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30 Days Of React: Introduction

_{Author:Asabeneh Yetayeh
October, 2020}

• Introduction
• Requirements
• Setup
  • Install Node.js
  • Browser
    • Installing Google Chrome
    • Opening Google Chrome Console
    • Writing Code on Browser Console
      • Console.log
      • Console.log with Multiple Arguments
      • Comments
      • Syntax
    • Arithmetics
  • Code Editor
    • Installing Visual Studio Code
    • How to Use Visual Studio Code
• Adding JavaScript to a Web Page
  • Inline Script
  • Internal Script
  • External Script
  • Multiple External Scripts
• JavaScript Refresher
• 1. Variables
• 2. Data types
• 3. Arrays
  • How to create an empty array
  • How to create an array with values
  • Creating an array using split
  • Accessing array items using index
  • Modifying array element
  • Methods to manipulate array
    • Array Constructor
    • Creating static values with fill
    • Concatenating array using concat
    • Getting array length
    • Getting index of an element in an array
    • Getting last index of an element in array
    • Checking array
    • Converting array to string
    • Joining array elements
    • Slice array elements
    • Splice method in array
    • Adding item to an array using push
    • Removing the end element using pop
    • Removing an element from the beginning
    • Add an element from the beginning
    • Reversing array order
    • Sorting elements in array
  • Array of arrays
  • 💻 Exercise
    • Exercise: Level 1
    • Exercise: Level 2
    • Exercise: Level 3
• 4. Conditionals
  • If
  • If Else
  • If Else if Else
  • Switch
  • Ternary Operators
  • 💻 Exercises
    • Exercises: Level 1
    • Exercises: Level 2
    • Exercises: Level 3
• 5. Loops
  • Types of Loops
    • 1. for
    • 2. while
    • 3. do while
    • 4. for of
    • 5. forEach
    • 6. for in
  • Interrupting a loop and skipping an item
    • break
    • continue
  • Conclusions
• 6. Scope
  • Window Scope
  • Global scope
  • Local scope
• 7. Object
  • Creating an empty object
  • Creating an objecting with values
  • Getting values from an object
  • Creating object methods
  • Setting new key for an object
  • Object Methods
    • Getting object keys using Object.keys()
    • Getting object values using Object.values()
    • Getting object keys and values using Object.entries()
    • Checking properties using hasOwnProperty()
  • 💻 Exercises
    • Exercises: Level 1
    • Exercises: Level 2
    • Exercises: Level 3
• 8. Functions
  • Function Declaration
  • Function without a parameter and return
  • Function returning value
  • Function with a parameter
  • Function with two parameters
  • Function with many parameters
  • Function with unlimited number of parameters
    • Unlimited number of parameters in regular function
    • Unlimited number of parameters in arrow function
  • Anonymous Function
  • Expression Function
  • Self Invoking Functions
  • Arrow Function
  • Function with default parameters
  • Function declaration versus Arrow function
  • 💻 Exercises
    • Exercises: Level 1
    • Exercises: Level 2
    • Exercises: Level 3
• 9. Higher Order Function
  • Callback
  • Returning function
  • setting time
    • setInterval
    • setTimeout
• 10. Destructuring and Spreading
  • What is Destructuring?
  • What can we destructure?
    • 1. Destructuring arrays
    • 2. Destructuring objects
  • Exercises
  • Spread or Rest Operator
    • Spread operator to get the rest of array elements
    • Spread operator to copy array
    • Spread operator to copy object
    • Spread operator with arrow function
• 11. Functional Programming
  • 1. forEach
  • 2. map
  • 3. filter
  • 4. reduce
  • 5. find
  • 6. findIndex
  • 7. some
  • 8. every
  • Exercises
• 12. Classes
  • Defining a classes
  • Class Instantiation
  • Class Constructor
  • Default values with constructor
  • Class methods
  • Properties with initial value
  • getter
  • setter
  • Static method
  • Inheritance
  • Overriding methods
  • Exercises
    • Exercises Level 1
    • Exercises Level 2
    • Exercises Level 3
• 13 Document Object Model(DOM)

Congratulations on deciding to participate in 30 days of React programming challenge. In this challenge you will learn everything you need to use to develop a React application. In the end of the challenge you will get a 30DaysOfReact programming challenge completion certificate. In case you need help or if you would like to help others you may join the telegram group.

A 30DaysOfReact challenge is a guide for both beginners and advanced JavaScript and React developers. Welcome to 30 Days Of React. React is a JavaScript library. I enjoy using and teaching React and I hope you will do so too. In this step by step 30 Days React challenge, you will learn React which is one of most popular user interface JavaScript libraries. React can do everything that JavaJavaScript can do. React can be used to add interactivity to websites, to develop mobile apps, desktop applications, games. I believe you will learn quite a lot in the next 30 days and your programming and problem solving skills will also be improved significantly.

I will use converstational English and less jargons to write this challenge. The content will be continousely updated. If you find a typo or grammar mistakes don't suprised because I don't do any proof read before I publish it. I would recommond you to focus on the main message of the challenge instead of the English and some minor mistakes. I really appreciate if you send me pull requests for improvement and remember to pull first from master before you send pull requests. Most of the images I have used in this challenge came from 30DaysOfJavaScript challenge therefore you may need to rename file names and folders 30DaysOfReact. If you are good at arrays, loops, functions, objects, functional programming, destructuring and spreading and class then you will be able to follow the challenge properly. Otherwise, I strongly recommend you to check 30DaysOfJavaScript.

To get along with the challenge you need to have the following:

1. Motivation
2. A computer
3. Internet
4. A browser
5. A code editor
6. HTML, CSS and JavaScript intermediate level skill

I believe you have the motivation and a strong desire to be a developer, a computer and Internet.In addition to that basic to intermidate lelev HTML, CSS and JS. If you have those, then you have everything to get started.

You may not need node.js right now but you may need it for later. Install node.js.

After downloading double click and install

We can check if node is installed on our local machine by opening our device terminal or command prompt.

asabeneh $ node -v
v12.14.0

When making this tutorial I was using node version 12.14.0, but now the recommended version of node.js for download is 12.17.0.

There are many browsers out there. However, I strongly recommend Google Chrome.

Install google chrome if you do not have one yet. We can write small JavaScript code on the browser console, but we do not use the browser console to develop applications.

You can open Google Chrome console either by clicking three dots at the top right corner of the browser, selecting More tools -> Developer tools or using a keyboard shortcut. I prefer using shortcuts.

To open the Chrome console using a keyboard shortcut. It is good to know the shortcut too as a JavaScript and React developer you will spend much time on a browser console and don't be lazy to open it during development.

Mac
Command+Option+J

Windows/Linux:
Ctl+Shift+J

After you open the Google Chrome console, try to explore the marked buttons. We will spend most of the time on the Console. The Console is the place where your JavaScript code goes. The Google Console V8 engine changes your JavaScript code to machine code. Let us write a JavaScript code on the Google Chrome console:

We can write any JavaScript code on the Google console or any browser console. However, for this challenge, we only focus on Google Chrome console. Open the console using:

Mac
Command+Option+I

Windows:
Ctl+Shift+I

To write our first JavaScript code, we used a built-in function console.log(). We passed an argument as input data, and the function displays the output. We passed 'Hello, World' as input data or argument in the console.log() function.

console.log('Hello, World!')

The console.log() function can take multiple parameters separated by comma. The syntax looks like as follows:console.log(param1, param2, param3)

console.log('Hello', 'World', '!')
console.log('HAPPY', 'NEW', 'YEAR', 2020)
console.log('Welcome', 'to', 30, 'Days', 'Of', 'JavaScript')

As you can see from the snippet code above, console.log() can take multiple arguments. It is recommended to use as many console.log() prints to check what is happening in your code but don't keep all console.log() tests on your code forever. Make your life easy by keeping the browser console open.

