erhantezer / javascript-basic

Best Practice

String Methods

Everything in JavaScript is an object. A string is a primitive data type that means we can not modify it once it is created. The string object has many string methods. There are different string methods that can help us to work with strings.

1. length: The string length method returns the number of characters in a string included empty space.

Example:

let js = 'JavaScript'
console.log(js.length)         // 10
let firstName = 'Asabeneh'
console.log(firstName.length)  // 8

2. Accessing characters in a string: We can access each character in a string using its index. In programming, counting starts from 0. The first index of the string is zero, and the last index is the length of the string minus one.

Let us access different characters in 'JavaScript' string.

let string = 'JavaScript'
let firstLetter = string[0]

console.log(firstLetter)           // J

let secondLetter = string[1]       // a
let thirdLetter = string[2]
let lastLetter = string[9]

console.log(lastLetter)            // t

let lastIndex = string.length - 1

console.log(lastIndex)  // 9
console.log(string[lastIndex])    // t

3. toUpperCase(): this method changes the string to uppercase letters.

let string = 'JavaScript'

console.log(string.toUpperCase())     // JAVASCRIPT

let firstName = 'Asabeneh'

console.log(firstName.toUpperCase())  // ASABENEH

let country = 'Finland'

console.log(country.toUpperCase())    // FINLAND

4. toLowerCase(): this method changes the string to lowercase letters.

let string = 'JavasCript'

console.log(string.toLowerCase())     // javascript

let firstName = 'Asabeneh'

console.log(firstName.toLowerCase())  // asabeneh

let country = 'Finland'

console.log(country.toLowerCase())   // finland

5. substr(): It takes two arguments, the starting index and number of characters to slice.

let string = 'JavaScript'
console.log(string.substr(4,6))    // Script

let country = 'Finland'
console.log(country.substr(3, 4))   // land

6. substring(): It takes two arguments, the starting index and the stopping index but it doesn't include the character at the stopping index.

let string = 'JavaScript'

console.log(string.substring(0,4))     // Java
console.log(string.substring(4,10))    // Script
console.log(string.substring(4))       // Script

let country = 'Finland'

console.log(country.substring(0, 3))   // Fin
console.log(country.substring(3, 7))   // land
console.log(country.substring(3))      // land

7. split(): The split method splits a string at a specified place.

let string = '30 Days Of JavaScript'

console.log(string.split())     // Changes to an array -> ["30 Days Of JavaScript"]
console.log(string.split(' '))  // Split to an array at space -> ["30", "Days", "Of", "JavaScript"]

let firstName = 'Asabeneh'

console.log(firstName.split())    // Change to an array - > ["Asabeneh"]
console.log(firstName.split(''))  // Split to an array at each letter ->  ["A", "s", "a", "b", "e", "n", "e", "h"]

let countries = 'Finland, Sweden, Norway, Denmark, and Iceland'

console.log(countries.split(','))  // split to any array at comma -> ["Finland", " Sweden", " Norway", " Denmark", " and Iceland"]
console.log(countries.split(', ')) //  ["Finland", "Sweden", "Norway", "Denmark", "and Iceland"]

8. trim(): Removes trailing space in the beginning or the end of a string.

let string = '   30 Days Of JavaScript   '

console.log(string)
console.log(string.trim(' '))

let firstName = ' Asabeneh '

console.log(firstName)
console.log(firstName.trim())  // still removes spaces at the beginning and the end of the string

   30 Days Of JavasCript   
30 Days Of JavasCript
  Asabeneh 
Asabeneh

9. includes(): It takes a substring argument and it checks if substring argument exists in the string. includes() returns a boolean. If a substring exist in a string, it returns true, otherwise it returns false.

let string = '30 Days Of JavaScript'

console.log(string.includes('Days'))     // true
console.log(string.includes('days'))     // false - it is case sensitive!
console.log(string.includes('Script'))   // true
console.log(string.includes('script'))   // false
console.log(string.includes('java'))     // false
console.log(string.includes('Java'))     // true

let country = 'Finland'

console.log(country.includes('fin'))     // false
console.log(country.includes('Fin'))     // true
console.log(country.includes('land'))    // true
console.log(country.includes('Land'))    // false

