type of a value

Truthy and Falsy assigment

An example of a conditional that evaluates if the variable is empty or not and assigns a default string if not:

let username = '';
let defaultName;

if (username) {
  defaultName = username;
} else {
  defaultName = 'Stranger';

console.log(defaultName); // Prints: Stranger

Now, if we combine logical operators, we can do the same with less typing:

let username = '';
let defaultName = username || 'Stranger';

console.log(defaultName); // Prints: Stranger

A || statement check the left condition first. If username had a string value, that one would be assigned to defaultName. Otherwise, if falsy, the Stranger value would be assigned. This concept is known as short-circuit-evaluation

Ternary operator

This is a way to simplify an if else statement:

let isNightTime = true;

if (isNightTime) {
  console.log('Turn on the lights!');
} else {
  console.log('Turn off the lights!');

Now, the way to apply the ternary operator is:

isNightTime ? console.log('Turn on the lights!') : console.log('Turn off the lights!');

This is another example:

let favoritePhrase = 'Love That!';

favoritePhrase === 'Love That!' ? console.log('I love that!') :
  console.log("I don't love that!");

Switch statement

Instead of typing a complet if else conditional statement, we can use a switch statement to check a series of conditions. Let's take a look on how to transform a series of statement created with if else to a switch statement:

let groceryItem = 'papaya';

if (groceryItem === 'tomato') {
  console.log('Tomatoes are $0.49');
} else if (groceryItem === 'papaya'){
  console.log('Papayas are $1.29');
} else {
  console.log('Invalid item');
let groceryItem = 'papaya';

switch (groceryItem) {
  case 'tomato':
    console.log('Tomatoes are $0.49');
  case 'lime':
    console.log('Limes are $1.49');
  case 'papaya':
    console.log('Papayas are $1.29');
    console.log('Invalid item');

// Prints 'Papayas are $1.29'

Pay attention to the break; and the last piece which is the default

Magic Eight Ball project

The idea of this project is to use control flow and somethings learned in the chapter:

let userName = ''
userName ? console.log(`Hello, ${userName}!`) : 

const userQuestion = 'Should I accept offer?' 

console.log(`The ${userName} asked: ` + userQuestion)

let randomNumber = Math.floor(Math.random() * 8);

let eightBall = ''

switch (randomNumber) {
  case 1:
    eightBall = 'It is certain'
  case 2:
    eightBall = 'It is decidedly so'
  case 3:
    eightBall = 'Reply hazy try again'
  case 4:
    eightBall = 'Cannot predict now'
  case 5:
    eightBall = 'Do not count on it'
  case 6:
    eightBall = 'My sources say no'
  case 7:
    eightBall = 'Outlook not so good'
  case 8:
    eightBall = 'Signs point to yes'


//This will print something like:
The  asked: Should I accept offer?
Outlook not so good


There are like 3 ways to create a function in js:

const plantNeedsWater = function(day) {
  if (day === 'Wednesday') {
    return true;
  } else {
    return false;

The same function but as an arrow function - The idea is to avoid the need to write the word function:

const plantNeedsWater = (day) => {
  if (day === 'Wednesday') {
    return true;
  } else {
    return false;

Consice body arrow functions

The most condensed form of a function are known as consice body

  • Functions that take only one parameter, do not need parethensis. If the arrow function have zero or more than one parameter, parenthesis are required.
const functionName = () => {};
const functionName = parameter => {};
const functionName = (A, B) => {};
  • Functions with a one line body do not need the curly braces. Without the curly braces, whatever the line evaluates, it will be returned inmediately. The body of the function should be inmediately after the => and we can remove the return. This is referred as implicit return
const sumNumber = number => number1 + number2;

const sumNumber = (number1, number2) => {
    const sum = (number1 + number2);
    return sum;

This is another example on how to transform a function to a consice one:

const plantNeedsWater = (day) => {
  return day === 'Wednesday' ? true : false;

const plantNeedsWater = day => day === 'Wednesday' ? true : false;

Callback functions

It's when a function is passed to another one as an argument. The function can be invoked during the execution of that higher order function. Remember that in JS functions are objects, so functions can be passed as arguments.

const isEven = (n) => {
    return n % 2 == 0

let printMsg = (evenFunc, num) => {
    const isNumEven = evenFunc(num);
    console.log(`The number ${num} is an even number: ${isNumEven}.`)

// Pass in isEven as the callback function
printMsg(isEven, 4); 

// Prints: The number 4 is an even number: True.

Scope pollution

Scoping rules are the same as in R. Nothing new. But there is one phenomena that I should be aware of: Scoping pollution

This is an example:

let num = 50;

const logNum = () => {
  num = 100; // Take note of this line of code

logNum(); // Prints 100
console.log(num); // Prints 100

So, a good practice is to not declare global variables!!!

Best practice for scoping pollution

It's called block scope. Using this method will improve the code in several ways:

  • Will make it more legible, since everything is organized in discrete sections.
  • Code will be more understandable, since it clarifies which parts are associated with specific parts of the program.
  • Code will be modular and easier to mantain.
  • It saves memory, since variables will cease to exist after the block finishes running.

An example on how to use the block scope:

const logSkyColor = () => {
  const dusk = true;
  let color = 'blue'; 
  if (dusk) {
    let color = 'pink';
    console.log(color); // Prints "pink"
  console.log(color); // Prints "blue"

console.log(color); // throws a ReferenceError

Setting the dev enviroment:

Installing nodejs

Downloaded from

Warning - Installing from apt, will install an outdated version of nodejs.

So, I downloaded the tar.gz file from the webpage, and then I run:

sudo tar -xf node-v18.13.0-linux-x64.tar.xz --directory=/usr/local --strip-components=1

After that, I checked the installed version:

node -v

First react app

Run the command create-react-app

create-react-app react-test

There is one image in the img folder with the terminal output after running this command.

This will install all the third party libraries needed to create the react app. Once everything is ready, I can run the template app with

cd react-test
npm start

Now that we have a template app, we can open VisualStudio to check the folder structure.

To start playing around with our first app, we can proceed to delete all the template files inside the src folder. We are going to build our own files from scratch.


  • On user settings, I can set up the formatting on save to apply formatting changes when saving the file.
  • To change several words that are the same (for example div) I can highlight one of the words and press ctrl + d. This will enable multi line editing.

A short game in js

The idea was to create the 'rock, paper, scissors' game using the concepts of creating a function and the ifelse.


const getUserChoice = (userInput) => {
  userInput = userInput.toLowerCase();
  if (userInput === 'rock') {
    return (userInput);
  } else if (userInput === 'paper') {
  } else if (userInput === 'scissors') {
  } else {
    console.log('Error, option not available');

const getComputerChoice = () => {
  compNumber = Math.floor(Math.random() * 3 )
  let compChoice = ''

  switch(compNumber) {
    case 0:
      compChoice = 'rock';
    case 1:
      compChoice = 'paper';
    case 2:
      compChoice = 'scissors';

const determineWinner = (userChoice, computerChoice) => {
  if (userChoice === computerChoice) {
    return('It\'s a tie!')

  if (userChoice === 'rock') {
    if (computerChoice === 'paper') {
      return('Computer won')
    } else {
      return('Human won')

   if (userChoice === 'paper') {
    if (computerChoice === 'scissors') {
      return('Computer won')
    } else {
      return('Human won')

   if (userChoice === 'scissors') {
    if (computerChoice === 'rock') {
      return('Computer won')
    } else {
      return('Human won')

const playGame = () => {
  let userChoice = getUserChoice('paper')
  let computerChoice = getComputerChoice()

  console.log(determineWinner(userChoice, computerChoice))