We add comments to our code. Comments are very important to make code more readable and to leave remarks in our code. JavaScript does not execute the comment part of our code.In JavaScript, any text line starting with // in JavaScript is a comment, and anything enclosed like this /* */ is also a comment.

Example: Single Line Comment

// This is the first comment
// This is the second comment
// I am a single line comment

Example: Multiline Comment

/* This is a multiline comment
Multiline comments can take multiple lines
JavaScript is the language of the web
*/

Programming languages are similar to human languages. English or many other language uses words, phrases, sentences,compound sentences and other more to convey a meaningful message. The English meaning of syntax is the arrangement of words and phrases to create well-formed sentences in a language. The technical definition of syntax is the structure of statements in a computer language. Programing languages have syntax. JavaScript is a programming language and like other programming languages it has its own syntax. If we do not write a syntax that JavaScript understands, it will raise different types of errors. We will explore different kinds of JavaScript errors later. For now, let us see syntax errors.

I made a deliberate mistake. As a result, the console raises syntax errors. Actually, the syntax is very informative. It informs what type of mistake was made. By reading the error feedback guideline, we can correct the syntax and fix the problem. The process of identifying and removing errors from a program is called debugging. Let us fix the errors:

console.log('Hello, World!')
console.log('Hello, World!')

So far, we saw how to display text using the console.log(). If we are printing text or string using console.log(), the text has to be inside the single quotes, double quotes, or a backtick quotes. Example:

console.log('Hello, World!')
console.log('Hello, World!')
console.log(`Hello, World!`)

Now, let us practice more writing JavaScript codes using console.log() on google chrome console for number data types. In addition to the text, we can also do mathematical calculations using JavaScript. Let us do the following simple calculations. The console can directly take arguments without the console.log() function. However, it is included in this introduction because most of this challenge would be taking place in a text editor where the usage of the function would be mandatory. You can play around directly with instructions on the console.

console.log(2 + 3) // Addition
console.log(3 - 2) // Subtraction
console.log(2 * 3) // Multiplication
console.log(3 / 2) // Division
console.log(3 % 2) // Modulus - finding remainder
console.log(3 ** 2) // Exponentiation 3 ** 2 == 3 * 3

We can write our codes on the browser console, but it won't do for bigger projects. In a real working environment, developers use different code editors to write their codes. In this 30 days JavaScript challenge, we will be using Visual Studio Code.

Visual studio code is a very popular open-source text editor. I would recommend to download Visual Studio Code, but if you are in favor of other editors, feel free to follow with what you have.

If you installed Visual Studio Code, let us start using it.

Open the Visual Studio Code by double-clicking its icon. When you open it, you will get this kind of interface. Try to interact with the labeled icons.

JavaScript can be added to a web page in three different ways:

• Inline script
• Internal script
• External script
• Multiple External scripts

The following sections show different ways of adding JavaScript code to your web page.

Create a project folder on your desktop or in any location, name it 30DaysOfJS and create an index.html file in the project folder. Then paste the following code and open it in a browser, for example Chrome.

<!DOCTYPE html>
<html>
  <head>
    <title>30DaysOfScript:Inline Script</title>
  </head>
  <body>
    <button onclick="alert('Welcome to 30DaysOfJavaScript!')">Click Me</button>
  </body>
</html>

Now, you just wrote your first inline script. We can create a pop up alert message using the alert() built-in function.

The internal script can be written in the head or the body, but it is preferred to put it on the body of the HTML document. First, let us write on the head part of the page.

<!DOCTYPE html>
<html>
  <head>
    <title>30DaysOfScript:Internal Script</title>
    <script>
      console.log('Welcome to 30DaysOfJavaScript')
    </script>
  </head>
  <body></body>
</html>

This is how we write an internal script most of the time. Writing the JavaScript code in the body section is the most preferred option. Open the browser console to see the output from the console.log()

<!DOCTYPE html>
<html>
  <head>
    <title>30DaysOfScript:Internal Script</title>
  </head>
  <body>
    <button onclick="alert('Welcome to 30DaysOfJavaScript!');">Click Me</button>
    <script>
      console.log('Welcome to 30DaysOfJavaScript')
    </script>
  </body>
</html>

Open the browser console to see the output from the console.log()

Similar to the internal script, the external script link can be on the header or body, but it is preferred to put it in the body. First, we should create an external JavaScript file with .js extension. All files ending with .js extension. All files ending with .js extension are JavaScript files. Create a file named introduction.js inside your project directory and write the following code and link this .js file at the bottom of the body.

console.log('Welcome to 30DaysOfJavaScript')

External scripts in the head:

<!DOCTYPE html>
<html>
  <head>
    <title>30DaysOfJavaScript:External script</title>
    <script src="introduction.js"></script>
  </head>
  <body></body>
</html>

External scripts in the body:

<!DOCTYPE html>
<html>
  <head>
    <title>30DaysOfJavaScript:External script</title>
  </head>
  <body>
    //it could be in the header or in the body // Here is the recommended place
    to put the external script
    <script src="introduction.js"></script>
  </body>
</html>

Open the browser console to see the output of the console.log()

We can also link multiple external JavaScript files to a web page. Create a helloworld.js file inside the 30DaysOfJS folder and write the following code.

console.log('Hello, World!')

<!DOCTYPE html>
<html>
  <head>
    <title>Multiple External Scripts</title>
  </head>
  <body>
    <script src="./helloworld.js"></script>
    <script src="./introduction.js"></script>
  </body>
</html>

Your main.js file should be below all other scripts. It is very important to remember this.

Congratulations! You have completed the setup you need to get started with React, but before we dive into React let's do JavaScript refresher. If you are very comfortable with JavaScript you may skip the JavaScript refresher. The JavaScript refresh section is vast and it may take more than one day. From my experience people usually get stuck in React because their JavaScript knowledge is very shallow therefore to fill that gap all the necessary JavaScript feature which can be use in React are covered in the JavaScript refresher. In the JavaScript refresher there are many questions and you are not required to solve them. Click here if you want to jump into React.

We use var, let and const to declare a variable. The var is functions scope, however let and const are block scope. In this challenge we use ES6 and above features of JavaScript. Avoid using var.

let firstName = 'Asabeneh'
firstName = 'Eyob'

const PI = 3.14 // Not allowed to reassign PI to a new value
// PI = 3.

If you do not feel comfortable with data types check the following link

In contrast to variables, an array can store multiple values. Each value in an array has an index, and each index has a reference in a memory address. Each value can be accessed by using their indexes. The index of an array starts from zero, and the index of the last element is less by one from the length of the array.

An array is a collection of different data types which are ordered and changeable(modifiable). An array allows storing duplicate elements and different data types. An array can be empty, or it may have different data type values.

In JavaScript, we can create an array in different ways. Let us see different ways to create an array. It is very common to use const instead of let to declare an array variable. If you ar using const it means you do not use that variable name again.