10. replace(): takes as a parameter the old substring and a new substring.

string.replace(oldsubstring, newsubstring)

let string = '30 Days Of JavaScript'
console.log(string.replace('JavaScript', 'Python')) // 30 Days Of Python

let country = 'Finland'
console.log(country.replace('Fin', 'Noman'))       // Nomanland

11. charAt(): Takes index and it returns the value at that index

string.charAt(index)

let string = '30 Days Of JavaScript'
console.log(string.charAt(0))        // 3

let lastIndex = string.length - 1
console.log(string.charAt(lastIndex)) // t

12. charCodeAt(): Takes index and it returns char code (ASCII number) of the value at that index

string.charCodeAt(index)

let string = '30 Days Of JavaScript'
console.log(string.charCodeAt(3))        // D ASCII number is 68

let lastIndex = string.length - 1
console.log(string.charCodeAt(lastIndex)) // t ASCII is 116

13. indexOf(): Takes a substring and if the substring exists in a string it returns the first position of the substring if does not exist it returns -1

string.indexOf(substring)

let string = '30 Days Of JavaScript'

console.log(string.indexOf('D'))          // 3
console.log(string.indexOf('Days'))       // 3
console.log(string.indexOf('days'))       // -1
console.log(string.indexOf('a'))          // 4
console.log(string.indexOf('JavaScript')) // 11
console.log(string.indexOf('Script'))     //15
console.log(string.indexOf('script'))     // -1

14. lastIndexOf(): Takes a substring and if the substring exists in a string it returns the last position of the substring if it does not exist it returns -1

//syntax
string.lastIndexOf(substring)

let string = 'I love JavaScript. If you do not love JavaScript what else can you love.'

console.log(string.lastIndexOf('love'))       // 67
console.log(string.lastIndexOf('you'))        // 63
console.log(string.lastIndexOf('JavaScript')) // 38

15. concat(): it takes many substrings and joins them.

string.concat(substring, substring, substring)

let string = '30'
console.log(string.concat("Days", "Of", "JavaScript")) // 30DaysOfJavaScript

let country = 'Fin'
console.log(country.concat("land")) // Finland

16. startsWith: it takes a substring as an argument and it checks if the string starts with that specified substring. It returns a boolean(true or false).

//syntax
string.startsWith(substring)

let string = 'Love is the best to in this world'

console.log(string.startsWith('Love'))   // true
console.log(string.startsWith('love'))   // false
console.log(string.startsWith('world'))  // false

let country = 'Finland'

console.log(country.startsWith('Fin'))   // true
console.log(country.startsWith('fin'))   // false
console.log(country.startsWith('land'))  //  false

17. endsWith: it takes a substring as an argument and it checks if the string ends with that specified substring. It returns a boolean(true or false).

string.endsWith(substring)

let string = 'Love is the most powerful feeling in the world'

console.log(string.endsWith('world'))         // true
console.log(string.endsWith('love'))          // false
console.log(string.endsWith('in the world')) // true

let country = 'Finland'

console.log(country.endsWith('land'))         // true
console.log(country.endsWith('fin'))          // false
console.log(country.endsWith('Fin'))          //  false

18. search: it takes a substring as an argument and it returns the index of the first match. The search value can be a string or a regular expression pattern.

string.search(substring)

let string = 'I love JavaScript. If you do not love JavaScript what else can you love.'
console.log(string.search('love'))          // 2
console.log(string.search(/javascript/gi))  // 7

19. match: it takes a substring or regular expression pattern as an argument and it returns an array if there is match if not it returns null. Let us see how a regular expression pattern looks like. It starts with / sign and ends with / sign.

let string = 'love'
let patternOne = /love/     // with out any flag
let patternTwo = /love/gi   // g-means to search in the whole text, i - case insensitive

Match syntax

// syntax
string.match(substring)

let string = 'I love JavaScript. If you do not love JavaScript what else can you love.'
console.log(string.match('love'))

