JavaScript

Question 1

What are Template Literals?

Answer 1

Template literals provide an easy way to interpolate variables and expressions into strings.

The method is called string interpolation.

The syntax is as follows:

${…}

Question 2

What are variables?

Answer 2

Variables are containers for storing data values.

Question 3

How many types of variables do you know in JavaScript?

Answer 3

There are 3 types of variables:
1. var - in QA we will not use “var”, because var is mostly used by developers. 1) var creates duplicate containers; 2. there is invisible var. var can be reached outside the block scope, i.e. has global access
2. let - when we use “let”, the value is mutable, i.e. you can reassign the value
3. const - we cannot reassign or redeclare the value; exists in a block scope. When using const, you need to assign the value once const is used, it is immutable.

Question 4

What are the data types in JavaScript?

Answer 4

*Primitive Data Types:
boolean - QA use
number - QA use
string - QA use
undefined
bigint
Symbol
null

* Non-Primitive Data Types
	Object
	Array

Question 5

What is the difference between == and ===?

Answer 5

== checks the value

=== checks data type and the value:
let a = ‘1’
let b = 1
console.log(a===b) // False

Question 6

What is switch statement?

Answer 6

The switch statement is used to perform different actions based on different conditions.

Use the switch statement to select one of many code blocks to be executed. This is how it works:
1. The switch expression is evaluated once.
2. The value of the expression is compared with the values of each case.
3. If there is a match, the associated block of code is executed.
4. If there is no match, the default code block is executed.

                                                     Example: let weather = "hot"

switch(weather){
case “hot”:{
console.log(“Go to the beach”)
break;
}
case “cold”: {
console.log(“Stay at home”)
break;
}
case “snow”:{
console.log(“Build a snowman”)
break;
}
default:{
console.log(“Invalid input”)
break;
}
}

Question 7

What is Conditional (Ternary) operator?

Answer 7

It is a conditional operator that assigns a value to a variable based on some condition.
Syntax
variablename = (condition) ? value1:value2

                                                Example let voteable = (age < 18) ? "Too young":"Old enough";

If the variable age is a value below 18, the value of the variable voteable will be “Too young”, otherwise the value of voteable will be “Old enough”.

Question 8

What are loops and how many types of loops are there in JavaScript?

Answer 8

Loops are handy, if you want to run the same code over and over again, each time with a different value.

JavaScript supports different kinds of loops:

for –> loops through a block of code a number of times
for/in –> loops through the properties of an object
for/of –> loops through the values of an iterable object
while –> loops through a block of code while a specified condition
is true
do/while –> also loops through a block of code while a specified
condition is true

The most commonly used ones are For, While, and Do While

Question 9

What are branching statements?

Answer 9

Branching statements allow us to add conditions in our Code.
There are mainly four kinds of branching statements available in JavaScript:

              1. If Statement
              2. If else Statement
              3. Else if Statement
              4. Switch Statement

Question 10

What are arrays?

Answer 10

• Array is a non primitive data type
• An array can hold many values under a single name, and you can access the values by referring to an index number.
• Each array has an index starting from 0 and we access an array element by referring to the index number:
  const cars = [“Saab”, “Volvo”, “BMW”];
  let car = cars[0];
• to access each element of an array we use brackets []

Question 11

ARRAY MANIPULATION

toString() method

Answer 11

toString() –> converts an array to a string of (comma separated) array values.

	const fruits = ["Banana", "Orange", "Apple", "Mango"];
	console.log(fruits.toString());// Banana,Orange,Apple,Mango

Question 12

How do you access the last element in an array?

Answer 12

The last element in an array can be accessed by subtracting 1 from from the length of an array:

const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
let fruit = fruits[fruits.length - 1];

Question 13

How do you add elements to the array?