• Using Array constructor

// syntax
const arr = Array()
// or
// let arr = new Array()
console.log(arr) // []

• Using square brackets([])

// syntax
// This the most recommended way to create an empty list
const arr = []
console.log(arr)

Array with initial values. We use length property to find the length of an array.

const numbers = [0, 3.14, 9.81, 37, 98.6, 100] // array of numbers
const fruits = ['banana', 'orange', 'mango', 'lemon'] // array of strings, fruits
const vegetables = ['Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot'] // array of strings, vegetables
const animalProducts = ['milk', 'meat', 'butter', 'yoghurt'] // array of strings, products
const webTechs = ['HTML', 'CSS', 'JS', 'React', 'Redux', 'Node', 'MongDB'] // array of web technologies
const countries = ['Finland', 'Denmark', 'Sweden', 'Norway', 'Iceland'] // array of strings, countries

// Print the array and its length

console.log('Numbers:', numbers)
console.log('Number of numbers:', numbers.length)

console.log('Fruits:', fruits)
console.log('Number of fruits:', fruits.length)

console.log('Vegetables:', vegetables)
console.log('Number of vegetables:', vegetables.length)

console.log('Animal products:', animalProducts)
console.log('Number of animal products:', animalProducts.length)

console.log('Web technologies:', webTechs)
console.log('Number of web technologies:', webTechs.length)

console.log('Countries:', countries)
console.log('Number of countries:', countries.length)

Numbers: [0, 3.14, 9.81, 37, 98.6, 100]
Number of numbers: 6
Fruits: ['banana', 'orange', 'mango', 'lemon']
Number of fruits: 4
Vegetables: ['Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot']
Number of vegetables: 5
Animal products: ['milk', 'meat', 'butter', 'yoghurt']
Number of animal products: 4
Web technologies: ['HTML', 'CSS', 'JS', 'React', 'Redux', 'Node', 'MongDB']
Number of web technologies: 7
Countries: ['Finland', 'Estonia', 'Denmark', 'Sweden', 'Norway']
Number of countries: 5

• Array can have items of different data types

const arr = [
  'Asabeneh',
  250,
  true,
  { country: 'Finland', city: 'Helsinki' },
  { skills: ['HTML', 'CSS', 'JS', 'React', 'Python'] },
] // arr containing different data types
console.log(arr)

As we have seen in the earlier section, we can split a string at different positions, and we can change to an array. Let us see the examples below.

let js = 'JavaScript'
const charsInJavaScript = js.split('')

console.log(charsInJavaScript) // ["J", "a", "v", "a", "S", "c", "r", "i", "p", "t"]

let companiesString = 'Facebook, Google, Microsoft, Apple, IBM, Oracle, Amazon'
const companies = companiesString.split(',')

console.log(companies) // ["Facebook", " Google", " Microsoft", " Apple", " IBM", " Oracle", " Amazon"]
let txt =
  'I love teaching and empowering people. I teach HTML, CSS, JS, React, Python.'
const words = txt.split(' ')

console.log(words)
// the text has special characters think how you can just get only the words
// ["I", "love", "teaching", "and", "empowering", "people.", "I", "teach", "HTML,", "CSS,", "JS,", "React,", "Python"]

We access each element in an array using their index. An array index starts from 0. The picture below clearly shows the index of each element in the array.

const fruits = ['banana', 'orange', 'mango', 'lemon']
let firstFruit = fruits[0] // we are accessing the first item using its index

console.log(firstFruit) // banana

secondFruit = fruits[1]
console.log(secondFruit) // orange

let lastFruit = fruits[3]
console.log(lastFruit) // lemon
// Last index can be calculated as follows

let lastIndex = fruits.length - 1
lastFruit = fruits[lastIndex]

console.log(lastFruit) // lemon

const numbers = [0, 3.14, 9.81, 37, 98.6, 100] // set of numbers

console.log(numbers.length) // => to know the size of the array, which is 6
console.log(numbers) // -> [0, 3.14, 9.81, 37, 98.6, 100]
console.log(numbers[0]) //  -> 0
console.log(numbers[5]) //  -> 100

let lastIndex = numbers.length - 1
console.log(numbers[lastIndex]) // -> 100

const webTechs = [
  'HTML',
  'CSS',
  'JavaScript',
  'React',
  'Redux',
  'Node',
  'MongoDB',
] // List of web technologies

console.log(webTechs) // all the array items
console.log(webTechs.length) // => to know the size of the array, which is 7
console.log(webTechs[0]) //  -> HTML
console.log(webTechs[6]) //  -> MongoDB

let lastIndex = webTechs.length - 1
console.log(webTechs[lastIndex]) // -> MongoDB

const countries = [
  'Albania',
  'Bolivia',
  'Canada',
  'Denmark',
  'Ethiopia',
  'Finland',
  'Germany',
  'Hungary',
  'Ireland',
  'Japan',
  'Kenya',
] // List of countries

console.log(countries) // -> all countries in array
console.log(countries[0]) //  -> Albania
console.log(countries[10]) //  -> Kenya

let lastIndex = countries.length - 1
console.log(countries[lastIndex]) //  -> Kenya

const shoppingCart = [
  'Milk',
  'Mango',
  'Tomato',
  'Potato',
  'Avocado',
  'Meat',
  'Eggs',
  'Sugar',
] // List of food products

console.log(shoppingCart) // -> all shoppingCart in array
console.log(shoppingCart[0]) //  -> Milk
console.log(shoppingCart[7]) //  -> Sugar

let lastIndex = shoppingCart.length - 1
console.log(shoppingCart[lastIndex]) //  -> Sugar

An array is mutable(modifiable). Once an array is created, we can modify the contents of the array elements.

const numbers = [1, 2, 3, 4, 5]
numbers[0] = 10 // changing 1 at index 0 to 10
numbers[1] = 20 // changing  2 at index 1 to 20

console.log(numbers) // [10, 20, 3, 4, 5]

const countries = [
  'Albania',
  'Bolivia',
  'Canada',
  'Denmark',
  'Ethiopia',
  'Finland',
  'Germany',
  'Hungary',
  'Ireland',
  'Japan',
  'Kenya',
]

countries[0] = 'Afghanistan' // Replacing Albania by Afghanistan
let lastIndex = countries.length - 1
countries[lastIndex] = 'Korea' // Replacing Kenya by Korea

console.log(countries)

["Afghanistan", "Bolivia", "Canada", "Denmark", "Ethiopia", "Finland", "Germany", "Hungary", "Ireland", "Japan", "Korea"]

There are different methods to manipulate an array. These are some of the available methods to deal with arrays:Array, length, concat, indexOf, slice, splice, join, toString, includes, lastIndexOf, isArray, fill, push, pop, shift, unshift

Array:To create an array.

const arr = Array() // creates an an empty array
console.log(arr)

const eightEmptyValues = Array(8) // it creates eight empty values
console.log(eightEmptyValues) // [empty x 8]

fill: Fill all the array elements with a static value

const arr = Array() // creates an an empty array
console.log(arr)

const eightXvalues = Array(8).fill('X') // it creates eight element values filled with 'X'
console.log(eightXvalues) // ['X', 'X','X','X','X','X','X','X']

const eight0values = Array(8).fill(0) // it creates eight element values filled with '0'
console.log(eight0values) // [0, 0, 0, 0, 0, 0, 0, 0]

const four4values = Array(4).fill(4) // it creates 4 element values filled with '4'
console.log(four4values) // [4, 4, 4, 4]

concat:To concatenate two arrays.

const firstList = [1, 2, 3]
const secondList = [4, 5, 6]
const thirdList = firstList.concat(secondList)

console.log(thirdList) // [1, 2, 3, 4, 5, 6]

const fruits = ['banana', 'orange', 'mango', 'lemon'] // array of fruits
const vegetables = ['Tomato', 'Potato', 'Cabbage', 'Onion', 'Carrot'] // array of vegetables
const fruitsAndVegetables = fruits.concat(vegetables) // concatenate the two arrays

console.log(fruitsAndVegetables)

["banana", "orange", "mango", "lemon", "Tomato", "Potato", "Cabbage", "Onion", "Carrot"]

Length:To know the size of the array

const numbers = [1, 2, 3, 4, 5]
console.log(numbers.length) // -> 5 is the size of the array

indexOf:To check if an item exist in an array. If it exists it returns the index else it returns -1.

const numbers = [1, 2, 3, 4, 5]

console.log(numbers.indexOf(5)) // -> 4
console.log(numbers.indexOf(0)) // -> -1
console.log(numbers.indexOf(1)) // -> 0
console.log(numbers.indexOf(6)) // -> -1

Check an element if it exist in an array.