["love", index: 2, input: "I love JavaScript. If you do not love JavaScript what else can you love.", groups: undefined]

let pattern = /love/gi
console.log(string.match(pattern))   // ["love", "love", "love"]

Let us extract numbers from text using a regular expression. This is not the regular expression section, do not panic! We will cover regular expressions later on.

let txt = 'In 2019, I ran 30 Days of Python. Now, in 2020 I am super exited to start this challenge'
let regEx = /\d+/

// d with escape character means d not a normal d instead acts a digit
// + means one or more digit numbers,
// if there is g after that it means global, search everywhere.

console.log(txt.match(regEx))  // ["2", "0", "1", "9", "3", "0", "2", "0", "2", "0"]
console.log(txt.match(/\d+/g)) // ["2019", "30", "2020"]

20. repeat(): it takes a number as argument and it returns the repeated version of the string.

string.repeat(n)

let string = 'love'
console.log(string.repeat(10)) // lovelovelovelovelovelovelovelovelovelove

Checking Data Types and Casting

Checking Data Types

To check the data type of a certain variable we use the typeof method.

Example:

// Different javascript data types
// Let's declare different data types

let firstName = 'Asabeneh'      // string
let lastName = 'Yetayeh'        // string
let country = 'Finland'         // string
let city = 'Helsinki'           // string
let age = 250                   // number, it is not my real age, do not worry about it
let job                         // undefined, because a value was not assigned

console.log(typeof 'Asabeneh')  // string
console.log(typeof firstName)   // string
console.log(typeof 10)          // number
console.log(typeof 3.14)        // number
console.log(typeof true)        // boolean
console.log(typeof false)       // boolean
console.log(typeof NaN)         // number
console.log(typeof job)         // undefined
console.log(typeof undefined)   // undefined
console.log(typeof null)        // object

Changing Data Type (Casting)

• Casting: Converting one data type to another data type. We use parseInt(), parseFloat(), Number(), + sign, str() When we do arithmetic operations string numbers should be first converted to integer or float if not it returns an error.

String to Int

We can convert string number to a number. Any number inside a quote is a string number. An example of a string number: '10', '5', etc. We can convert string to number using the following methods:

• parseInt()
• Number()
• Plus sign(+)

let num = '10'
let numInt = parseInt(num)
console.log(numInt) // 10

let num = '10'
let numInt = Number(num)

console.log(numInt) // 10

let num = '10'
let numInt = +num

console.log(numInt) // 10

String to Float

We can convert string float number to a float number. Any float number inside a quote is a string float number. An example of a string float number: '9.81', '3.14', '1.44', etc. We can convert string float to number using the following methods:

• parseFloat()
• Number()
• Plus sign(+)

let num = '9.81'
let numFloat = parseFloat(num)

console.log(numFloat) // 9.81

let num = '9.81'
let numFloat = Number(num)

console.log(numFloat) // 9.81

let num = '9.81'
let numFloat = +num

console.log(numFloat) // 9.81

Float to Int

We can convert float numbers to integers. We use the following method to convert float to int:

• parseInt()

let num = 9.81
let numInt = parseInt(num)

console.log(numInt) // 9

🌕 You are awesome. You have just completed day 2 challenges and you are two steps ahead on your way to greatness. Now do some exercises for your brain and for your muscle.

