Javascript

Question 1

WithOUT ES6, how are class properties inherited from the parent?

Answer 1

Parent.call(property1, …propertyN)

Question 2

WITH ES6, how are class properties inherited from the parent?

Answer 2

class Child extends Parent, call super() in constructor

Question 3

WithOUT ES6, how are class methods inherited from the parent?

Answer 3

Child.prototype = Object.create(Parent.prototype)

Question 4

How do you access class properties?

Answer 4

this

Question 5

How do you access class methods?

Answer 5

prototype

Question 6

How to make an AJAX or async call from the browser using vanilla JS?

Answer 6

new XMLHttpRequest();

req. addEventListener(“load”, reqListener);
req. open(“GET”, “http://www.example.org/example.txt”);
req. send();

Question 7

How to create a new regular expression

Answer 7

/something/ or new RegExp()

Question 8

How to specify a “character” in regex?

Answer 8

\w

Question 9

How to specify a number in regex?

Answer 9

\d - Decimal

\D - non-decimal

Question 10

How to test a regex?

Answer 10

String.prototype.match or RegExp.prototype.exec

Question 11

What does the i modifier do in a regex?

Answer 11

case-insensitive

Question 12

What is the map function?

Answer 12

Get a new array by applying a transformation function on each and every element in the array (immutable)

Question 13

What does reduce function do?

Answer 13

Reduce function reduces a given list to one final result (immutable)

Question 14

What are the arguments to the reduce function?

Answer 14

function(accumulator, currentEl, index), initialAccumulatorValue

Question 15

What does filter do?

Answer 15

Evaluates each element of the array against a criteria, and returns a new array with only elements that return true (immutable)

Question 16

What is a Pure Function?

Answer 16

A function which:

• Given the same input, will always return the same output.
• Produces no side effects.

Question 17

How to check whether a variable is valid and not null (or) undefined?

Answer 17

Boolean(variable) or nullish coalescing operator (??) from ES2020

Question 18

What is “hoisting”?

Answer 18

Variable and function declarations are put into memory during the compile phase, but stay exactly where you typed them in your code. This it allows you to use a function before you declare it in your code. JavaScript only hoists declarations, not initializations.

Question 19

What is Event bubbling?

Answer 19

The event is handled by the innermost element first and it propagates outwards till it reaches the parent element

Question 20

When to use .bind()?

Answer 20

Use .bind() when you want that function to later be called with a certain context (useful in events)

Question 21

What does call() or apply() do?

Answer 21

Overwrites the function context

Question 22

What are the three kinds of scopes?

Answer 22

Global - available everywhere,
Block - available within a bracket enclosure (let),
Local/Function - available within a function (var)

Question 23

What is closure?

Answer 23

A function that returns another function and wraps data
A closure is an expression (most commonly, a function) that can have free variables together with an environment that binds those variables (that “closes” the expression).

Question 24

What does “this” refer to in the browser?

Answer 24

The window object

Question 25

How to iterate over an object?

Answer 25

Object.keys or a for/in loop

Question 26

What does Object.freeze do?

Answer 26

Prevent properties from being changed, added, or removed

Question 27

What does Object.seal do?

Answer 27

Prevent properties from being added, or removed but they can still be changed

Question 28

What are some Object prototype functions?

Answer 28

hasOwnProperty(), instanceof(), toString()

Question 29

What’s the difference between let and var?

Answer 29

Var has local scope, while Let has block scope

Variables declared with var keyword are hoisted (initialized with undefined) before the code is run

Question 30

Is JS pass-by-reference or pass-by-value?

Answer 30

It’s always pass by value, but for objects the value of the variable is a reference. Because of this, when you pass an object and change its members, those changes persist outside of the function.

Question 31

What is the rest parameter?

Answer 31

function(…args) allows a function to take an indefinite number or arguments as an array

Question 32

What are the types of loops?

Answer 32

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

Question 33

What are the 7 primitive data types?

Answer 33

string, number, bigint, boolean, undefined, symbol, and null

Question 34

What is a method

Answer 34

A method is a function that is a property of an object