• Check items in a list

// let us check if a banana exist in the array

const fruits = ['banana', 'orange', 'mango', 'lemon']
let index = fruits.indexOf('banana') // 0

if (index != -1) {
  console.log('This fruit does exist in the array')
} else {
  console.log('This fruit does not exist in the array')
}
// This fruit does exist in the array

// we can use also ternary here
index != -1
  ? console.log('This fruit does exist in the array')
  : console.log('This fruit does not exist in the array')

// let us check if a avocado exist in the array
let indexOfAvocado = fruits.indexOf('avocado') // -1, if the element not found index is -1
if (indexOfAvocado != -1) {
  console.log('This fruit does exist in the array')
} else {
  console.log('This fruit does not exist in the array')
}
// This fruit does not exist in the array

lastIndexOf: It gives the position of the last item in the array. If it exist, it returns the index else it returns -1.

const numbers = [1, 2, 3, 4, 5, 3, 1, 2]

console.log(numbers.lastIndexOf(2)) // 7
console.log(numbers.lastIndexOf(0)) // -1
console.log(numbers.lastIndexOf(1)) //  6
console.log(numbers.lastIndexOf(4)) //  3
console.log(numbers.lastIndexOf(6)) // -1

includes:To check if an item exist in an array. If it exist it returns the true else it returns false.

const numbers = [1, 2, 3, 4, 5]

console.log(numbers.includes(5)) // true
console.log(numbers.includes(0)) // false
console.log(numbers.includes(1)) // true
console.log(numbers.includes(6)) // false

const webTechs = [
  'HTML',
  'CSS',
  'JavaScript',
  'React',
  'Redux',
  'Node',
  'MongoDB',
] // List of web technologies

console.log(webTechs.includes('Node')) // true
console.log(webTechs.includes('C')) // false

Array.isArray:To check if the data type is an array

const numbers = [1, 2, 3, 4, 5]
console.log(Array.isArray(numbers)) // true

const number = 100
console.log(Array.isArray(number)) // false

toString:Converts array to string

const numbers = [1, 2, 3, 4, 5]
console.log(numbers.toString()) // 1,2,3,4,5

const names = ['Asabeneh', 'Mathias', 'Elias', 'Brook']
console.log(names.toString()) // Asabeneh,Mathias,Elias,Brook

join: It is used to join the elements of the array, the argument we passed in the join method will be joined in the array and return as a string. By default, it joins with a comma, but we can pass different string parameter which can be joined between the items.

const numbers = [1, 2, 3, 4, 5]
console.log(numbers.join()) // 1,2,3,4,5

const names = ['Asabeneh', 'Mathias', 'Elias', 'Brook']

console.log(names.join()) // Asabeneh,Mathias,Elias,Brook
console.log(names.join('')) //AsabenehMathiasEliasBrook
console.log(names.join(' ')) //Asabeneh Mathias Elias Brook
console.log(names.join(', ')) //Asabeneh, Mathias, Elias, Brook
console.log(names.join(' # ')) //Asabeneh # Mathias # Elias # Brook

const webTechs = [
  'HTML',
  'CSS',
  'JavaScript',
  'React',
  'Redux',
  'Node',
  'MongoDB',
] // List of web technologies

console.log(webTechs.join()) // "HTML,CSS,JavaScript,React,Redux,Node,MongoDB"
console.log(webTechs.join(' # ')) // "HTML # CSS # JavaScript # React # Redux # Node # MongoDB"

Slice: To cut out a multiple items in range. It takes two parameters:starting and ending position. It doesn't include the ending position.

const numbers = [1, 2, 3, 4, 5]

console.log(numbers.slice()) // -> it copies all  item
console.log(numbers.slice(0)) // -> it copies all  item
console.log(numbers.slice(0, numbers.length)) // it copies all  item
console.log(numbers.slice(1, 4)) // -> [2,3,4] // it doesn't include the ending position

Splice: It takes three parameters:Starting position, number of times to be removed and number of items to be added.

const numbers = [1, 2, 3, 4, 5]

console.log(numbers.splice()) // -> remove all items

const numbers = [1, 2, 3, 4, 5]
console.log(numbers.splice(0, 1)) // remove the first item

const numbers = [1, 2, 3, 4, 5, 6]
console.log(numbers.splice(3, 3, 7, 8, 9)) // -> [1, 2, 3, 7, 8, 9] //it removes three item and replace three items

Push: adding item in the end. To add item to the end of an existing array we use the push method.

// syntax
const arr = ['item1', 'item2', 'item3']
arr.push('new item')

console.log(arr)
// ['item1', 'item2','item3','new item']

const numbers = [1, 2, 3, 4, 5]
numbers.push(6)

console.log(numbers) // -> [1,2,3,4,5,6]

numbers.pop() // -> remove one item from the end
console.log(numbers) // -> [1,2,3,4,5]

let fruits = ['banana', 'orange', 'mango', 'lemon']
fruits.push('apple')

console.log(fruits) // ['banana', 'orange', 'mango', 'lemon', 'apple']

fruits.push('lime')
console.log(fruits) // ['banana', 'orange', 'mango', 'lemon', 'apple', 'lime']

pop: Removing item in the end.

const numbers = [1, 2, 3, 4, 5]
numbers.pop() // -> remove one item from the end

console.log(numbers) // -> [1,2,3,4]

shift: Removing one array element in the beginning of the array.

const numbers = [1, 2, 3, 4, 5]
numbers.shift() // -> remove one item from the beginning

console.log(numbers) // -> [2,3,4,5]

unshift: Adding array element in the beginning of the array.

const numbers = [1, 2, 3, 4, 5]
numbers.unshift(0) // -> add one item from the beginning

console.log(numbers) // -> [0,1,2,3,4,5]

reverse: reverse the order of an array.

const numbers = [1, 2, 3, 4, 5]
numbers.reverse() // -> reverse array order

console.log(numbers) // [5, 4, 3, 2, 1]

numbers.reverse()
console.log(numbers) // [1, 2, 3, 4, 5]

sort: arrange array elements in ascending order. Sort takes a call back function, we will see how we use sort with a call back function in the coming sections.

const webTechs = [
  'HTML',
  'CSS',
  'JavaScript',
  'React',
  'Redux',
  'Node',
  'MongoDB',
]

webTechs.sort()
console.log(webTechs) // ["CSS", "HTML", "JavaScript", "MongoDB", "Node", "React", "Redux"]

webTechs.reverse() // after sorting we can reverse it
console.log(webTechs) // ["Redux", "React", "Node", "MongoDB", "JavaScript", "HTML", "CSS"]

Array can store different data types including an array itself. Let us create an array of arrays

const firstNums = [1, 2, 3]
const secondNums = [1, 4, 9]

const arrayOfArray = [
  [1, 2, 3],
  [1, 2, 3],
]
console.log(arrayOfArray[0]) // [1, 2, 3]

const frontEnd = ['HTML', 'CSS', 'JS', 'React', 'Redux']
const backEnd = ['Node', 'Express', 'MongoDB']
const fullStack = [frontEnd, backEnd]
console.log(fullStack) // [["HTML", "CSS", "JS", "React", "Redux"], ["Node", "Express", "MongoDB"]]
console.log(fullStack.length) // 2
console.log(fullStack[0]) // ["HTML", "CSS", "JS", "React", "Redux"]
console.log(fullStack[1]) // ["Node", "Express", "MongoDB"]

const countries = [
  'Albania',
  'Bolivia',
  'Canada',
  'Denmark',
  'Ethiopia',
  'Finland',
  'Germany',
  'Hungary',
  'Ireland',
  'Japan',
  'Kenya',
]

const webTechs = [
  'HTML',
  'CSS',
  'JavaScript',
  'React',
  'Redux',
  'Node',
  'MongoDB',
]