Exercise: Level 1

1. Declare a variable named challenge and assign it to an initial value '30 Days Of JavaScript'.
2. Print the string on the browser console using console.log()
3. Print the length of the string on the browser console using console.log()
4. Change all the string characters to capital letters using toUpperCase() method
5. Change all the string characters to lowercase letters using toLowerCase() method
6. Cut (slice) out the first word of the string using substr() or substring() method
7. Slice out the phrase Days Of JavaScript from 30 Days Of JavaScript.
8. Check if the string contains a word Script using includes() method
9. Split the string into an array using split() method
10. Split the string 30 Days Of JavaScript at the space using split() method
11. 'Facebook, Google, Microsoft, Apple, IBM, Oracle, Amazon' split the string at the comma and change it to an array.
12. Change 30 Days Of JavaScript to 30 Days Of Python using replace() method.
13. What is character at index 15 in '30 Days Of JavaScript' string? Use charAt() method.
14. What is the character code of J in '30 Days Of JavaScript' string using charCodeAt()
15. Use indexOf to determine the position of the first occurrence of a in 30 Days Of JavaScript
16. Use lastIndexOf to determine the position of the last occurrence of a in 30 Days Of JavaScript.
17. Use indexOf to find the position of the first occurrence of the word because in the following sentence:'You cannot end a sentence with because because because is a conjunction'
18. Use lastIndexOf to find the position of the last occurrence of the word because in the following sentence:'You cannot end a sentence with because because because is a conjunction'
19. Use search to find the position of the first occurrence of the word because in the following sentence:'You cannot end a sentence with because because because is a conjunction'
20. Use trim() to remove any trailing whitespace at the beginning and the end of a string.E.g ' 30 Days Of JavaScript '.
21. Use startsWith() method with the string 30 Days Of JavaScript and make the result true
22. Use endsWith() method with the string 30 Days Of JavaScript and make the result true
23. Use match() method to find all the a’s in 30 Days Of JavaScript
24. Use concat() and merge '30 Days of' and 'JavaScript' to a single string, '30 Days Of JavaScript'
25. Use repeat() method to print 30 Days Of JavaScript 2 times

Exercise: Level 2

1. Using console.log() print out the following statement:

   The quote 'There is no exercise better for the heart than reaching down and lifting people up.' by John Holmes teaches us to help one another.

2. Using console.log() print out the following quote by Mother Teresa:

   "Love is not patronizing and charity isn't about pity, it is about love. Charity and love are the same -- with charity you give love, so don't just give money but reach out your hand instead."

3. Check if typeof '10' is exactly equal to 10. If not make it exactly equal.

4. Check if parseFloat('9.8') is equal to 10 if not make it exactly equal with 10.

5. Check if 'on' is found in both python and jargon

6. I hope this course is not full of jargon. Check if jargon is in the sentence.

7. Generate a random number between 0 and 100 inclusively.

8. Generate a random number between 50 and 100 inclusively.

9. Generate a random number between 0 and 255 inclusively.

10. Access the 'JavaScript' string characters using a random number.

11. Use console.log() and escape characters to print the following pattern.

    1 1 1 1 1
    2 1 2 4 8
    3 1 3 9 27
    4 1 4 16 64
    5 1 5 25 125

12. Use substr to slice out the phrase because because because from the following sentence:'You cannot end a sentence with because because because is a conjunction'

Exercises: Level 3

1. 'Love is the best thing in this world. Some found their love and some are still looking for their love.' Count the number of word love in this sentence.

2. Use match() to count the number of all because in the following sentence:'You cannot end a sentence with because because because is a conjunction'

3. Clean the following text and find the most frequent word (hint, use replace and regular expressions).

       const sentence = '%I $am@% a %tea@cher%, &and& I lo%#ve %te@a@ching%;. The@re $is no@th@ing; &as& mo@re rewarding as educa@ting &and& @emp%o@weri@ng peo@ple. ;I found tea@ching m%o@re interesting tha@n any ot#her %jo@bs. %Do@es thi%s mo@tiv#ate yo@u to be a tea@cher!? %Th#is 30#Days&OfJavaScript &is al@so $the $resu@lt of &love& of tea&ching'

4. Calculate the total annual income of the person by extracting the numbers from the following text. 'He earns 5000 euro from salary per month, 10000 euro annual bonus, 15000 euro online courses per month.'

Booleans

A boolean data type represents one of the two values:true or false. Boolean value is either true or false. The use of these data types will be clear when you start the comparison operator. Any comparisons return a boolean value which is either true or false.

Example: Boolean Values

let isLightOn = true
let isRaining = false
let isHungry = false
let isMarried = true
let truValue = 4 > 3    // true
let falseValue = 4 < 3  // false

We agreed that boolean values are either true or false.