Answer 13

The easiest way to add a new element to an array is using the push() method:

	const fruits = ["Banana", "Orange", "Apple"];
	fruits.push("Lemon");  // Adds a new element (Lemon) to the end of fruits array 

New element can also be added to an array using the length property:

	const fruits = ["Banana", "Orange", "Apple"];
	fruits[fruits.length] = "Lemon";  // Adds "Lemon" to fruits

Question 14

pop()

Answer 14

removes the last element

Question 15

push()

Answer 15

adds an element to the end of an array

Question 16

shift()

Answer 16

removes the first element from the array

Question 17

unshift()

Answer 17

adds an element to the beginning

Question 18

splice()

Answer 18

It adds new items to an array. You can delete elements in an array by passing TWO arguments. Starting index, number of elements to be deleted.

let cars = [“Toyota”, “BMW”, “Tesla”, “Ford”, “Buick”, “Rivian”]
cars.splice(0,3)
// [ ‘Ford’, ‘Buick’, ‘Rivian’ ]
console.log(cars)

with 3 arguments–>

				let cars = ["Toyota", "BMW", "Tesla", "Ford", "Buick", "Rivian"]
				cars.splice(1,2, "Subaru")// in index 1 ("BMW"), remove 2 elements ("BMW" and "Tesla"), and add "Subaru"
				console.log(cars)//[ 'Toyota', 'Subaru', 'Ford', 'Buick', 'Rivian' ]

Question 19

indexOf()

Answer 19

returns the first index at which a given element can be found in the array. “-1” if the item is not found

Question 20

includes()

Answer 20

determines whether an array includes a certain value among its entries, returning true or false

let animals = [“ant”, “bison”, “camel”, “duck”, “elephant”]
console.log(animals.includes(“ant”)) // true

Question 21

What is a multidimensional array?

Answer 21

Multidimensional array in JavaScript is known as array inside another array.

Question 22

What is a function?

Answer 22

Functions let you group a series of statements together to perform a specific task. It is a piece of reusable code.

Question 23

What is a global variable?

Answer 23

Variable that is created outside of the function

Question 24

What is a local variable?

Answer 24

Variable that is created inside the function

Question 25

What is hoisting in JavaScript?

Answer 25

Hoisting is JavaScript’s default behavior of moving all declarations to the top of the current scope. In relation to functions, it is calling a function even before creating it.

Question 26

What is a callback function?

Answer 26

A callback is a function passed as an argument to another function. This technique allows a function to call another function. A callback function can run after another function has finished.
SYNTAX:

function dosomething(x) {
let first_name = “Mitko”
x(first_name)
}

function test(result) {

console.log("My name is " + " " + result) }

dosomething(test)

Question 27

What are closures?

Answer 27

When your inner function has access to variables and parameters

For more info https://www.w3schools.com/js/js_function_closures.asp

Question 28

What are arrow functions?

Answer 28

Arrow functions allow us to write shorter function

syntax:

let myFunction = (a, b) => a * b;

Question 29

What is a map?

Answer 29

• map() creates a new array from calling a function for every array element.
• map() does not execute the function for empty elements.
• map() does not change the original array.

let a = [“a”, “b”, “c”, “d”, “f”]

let test = a.map((x, y) => { // x = first argument is an element, y = second argument is an index of the element
return y
})
console.log(test)

Array.from() method
let a = “Javascript”
let b = Array.from(a)// [ ‘J’, ‘a’, ‘v’, ‘a’, ‘s’, ‘c’, ‘r’, ‘i’, ‘p’, ‘t’]

Question 30

forEach() loop

Answer 30

forEach loop works only with arrays

forEach () method and map() method work very similarly. However, map () method returns an array from the array, while
forEach() method is just a loop which iterates throgh an array. That is why we should assign map() method to a variable.

Question 31

Spread Operator

Answer 31

The spread operator (…) expands an iterable (like an array) into more elements.

This allows us to quickly copy all or parts of an existing array into another array.

Question 32

What is a Map Object?

Answer 32

Map object can be created by calling new map object. It has a key and value. The key must be in a string format.

let test = new Map()
let account = “account”

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.

Question 33

What is a Set Object?

Answer 33

Set object is similar to the Map object, however the Set object stores only unique values

let mySet = new Set()
my.set.add(1)
my.set.add(“a”)
my.set.add(false)
console.log(mySet)// { 1, ‘a’, false }