1. Declare an empty array;
2. Declare an array with more than 5 number of elements
3. Find the length of your array
4. Get the first item, the middle item and the last item of the array
5. Declare an array called mixedDataTypes, put different data types in the array and find the length of the array. The array size should be greater than 5
6. Declare an array variable name itCompanies and assign initial values Facebook, Google, Microsoft, Apple, IBM, Oracle and Amazon
7. Print the array using console.log()
8. Print the number of companies in the array
9. Print the first company, middle and last company
10. Print out each company
11. Change each company name to uppercase one by one and print them out
12. Print the array like as a sentence: Facebook, Google, Microsoft, Apple, IBM,Oracle and Amazon are big IT companies.
13. Check if a certain company exists in the itCompanies array. If it exist return the company else return a company is not found
14. Filter out companies which have more than one 'o' without the filter method
15. Sort the array using sort() method
16. Reverse the array using reverse() method
17. Slice out the first 3 companies from the array
18. Slice out the last 3 companies from the array
19. Slice out the middle IT company or companies from the array
20. Remove the first IT company from the array
21. Remove the middle IT company or companies from the array
22. Remove the last IT company from the array
23. Remove all IT companies

1. Create a separate countries.js file and store the countries array into this file, create a separate file web_techs.js and store the webTechs array into this file. Access both file in main.js file

2. First remove all the punctuations and change the string to array and count the number of words in the array

   let text =
     'I love teaching and empowering people. I teach HTML, CSS, JS, React, Python.'
   console.log(words)
   console.log(words.length)

   ["I", "love", "teaching", "and", "empowering", "people", "I", "teach", "HTML", "CSS", "JS", "React", "Python"]
   
   13

3. In the following shopping cart add, remove, edit items

   const shoppingCart = ['Milk', 'Coffee', 'Tea', 'Honey']

   • add 'Meat' in the beginning of your shopping cart if it has not been already added
   • add Sugar at the end of you shopping cart if it has not been already added
   • remove 'Honey' if you are allergic to honey
   • modify Tea to 'Green Tea'

4. In countries array check if 'Ethiopia' exists in the array if it exists print 'ETHIOPIA'. If it does not exist add to the countries list.

5. In the webTechs array check if Sass exists in the array and if it exists print 'Sass is a CSS preprocess'. If it does not exist add Sass to the array and print the array.

6. Concatenate the following two variables and store it in a fullStack variable.

   const frontEnd = ['HTML', 'CSS', 'JS', 'React', 'Redux']
   const backEnd = ['Node', 'Express', 'MongoDB']
   
   console.log(fullStack)

   ["HTML", "CSS", "JS", "React", "Redux", "Node", "Express", "MongoDB"]

1. The following is an array of 10 students ages: js const ages = [19, 22, 19, 24, 20, 25, 26, 24, 25, 24] - Sort the array and find the min and max age - Find the median age(one middle item or two middle items divided by two) - Find the average age(all items divided by number of items) - Find the range of the ages(max minus min) - Compare the value of (min - average) and (max - average), use abs() method

   1.Slice the first ten countries from the countries array

2. Find the middle country(ies) in the countries array

3. Divide the countries array into two equal arrays if it is even. If countries array is not even , one more country for the first half.

Conditional statements are used for make decisions based on different conditions. By default , statements in JavaScript script executed sequentially from top to bottom. If the processing logic require so, the sequential flow of execution can be altered in two ways:

• Conditional execution: a block of one or more statements will be executed if a certain expression is true
• Repetitive execution: a block of one or more statements will be repetitively executed as long as a certain expression is true. In this section, we will cover if, else , else if statements. The comparison and logical operators we learned in the previous sections will be useful in here.

Conditions can be implementing using the following ways:

• if
• if else
• if else if else
• switch
• ternary operator

In JavaScript and other programming languages the key word if is to used check if a condition is true and to execute the block code. To create an if condition, we need if keyword, condition inside a parenthesis and block of code inside a curly bracket({}).

// syntax
if (condition) {
  //this part of code runs for truthy condition
}

Example:

let num = 3
if (num > 0) {
  console.log(`${num} is a positive number`)
}
//  3 is a positive number

As you can see in the condition example above, 3 is greater than 0, so it is a positive number. The condition was true and the block of code was executed. However, if the condition is false, we won't see any results.

let isRaining = true
if (isRaining) {
  console.log('Remember to take your rain coat.')
}

The same goes for the second condition, if isRaining is false the if block will not be executed and we do not see any output. In order to see the result of a falsy condition, we should have another block, which is going to be else.

If condition is true the first block will be executed, if not the else condition will be executed.

// syntax
if (condition) {
  // this part of code runs for truthy condition
} else {
  // this part of code runs for false condition
}

let num = 3
if (num > 0) {
  console.log(`${num} is a positive number`)
} else {
  console.log(`${num} is a negative number`)
}
//  3 is a positive number

num = -3
if (num > 0) {
  console.log(`${num} is a positive number`)
} else {
  console.log(`${num} is a negative number`)
}
//  -3 is a negative number

let isRaining = true
if (isRaining) {
  console.log('You need a rain coat.')
} else {
  console.log('No need for a rain coat.')
}
// You need a rain coat.

isRaining = false
if (isRaining) {
  console.log('You need a rain coat.')
} else {
  console.log('No need for a rain coat.')
}
// No need for a rain coat.

The last condition is false, therefore the else block was executed. What if we have more than two conditions? In that case, we would use else if conditions.

On our daily life, we make decisions on daily basis. We make decisions not by checking one or two conditions instead we make decisions based on multiple conditions. As similar to our daily life, programming is also full of conditions. We use else if when we have multiple conditions.

// syntax
if (condition) {
  // code
} else if (condition) {
  // code
} else {
  //  code
}

Example:

let a = 0
if (a > 0) {
  console.log(`${a} is a positive number`)
} else if (a < 0) {
  console.log(`${a} is a negative number`)
} else if (a == 0) {
  console.log(`${a} is zero`)
} else {
  console.log(`${a} is not a number`)
}

// if else if else
let weather = 'sunny'
if (weather === 'rainy') {
  console.log('You need a rain coat.')
} else if (weather === 'cloudy') {
  console.log('It might be cold, you need a jacket.')
} else if (weather === 'sunny') {
  console.log('Go out freely.')
} else {
  console.log('No need for rain coat.')
}

Switch is an alternative for if else if else else. The switch statement starts with a switch keyword followed by a parenthesis and code block. Inside the code block we will have different cases. Case block runs if the value in the switch statement parenthesis matches with the case value. The break statement is to terminate execution so the code execution does not go down after the condition is satisfied. The default block runs if all the cases don't satisfy the condition.

switch (caseValue) {
  case 1:
    // code
    break
  case 2:
    // code
    break
  case 3:
  // code
  default:
  // code
}

let weather = 'cloudy'
switch (weather) {
  case 'rainy':
    console.log('You need a rain coat.')
    break
  case 'cloudy':
    console.log('It might be cold, you need a jacket.')
    break
  case 'sunny':
    console.log('Go out freely.')
    break
  default:
    console.log(' No need for rain coat.')
}

// Switch More Examples
let dayUserInput = prompt('What day is today ?')
let day = dayUserInput.toLowerCase()

switch (day) {
  case 'monday':
    console.log('Today is Monday')
    break
  case 'tuesday':
    console.log('Today is Tuesday')
    break
  case 'wednesday':
    console.log('Today is Wednesday')
    break
  case 'thursday':
    console.log('Today is Thursday')
    break
  case 'friday':
    console.log('Today is Friday')
    break
  case 'saturday':
    console.log('Today is Saturday')
    break
  case 'sunday':
    console.log('Today is Sunday')
    break
  default:
    console.log('It is not a week day.')
}

// Examples to use conditions in the cases

let num = prompt('Enter number')
switch (true) {
  case num > 0:
    console.log('Number is positive')
    break
  case num == 0:
    console.log('Numbers is zero')
    break
  case num < 0:
    console.log('Number is negative')
    break
  default:
    console.log('Entered value was not a number')
}

Ternary operator is very common in React. It is a short way to write if else statement. In React we use ternary operator in many cases.

To generalize, ternary operator is another way to write conditionals.

let isRaining = true
isRaining
  ? console.log('You need a rain coat.')
  : console.log('No need for a rain coat.')

1. Get user input using prompt(“Enter your age:”). If user is 18 or older , give feedback:'You are old enough to drive' but if not 18 give another feedback stating to wait for the number of years he needs to turn 18.