Truthy values

• All numbers(positive and negative) are truthy except zero
• All strings are truthy except an empty string ('')
• The boolean true

Falsy values

• 0
• 0n
• null
• undefined
• NaN
• the boolean false
• '', "", ``, empty string

It is good to remember those truthy values and falsy values. In later section, we will use them with conditions to make decisions.

Undefined

If we declare a variable and if we do not assign a value, the value will be undefined. In addition to this, if a function is not returning the value, it will be undefined.

let firstName
console.log(firstName) //not defined, because it is not assigned to a value yet

Null

let empty = null
console.log(empty) // -> null , means no value

Operators

Assignment operators

An equal sign in JavaScript is an assignment operator. It uses to assign a variable.

let firstName = 'Asabeneh'
let country = 'Finland'

Assignment Operators

Arithmetic Operators

Arithmetic operators are mathematical operators.

• Addition(+): a + b
• Subtraction(-): a - b
• Multiplication(*): a * b
• Division(/): a / b
• Modulus(%): a % b
• Exponential(**): a ** b

let numOne = 4
let numTwo = 3
let sum = numOne + numTwo
let diff = numOne - numTwo
let mult = numOne * numTwo
let div = numOne / numTwo
let remainder = numOne % numTwo
let powerOf = numOne ** numTwo

console.log(sum, diff, mult, div, remainder, powerOf) // 7,1,12,1.33,1, 64

const PI = 3.14
let radius = 100          // length in meter

//Let us calculate area of a circle
const areaOfCircle = PI * radius * radius
console.log(areaOfCircle)  //  314 m


const gravity = 9.81      // in m/s2
let mass = 72             // in Kilogram

// Let us calculate weight of an object
const weight = mass * gravity
console.log(weight)        // 706.32 N(Newton)

const boilingPoint = 100  // temperature in oC, boiling point of water
const bodyTemp = 37       // body temperature in oC


// Concatenating string with numbers using string interpolation
/*
 The boiling point of water is 100 oC.
 Human body temperature is 37 oC.
 The gravity of earth is 9.81 m/s2.
 */
console.log(
  `The boiling point of water is ${boilingPoint} oC.\nHuman body temperature is ${bodyTemp} oC.\nThe gravity of earth is ${gravity} m / s2.`
)

Comparison Operators

In programming we compare values, we use comparison operators to compare two values. We check if a value is greater or less or equal to other value.

Example: Comparison Operators

console.log(3 > 2)              // true, because 3 is greater than 2
console.log(3 >= 2)             // true, because 3 is greater than 2
console.log(3 < 2)              // false,  because 3 is greater than 2
console.log(2 < 3)              // true, because 2 is less than 3
console.log(2 <= 3)             // true, because 2 is less than 3
console.log(3 == 2)             // false, because 3 is not equal to 2
console.log(3 != 2)             // true, because 3 is not equal to 2
console.log(3 == '3')           // true, compare only value
console.log(3 === '3')          // false, compare both value and data type
console.log(3 !== '3')          // true, compare both value and data type
console.log(3 != 3)             // false, compare only value
console.log(3 !== 3)            // false, compare both value and data type
console.log(0 == false)         // true, equivalent
console.log(0 === false)        // false, not exactly the same
console.log(0 == '')            // true, equivalent
console.log(0 == ' ')           // true, equivalent
console.log(0 === '')           // false, not exactly the same
console.log(1 == true)          // true, equivalent
console.log(1 === true)         // false, not exactly the same
console.log(undefined == null)  // true
console.log(undefined === null) // false
console.log(NaN == NaN)         // false, not equal
console.log(NaN === NaN)        // false
console.log(typeof NaN)         // number

console.log('mango'.length == 'avocado'.length)  // false
console.log('mango'.length != 'avocado'.length)  // true
console.log('mango'.length < 'avocado'.length)   // true
console.log('milk'.length == 'meat'.length)      // true
console.log('milk'.length != 'meat'.length)      // false
console.log('tomato'.length == 'potato'.length)  // true
console.log('python'.length > 'dragon'.length)   // false

Try to understand the above comparisons with some logic. Remembering without any logic might be difficult. JavaScript is somehow a wired kind of programming language. JavaScript code run and give you a result but unless you are good at it may not be the desired result.

Logical Operators

The following symbols are the common logical operators: &&(ampersand) , ||(pipe) and !(negation). The && operator gets true only if the two operands are true. The || operator gets true either of the operand is true. The ! operator negates true to false and false to true.

// && ampersand operator example

const check = 4 > 3 && 10 > 5         // true && true -> true
const check = 4 > 3 && 10 < 5         // true && false -> false
const check = 4 < 3 && 10 < 5         // false && false -> false

// || pipe or operator, example

const check = 4 > 3 || 10 > 5         // true  || true -> true
const check = 4 > 3 || 10 < 5         // true  || false -> true
const check = 4 < 3 || 10 < 5         // false || false -> false

//! Negation examples

let check = 4 > 3                     // true
let check = !(4 > 3)                  //  false
let isLightOn = true
let isLightOff = !isLightOn           // false
let isMarried = !false                // true

Increment Operator

In JavaScript we use the increment operator to increase a value stored in a variable. The increment could be pre or post increment. Let us see each of them:

1. Pre-increment

let count = 0
console.log(++count)        // 1
console.log(count)          // 1

1. Post-increment

let count = 0
console.log(count++)        // 0
console.log(count)          // 1

We use most of the time post-increment. At least you should remember how to use post-increment operator.

Decrement Operator

In JavaScript we use the decrement operator to decrease a value stored in a variable. The decrement could be pre or post decrement. Let us see each of them:

1. Pre-decrement

let count = 0
console.log(--count) // -1
console.log(count)  // -1

2. Post-decrement

let count = 0
console.log(count--) // 0
console.log(count)   // -1

Ternary Operators

Ternary operator allows to write a condition. Another way to write conditionals is using ternary operators. Look at the following examples:

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

isRaining = false

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

You need a rain coat.
No need for a rain coat.

let number = 5
number > 0
  ? console.log(`${number} is a positive number`)
  : console.log(`${number} is a negative number`)
number = -5

number > 0
  ? console.log(`${number} is a positive number`)
  : console.log(`${number} is a negative number`)

5 is a positive number
-5 is a negative number

Operator Precedence

I would like to recommend you to read about operator precedence from this link

Window Methods

Window alert() method

As you have seen at very beginning alert() method displays an alert box with a specified message and an OK button. It is a builtin method and it takes on argument.

alert(message)

alert('Welcome to 30DaysOfJavaScript')

Do not use too much alert because it is destructing and annoying, use it just to test.

Window prompt() method

The window prompt methods display a prompt box with an input on your browser to take input values and the input data can be stored in a variable. The prompt() method takes two arguments. The second argument is optional.

prompt('required text', 'optional text')

let number = prompt('Enter number', 'number goes here')
console.log(number)

Window confirm() method

The confirm() method displays a dialog box with a specified message, along with an OK and a Cancel button. A confirm box is often used to ask permission from a user to execute something. Window confirm() takes a string as an argument. Clicking the OK yields true value, whereas clicking the Cancel button yields false value.

const agree = confirm('Are you sure you like to delete? ')
console.log(agree) // result will be true or false based on what you click on the dialog box

These are not all the window methods we will have a separate section to go deep into window methods.

Date Object

Time is an important thing. We like to know the time a certain activity or event. In JavaScript current time and date is created using JavaScript Date Object. The object we create using Date object provides many methods to work with date and time.The methods we use to get date and time information from a date object values are started with a word get because it provide the information. getFullYear(), getMonth(), getDate(), getDay(), getHours(), getMinutes, getSeconds(), getMilliseconds(), getTime(), getDay()

Creating a time object

Once we create time object. The time object will provide information about time. Let us create a time object

const now = new Date()
console.log(now) // Sat Jan 04 2020 00:56:41 GMT+0200 (Eastern European Standard Time)

We have created a time object and we can access any date time information from the object using the get methods we have mentioned on the table.