   Enter your age: 30
   You are old enough to drive.
   
   Enter your age:15
   You are left with 3 years to drive.

2. Compare the values of myAge and yourAge using if … else. Based on the comparison and log the result to console stating who is older (me or you). Use prompt(“Enter your age:”) to get the age as input.

   Enter your age: 30
   You are 5 years older than me.

3. If a is greater than b return 'a is greater than b' else 'a is less than b'. Try to implement it in two ways

   • using if else
   • ternary operator.

   let a = 4
   let b = 3

     4 is greater than 3

4. Even numbers are divisible by 2 and the remainder is zero. How do you check, if a number is even or not using JavaScript?

   Enter a number: 2
   2 is an even number
   
   Enter a number: 9
   9 is is an odd number.

1. Write a code which can give grades to students according to theirs scores:
   • 80-100, A
   • 70-89, B
   • 60-69, C
   • 50-59, D
   • 0-49, F
2. Check if the season is Autumn, Winter, Spring or Summer. If the user input is :
   • September, October or November, the season is Autumn.
   • December, January or February, the season is Winter.
   • March, April or May, the season is Spring
   • June, July or August, the season is Summer
3. Check if a day is weekend day or a working day. Your script will take day as an input.

    What is the day  today? Saturday
    Saturday is a weekend.

    What is the day today? saturDaY
    Saturday is a weekend.

    What is the day today? Friday
    Friday is a working day.

    What is the day today? FrIDAy
    Friday is a working day.

1. Write a program which tells the number of days in a month.

  Enter a month: January
  January has 31 days.

  Enter a month: JANUARY
  January has 31 day

  Enter a month: February
  February has 28 days.

  Enter a month: FEbruary
  February has 28 days.

1. Write a program which tells the number of days in a month, now consider leap year.

In programming we use different loops to carry out repetitive tasks. Therefore, loop can help us to automate tedious and repetitive task. JavaScript has also different types of loops which we can use to work on repetitive task.

Imagine if your are asked to print Hello world one thousand times without a loop, it may take an hour or two to do this tedious task. However, using loop we can print it in less than a second.

Loops:

• for
• while
• do while
• for of
• forEach
• for in

A loop usually goes until the condition gets false. But sometimes we like to interrupt the loop or skip an item during iteration. We use break to interrupt the loop and continue to skip an item during iteration.

We use for loop when we know how many iteration we go. Let's see the following example

// for loop syntax

for (initialization, condition, increment/decrement) {
    code goes here
}

This code prints from 0 to 5.

for (let i = 0; i < 6; i++) {
  console.log(i)
}

For example if we want to sum all the numbers from 0 to 100.

let sum = 0
for (let i = 0; i < 101; i++) {
  sum += i
}

console.log(sum)

If we want to sum only even numbers:

let sum = 0
for (let i = 0; i < 101; i += 2) {
  sum += i
}

console.log(sum)

// or another way

let total = 0
for (let i = 0; i < 101; i++) {
  if (i % 2 == 0) {
    total += i
  }
}
console.log(total)

This code iterates through the array

const nums = [1, 2, 3, 4, 5]
for (let i = 0; i < 6; i++) {
  console.log(nums[i])
}

This code prints 5 to 0. Looping in reverse order

for (let i = 5; i >= 0; i--) {
  console.log(i)
}

The Code below can reverse an array.

const nums = [1, 2, 3, 4, 5]
const lastIndex = nums.length - 1
const newArray = []
for (let i = lastIndex; i >= 0; i--) {
  newArray.push(nums[i])
}

console.log(newArray)

We use the while loop when we do not know how man iteration we go in advance.

let count = prompt('Enter a positive number: ')
while (count > 0) {
  console.log(count)
  count--
}

Do while run at least once if the condition is true or false

let count = 0
do {
  console.log(count)
  count++
} while (count < 11)

The code below runs ones though the condition is false

let count = 11
do {
  console.log(count)
  count++
} while (count < 11)

While loop is the least important loop in many programming languages.

The for of loop is very handy to use it with array. If we are not interested in the index of the array a for of loop is preferable to regular for loop or forEach loop.

const numbers = [1, 2, 3, 4, 5]
for (const number of numbers) {
  console.log(number)
}

const countries = ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']
for (const country of countries) {
  console.log(country.toUpperCase())
}

If we are interested in the index of the array forEach is preferable to for of loop. The forEach array method takes a callback function, the callback function takes three arguments: the item, the index and the array itself.

const numbers = [1, 2, 3, 4, 5]
numbers.forEach((number, i) => {
  console.log(number, i)
})

const countries = ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']
countries.forEach((country, i, arr) => {
  console.log(i, country.toUpperCase())
})

The for in loop can be used with object literals to get the keys of the object.

const user = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  skills: ['HTML', 'CSS', 'JS', 'React', 'Node', 'Python', 'D3.js'],
}

for (const key in user) {
  console.log(key, user[key])
}

Break is used to interrupt a loop.

for (let i = 0; i <= 5; i++) {
  if (i == 3) {
    break
  }
  console.log(i)
}

// 0 1 2

The above code stops if 3 found in the iteration process.

We use the keyword continue to skip a certain iterations.

for (let i = 0; i <= 5; i++) {
  if (i == 3) {
    continue
  }
  console.log(i)
}
// 0 1 2 4 5

• Regular for loop can be used anywhere when the number of iteration is known.
• While loop when the number of iteration is not know
• Do while loop and while loop are almost the same but do while loop run at least once even when the condition is false
• for of is used only for array
• forEach is used for array
• for in is used for object

Variable is the fundamental part in programming. We declare variable to store different data types. To declare a variable we use the key word var, let and const. A variable can declared at different scope. In this section we will see the scope, scope of variables when we use var or let. Variables scopes can be:

• Window
• Global
• Local

Variable can be declared globally or locally or window scope. We will see both global and local scope. Anything declared without let, var or const is scoped at window level.

Let us image we have a scope.js file.

Without using console.log() open your browser and check, you will see the value of a and b if you write a or b on the browser. That means a and b are already available in the window.

//scope.js
a = 'JavaScript' // is a window scope this found anywhere
b = 10 // this is a window scope variable
function letsLearnScope() {
  console.log(a, b)
  if (true) {
    console.log(a, b)
  }
}
console.log(a, b) // accessible

A globally declared variable can be accessed every where in the same file. But the term global is relative. It can be global to the file or it can be global relative to some block of codes.

//scope.js
let a = 'JavaScript' // is a global scope it will be found anywhere in this file
let b = 10 // is a global scope it will be found anywhere in this file
function letsLearnScope() {
  console.log(a, b) // JavaScript 10, accessible
  if (true) {
    let a = 'Python'
    let b = 100
    console.log(a, b) // Python 100
  }
  console.log(a, b)
}
letsLearnScope()
console.log(a, b) // JavaScript 10, accessible

A variable declared as local can be accessed only in certain block code.

//scope.js
let a = 'JavaScript' // is a global scope it will be found anywhere in this file
let b = 10 // is a global scope it will be found anywhere in this file
function letsLearnScope() {
  console.log(a, b) // JavaScript 10, accessible
  let c = 30
  if (true) {
    // we can access from the function and outside the function but
    // variables declared inside the if will not be accessed outside the if block
    let a = 'Python'
    let b = 20
    let d = 40
    console.log(a, b, c) // Python 20 30
  }
  // we can not access c because c's scope is only the if block
  console.log(a, b) // JavaScript 10
}
letsLearnScope()
console.log(a, b) // JavaScript 10, accessible

Now, you have an understanding of scope. A variable declared with var only scoped to function but variable declared with let or const is block scope(function block, if block, loop etc). Block in JavaScript is a code in between two curly brackets ({}).