Getting full year

Let's extract or get the full year from a time object.

const now = new Date()
console.log(now.getFullYear()) // 2020

Getting month

Let's extract or get the month from a time object.

const now = new Date()
console.log(now.getMonth()) // 0, because the month is January,  month(0-11)

Getting date

Let's extract or get the date of the month from a time object.

const now = new Date()
console.log(now.getDate()) // 4, because the day of the month is 4th,  day(1-31)

Getting day

Let's extract or get the day of the week from a time object.

const now = new Date()
console.log(now.getDay()) // 6, because the day is Saturday which is the 7th day
//  Sunday is 0, Monday is 1 and Saturday is 6
// Getting the weekday as a number (0-6)

Getting hours

Let's extract or get the hours from a time object.

const now = new Date()
console.log(now.getHours()) // 0, because the time is 00:56:41

Getting minutes

Let's extract or get the minutes from a time object.

const now = new Date()
console.log(now.getMinutes()) // 56, because the time is 00:56:41

Getting seconds

Let's extract or get the seconds from a time object.

const now = new Date()
console.log(now.getSeconds()) // 41, because the time is 00:56:41

Getting time

This method give time in milliseconds starting from January 1, 1970. It is also know as Unix time. We can get the unix time in two ways:

1. Using getTime()

const now = new Date() //
console.log(now.getTime()) // 1578092201341, this is the number of seconds passed from January 1, 1970 to January 4, 2020 00:56:41

1. Using Date.now()

const allSeconds = Date.now() //
console.log(allSeconds) // 1578092201341, this is the number of seconds passed from January 1, 1970 to January 4, 2020 00:56:41

const timeInSeconds = new Date().getTime()
console.log(allSeconds == timeInSeconds) // true

Let us format these values to a human readable time format. Example:

const now = new Date()
const year = now.getFullYear() // return year
const month = now.getMonth() + 1 // return month(0 - 11)
const date = now.getDate() // return date (1 - 31)
const hours = now.getHours() // return number (0 - 23)
const minutes = now.getMinutes() // return number (0 -59)

console.log(`${date}/${month}/${year} ${hours}:${minutes}`) // 4/1/2020 0:56

🌕 You have boundless energy. You have just completed day 3 challenges and you are three steps a head in to your way to greatness. Now do some exercises for your brain and for your muscle.

Exercises: Level 1

1. Declare firstName, lastName, country, city, age, isMarried, year variable and assign value to it and use the typeof operator to check different data types.

2. Check if type of '10' is equal to 10

3. Check if parseInt('9.8') is equal to 10

4. Boolean value is either true or false.

   1. Write three JavaScript statement which provide truthy value.
   2. Write three JavaScript statement which provide falsy value.

5. Figure out the result of the following comparison expression first without using console.log(). After you decide the result confirm it using console.log()

   1. 4 > 3
   2. 4 >= 3
   3. 4 < 3
   4. 4 <= 3
   5. 4 == 4
   6. 4 === 4
   7. 4 != 4
   8. 4 !== 4
   9. 4 != '4'
   10. 4 == '4'
   11. 4 === '4'
   12. Find the length of python and jargon and make a falsy comparison statement.

6. Figure out the result of the following expressions first without using console.log(). After you decide the result confirm it by using console.log()

   1. 4 > 3 && 10 < 12
   2. 4 > 3 && 10 > 12
   3. 4 > 3 || 10 < 12
   4. 4 > 3 || 10 > 12
   5. !(4 > 3)
   6. !(4 < 3)
   7. !(false)
   8. !(4 > 3 && 10 < 12)
   9. !(4 > 3 && 10 > 12)
   10. !(4 === '4')
   11. There is no 'on' in both dragon and python

7. Use the Date object to do the following activities

   1. What is the year today?
   2. What is the month today as a number?
   3. What is the date today?
   4. What is the day today as a number?
   5. What is the hours now?
   6. What is the minutes now?
   7. Find out the numbers of seconds elapsed from January 1, 1970 to now.