//scope.js
function letsLearnScope() {
  var gravity = 9.81
  console.log(gravity)
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined

if (true) {
  var gravity = 9.81
  console.log(gravity) // 9.81
}
console.log(gravity) // 9.81

for (var i = 0; i < 3; i++) {
  console.log(i) // 1, 2, 3
}
console.log(i)

In ES6 and above there is let and const, so you will not suffer from the sneakiness of var. When we use let our variable is block scoped and it will not infect other parts of our code.

//scope.js
function letsLearnScope() {
  // you can use let or const, but gravity is constant I prefer to use const
  const gravity = 9.81
  console.log(gravity)
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined

if (true) {
  const gravity = 9.81
  console.log(gravity) // 9.81
}
// console.log(gravity), Uncaught ReferenceError: gravity is not defined

for (let i = 0; i < 3; i++) {
  console.log(i) // 1, 2, 3
}
// console.log(i), Uncaught ReferenceError: gravity is not defined

The scope let and const is the same. The difference is only reassigning. We can not change or reassign the value of const variable. I would strongly suggest you to use let and const, by using let and const you will writ clean code and avoid hard to debug mistakes. As a rule of thumb, you can use let for any value which change, const for any constant value, and for array, object, arrow function and function expression.

Everything can be an object and objects do have properties and properties have values, so an object is a key value pair. The order of the key is not reserved, or there is no order. To create an object literal, we use two curly brackets.

An empty object

const person = {}

Now, the person object has firstName, lastName, age, location, skills and isMarried properties. The value of properties or keys could be a string, number, boolean, an object, null, undefined or a function.

Let us see some examples of object. Each key has a value in the object.

const rectangle = {
  length: 20,
  width: 20,
}
console.log(rectangle) // {length: 20, width: 20}

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js',
  ],
  isMarried: true,
}
console.log(person)

We can access values of object using two methods:

• using . followed by key name if the key-name is a one word
• using square bracket and a quote

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js',
  ],
  getFullName: function () {
    return `${this.firstName}${this.lastName}`
  },
  'phone number': '+3584545454545',
}

// accessing values using .
console.log(person.firstName)
console.log(person.lastName)
console.log(person.age)
console.log(person.location)

// value can be accessed using square bracket and key name
console.log(person['firstName'])
console.log(person['lastName'])
console.log(person['age'])
console.log(person['age'])
console.log(person['location'])

// for instance to access the phone number we only use the square bracket method
console.log(person['phone number'])

Now, the person object has getFullName properties. The getFullName is function inside the person object and we call it an object method. The this key word refers to the object itself. We can use the word this to access the values of different properties of the object. We can not use an arrow function as object method because the word this refers to the window inside an arrow function instead of the object itself. Example of object:

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js',
  ],
  getFullName: function () {
    return `${this.firstName} ${this.lastName}`
  },
}

console.log(person.getFullName())
// Asabeneh Yetayeh

An object is a mutable data structure and we can modify the content of an object after it gets created.

Setting a new keys in an object

const person = {
  firstName: 'Asabeneh',
  lastName: 'Yetayeh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: [
    'HTML',
    'CSS',
    'JavaScript',
    'React',
    'Node',
    'MongoDB',
    'Python',
    'D3.js',
  ],
  getFullName: function () {
    return `${this.firstName} ${this.lastName}`
  },
}
person.nationality = 'Ethiopian'
person.country = 'Finland'
person.title = 'teacher'
person.skills.push('Meteor')
person.skills.push('SasS')
person.isMarried = true

person.getPersonInfo = function () {
  let skillsWithoutLastSkill = this.skills
    .splice(0, this.skills.length - 1)
    .join(', ')
  let lastSkill = this.skills.splice(this.skills.length - 1)[0]

  let skills = `${skillsWithoutLastSkill}, and ${lastSkill}`
  let fullName = this.getFullName()
  let statement = `${fullName} is a ${this.title}.\nHe lives in ${this.country}.\nHe teaches ${skills}.`
  return statement
}
console.log(person)
console.log(person.getPersonInfo())

Asabeneh Yetayeh is a teacher.
He lives in Finland.
He teaches HTML, CSS, JavaScript, React, Node, MongoDB, Python, D3.js, Meteor, and SasS.

There are different methods to manipulate an object. Let us see some of the available methods.

Object.assign: To copy an object without modifying the original object

const person = {
  firstName: 'Asabeneh',
  age: 250,
  country: 'Finland',
  city: 'Helsinki',
  skills: ['HTML', 'CSS', 'JS'],
  title: 'teacher',
  address: {
    street: 'Heitamienkatu 16',
    pobox: 2002,
    city: 'Helsinki',
  },
  getPersonInfo: function () {
    return `I am ${this.firstName} and I live in ${this.city}, ${this.country}. I am ${this.age}.`
  },
}

//Object methods: Object.assign, Object.keys, Object.values, Object.entries
//hasOwnProperty

const copyPerson = Object.assign({}, person)
console.log(copyPerson)

Object.keys: To get the keys or properties of an object as an array

const keys = Object.keys(copyPerson)
console.log(keys) //['name', 'age', 'country', 'skills', 'address', 'getPersonInfo']
const address = Object.keys(copyPerson.address)
console.log(address) //['street', 'pobox', 'city']

Object.values:To get values of an object as an array

const values = Object.values(copyPerson)
console.log(values)

Object.entries:To get the keys and values in an array

const entries = Object.entries(copyPerson)
console.log(entries)

hasOwnProperty: To check if a specific key or property exist in an object

console.log(copyPerson.hasOwnProperty('name'))
console.log(copyPerson.hasOwnProperty('score'))

🌕 You are astonishing. Now, you are super charged with the power of objects. You have just completed day 8 challenges and you are 8 steps a head into your way to greatness. Now do some exercises for your brain and for your muscle.

1. Create an empty object called dog
2. Print the the dog object on the console
3. Add name, legs, color, age and bark properties for the dog object. The bark property is a method which return woof woof
4. Get name, legs, color, age and bark value from the dog object
5. Set new properties the dog object: breed, getDogInfo

1. Find the person who has many skills in the users object.

2. Count logged in users,count users having greater than equal to 50 points from the following object.

   const users = {
     Alex: {
       email: 'alex@alex.com',
       skills: ['HTML', 'CSS', 'JavaScript'],
       age: 20,
       isLoggedIn: false,
       points: 30
     },
     Asab: {
       email: 'asab@asab.com',
       skills: ['HTML', 'CSS', 'JavaScript', 'Redux', 'MongoDB', 'Express', 'React', 'Node'],
       age: 25,
       isLoggedIn: false,
       points: 50
     },
     Brook: {
       email: 'daniel@daniel.com',
       skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux'],
       age: 30,
       isLoggedIn: true,
       points: 50
     },
     Daniel: {
       email: 'daniel@alex.com',
       skills: ['HTML', 'CSS', 'JavaScript', 'Python'],
       age: 20,
       isLoggedIn: false,
       points: 40
     },
     John: {
       email: 'john@john.com',
       skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux', 'Node.js'],
       age: 20,
       isLoggedIn: true,
       points: 50
     },
     Thomas: {
       email: 'thomas@thomas.com',
       skills: ['HTML', 'CSS', 'JavaScript', 'React'],
       age: 20,
       isLoggedIn: false,
       points: 40
     },
     Paul: {
       email: 'paul@paul.com',
       skills: ['HTML', 'CSS', 'JavaScript', 'MongoDB', 'Express', 'React', 'Node'],
       age: 20,
       isLoggedIn: false,
       points: 40
     }
   }```