Exercises: Level 2

1. Write a script that prompt the user to enter base and height of the triangle and calculate an area of a triangle (area = 0.5 x b x h).

   Enter base: 20
   Enter height: 10
   The area of the triangle is 100

2. Write a script that prompt the user to enter side a, side b, and side c of the triangle and and calculate the perimeter of triangle (perimeter = a + b + c)

   Enter side a: 5
   Enter side b: 4
   Enter side c: 3
   The perimeter of the triangle is 12

3. Get length and width using prompt and calculate an area of rectangle (area = length x width and the perimeter of rectangle (perimeter = 2 x (length + width))

4. Get radius using prompt and calculate the area of a circle (area = pi x r x r) and circumference of a circle(c = 2 x pi x r) where pi = 3.14.

5. Calculate the slope, x-intercept and y-intercept of y = 2x -2

6. Slope is m = (y₂-y₁)/(x₂-x₁). Find the slope between point (2, 2) and point(6,10)

7. Compare the slope of above two questions.

8. Calculate the value of y (y = x² + 6x + 9). Try to use different x values and figure out at what x value y is 0.

9. Writ a script that prompt a user to enter hours and rate per hour. Calculate pay of the person?

   Enter hours: 40
   Enter rate per hour: 28
   Your weekly earning is 1120

10. If the length of your name is greater than 7 say, your name is long else say your name is short.

11. Compare your first name length and your family name length and you should get this output.

    let firstName = 'Asabeneh'
    let lastName = 'Yetayeh'

    Your first name, Asabeneh is longer than your family name, Yetayeh

12. Declare two variables myAge and yourAge and assign them initial values and myAge and yourAge.

    let myAge = 250
    let yourAge = 25

    I am 225 years older than you.

13. Using prompt get the year the user was born and if the user is 18 or above allow the user to drive if not tell the user to wait a certain amount of years.

    Enter birth year: 1995
    You are 25. You are old enough to drive
    
    Enter birth year: 2005
    You are 15. You will be allowed to drive after 3 years.

14. Write a script that prompt the user to enter number of years. Calculate the number of seconds a person can live. Assume some one lives just hundred years

    Enter number of years you live: 100
    You lived 3153600000 seconds.

15. Create a human readable time format using the Date time object

    1. YYYY-MM-DD HH:mm
    2. DD-MM-YYYY HH:mm
    3. DD/MM/YYYY HH:mm

Exercises: Level 3

1. Create a human readable time format using the Date time object. The hour and the minute should be all the time two digits(7 hours should be 07 and 5 minutes should be 05 )
   1. YYY-MM-DD HH:mm eg. 20120-01-02 07:05

Conditionals

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

If

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 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.

If Else if Else

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

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
   break
  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 Operators

Another way to write conditionals is using ternary operators. We have covered this in other sections, but we should also mention it here.

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

🌕 You are extraordinary and you have a remarkable potential. You have just completed day 4 challenges and you are four steps ahead to your way to greatness. Now do some exercises for your brain and muscle.

💻 Exercises

Exercises: Level 1

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 to 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.

Exercises: Level 2

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.

Exercises: Level 3

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.

Methods to manipulate array

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 Constructor

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]

Creating static values with fill

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]

Concatenating array using concat

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"]

Getting array length

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

Getting index an element in arr 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 not exist in the array')  
} else {
    console.log('This fruit does exist in the array')
}
// This fruit does exist in the array

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

// let us check if an 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 not exist in the array')  
} else {
    console.log('This fruit does exist in the array')
}
// This fruit does not exist in the array

Getting last index of an element in 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

Checking array

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

Converting array to string

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

Joining array elements

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 array elements

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 method in array

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

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

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

Adding item to an array using push

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']

Removing the end element using pop

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]

Removing an element from the beginning

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]

Add an element from the beginning

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]

Reversing array order

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]

Sorting elements in array

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 of arrays

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

Exercise: Level 2

1. Create a separate countries.js file and store the countries array in to this file, create a separate file web_techs.js and store the webTechs array in to 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"]

Exercise: Level 3

1. The following is an array of 10 students ages:

   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