3. Find people who are MERN stack developer from the users object

4. Set your name in the users object without modifying the original users object

5. Get all keys or properties of users object

6. Get all the values of users object

7. Use the countries object to print a country name, capital, populations and languages.

1. Create an object literal called personAccount. It has firstName, lastName, incomes, expenses properties and it has totalIncome, totalExpense, accountInfo,addIncome, addExpense and accountBalance methods. Incomes is a set of incomes and its description and expenses is a set of incomes and its description.
2. **** Questions:2, 3 and 4 are based on the following two arrays:users and products ()

const users = [
  {
    _id: 'ab12ex',
    username: 'Alex',
    email: 'alex@alex.com',
    password: '123123',
    createdAt: '08/01/2020 9:00 AM',
    isLoggedIn: false,
  },
  {
    _id: 'fg12cy',
    username: 'Asab',
    email: 'asab@asab.com',
    password: '123456',
    createdAt: '08/01/2020 9:30 AM',
    isLoggedIn: true,
  },
  {
    _id: 'zwf8md',
    username: 'Brook',
    email: 'brook@brook.com',
    password: '123111',
    createdAt: '08/01/2020 9:45 AM',
    isLoggedIn: true,
  },
  {
    _id: 'eefamr',
    username: 'Martha',
    email: 'martha@martha.com',
    password: '123222',
    createdAt: '08/01/2020 9:50 AM',
    isLoggedIn: false,
  },
  {
    _id: 'ghderc',
    username: 'Thomas',
    email: 'thomas@thomas.com',
    password: '123333',
    createdAt: '08/01/2020 10:00 AM',
    isLoggedIn: false,
  },
]

const products = [
  {
    _id: 'eedfcf',
    name: 'mobile phone',
    description: 'Huawei Honor',
    price: 200,
    ratings: [
      { userId: 'fg12cy', rate: 5 },
      { userId: 'zwf8md', rate: 4.5 },
    ],
    likes: [],
  },
  {
    _id: 'aegfal',
    name: 'Laptop',
    description: 'MacPro: System Darwin',
    price: 2500,
    ratings: [],
    likes: ['fg12cy'],
  },
  {
    _id: 'hedfcg',
    name: 'TV',
    description: 'Smart TV:Procaster',
    price: 400,
    ratings: [{ userId: 'fg12cy', rate: 5 }],
    likes: ['fg12cy'],
  },
]

Imagine you are getting the above users collection from a MongoDB database. a. Create a function called signUp which allows user to add to the collection. If user exists, inform the user that he has already an account.
b. Create a function called signIn which allows user to sign in to the application

3. The products array has three elements and each of them has six properties. a. Create a function called rateProduct which rates the product b. Create a function called averageRating which calculate the average rating of a product

4. Create a function called likeProduct. This function will helps to like to the product if it is not liked and remove like if it was liked.

So far we have seen many builtin JavaScript functions. In this section, we will focus on custom functions. What is a function? Before we start making functions, lets understand what function is and why we need function?

A function is a reusable block of code or programming statements designed to perform a certain task. A function is declared by a function key word followed by a name, followed by parentheses (). A parentheses can take a parameter. If a function take a parameter it will be called with argument. A function can also take a default parameter. To store a data to a function, a function has to return certain data types. To get the value we call or invoke a function. Function makes code:

• clean and easy to read
• reusable
• easy to test

A function can be declared or created in couple of ways:

• Declaration function
• Expression function
• Anonymous function
• Arrow function

Let us see how to declare a function and how to call a function.

//declaring a function without a parameter
function functionName() {
  // code goes here
}
functionName() // calling function by its name and with parentheses

Function can be declared without a parameter.

Example:

// function without parameter,  a function which make a number square
function square() {
  let num = 2
  let sq = num * num
  console.log(sq)
}

square() // 4

// function without parameter
function addTwoNumbers() {
  let numOne = 10
  let numTwo = 20
  let sum = numOne + numTwo

  console.log(sum)
}

addTwoNumbers() // a function has to be called by its name to be executed

function printFullName() {
  let firstName = 'Asabeneh'
  let lastName = 'Yetayeh'
  let space = ' '
  let fullName = firstName + space + lastName
  console.log(fullName)
}

printFullName() // calling a function

Function can also return values, if a function does not return values the value of the function is undefined. Let us write the above functions with return. From now on, we return value to a function instead of printing it.

function printFullName() {
  let firstName = 'Asabeneh'
  let lastName = 'Yetayeh'
  let space = ' '
  let fullName = firstName + space + lastName
  return fullName
}
console.log(printFullName())

function addTwoNumbers() {
  let numOne = 2
  let numTwo = 3
  let total = numOne + numTwo
  return total
}

console.log(addTwoNumbers())

In a function we can pass different data types(number, string, boolean, object, function) as a parameter.

// function with one parameter
function functionName(parm1) {
  //code goes her
}
functionName(parm1) // during calling or invoking one argument needed

function areaOfCircle(r) {
  let area = Math.PI * r * r
  return area
}

console.log(areaOfCircle(10)) // should be called with one argument

function square(number) {
  return number * number
}

console.log(square(10))

// function with two parameters
function functionName(parm1, parm2) {
  //code goes her
}
functionName(parm1, parm2) // during calling or invoking two arguments needed
// Function without parameter doesn't take input, so lets make a function with parameters
function sumTwoNumbers(numOne, numTwo) {
  let sum = numOne + numTwo
  console.log(sum)
}
sumTwoNumbers(10, 20) // calling functions
// If a function doesn't return it doesn't store data, so it should return

function sumTwoNumbers(numOne, numTwo) {
  let sum = numOne + numTwo
  return sum
}

console.log(sumTwoNumbers(10, 20))
function printFullName(firstName, lastName) {
  return `${firstName} ${lastName}`
}
console.log(printFullName('Asabeneh', 'Yetayeh'))

// function with multiple parameters
function functionName(parm1, parm2, parm3,...){
  //code goes here
}
functionName(parm1,parm2,parm3,...) // during calling or invoking three arguments needed


// this function takes array as a parameter and sum up the numbers in the array
function sumArrayValues(arr) {
  let sum = 0;
  for (let i = 0; i < arr.length; i++) {
    sum = sum + arr[i];
  }
  return sum;
}
const numbers = [1, 2, 3, 4, 5];
    //calling a function
console.log(sumArrayValues(numbers));


    const areaOfCircle = (radius) => {
      let area = Math.PI * radius * radius;
      return area;
    }
console.log(areaOfCircle(10))

Sometimes we do not know how many arguments the user going to pass. Therefore, we should know how to write a function which can take unlimited number of arguments. The way we do it has a significant difference between a function declaration(regular function) and arrow function. Let us see examples both in function declaration and arrow function.

A function declaration provides a function scoped arguments array like object. Any thing we passed as argument in the function can be accessed from arguments object inside the functions. Let us see an example

// Let us access the arguments object
​
function sumAllNums() {
 console.log(arguments)
}

sumAllNums(1, 2, 3, 4))
// Arguments(4) [1, 2, 3, 4, callee: ƒ, Symbol(Symbol.iterator): ƒ]

// function declaration
​
function sumAllNums() {
  let sum = 0
  for (let i = 0; i < arguments.length; i++) {
    sum += arguments[i]
  }
  return sum
}

console.log(sumAllNums(1, 2, 3, 4)) // 10
console.log(sumAllNums(10, 20, 13, 40, 10))  // 93
console.log(sumAllNums(15, 20, 30, 25, 10, 33, 40))  // 173

Arrow function does not have the function scoped arguments object. To implement a function which takes unlimited number of arguments in an arrow function we use spread operator followed by any parameter name. Any thing we passed as argument in the function can be accessed as array in the arrow function. Let us see an example

// Let us access the arguments object
​
const sumAllNums = (...args) => {
 // console.log(arguments), arguments object not found in arrow function
 // instead we use an a parameter followed by spread operator
 console.log(args)
}

sumAllNums(1, 2, 3, 4))
// [1, 2, 3, 4]

// function declaration
​
const sumAllNums = (...args) => {
  let sum = 0
  for (const element of args) {
    sum += element
  }
  return sum
}

console.log(sumAllNums(1, 2, 3, 4)) // 10
console.log(sumAllNums(10, 20, 13, 40, 10))  // 93
console.log(sumAllNums(15, 20, 30, 25, 10, 33, 40))  // 173