JS MUST KNOW

Question 1

How many “Scopes” are there in JS?

Answer 1

In JavaScript, there can be following types of scopes:

Global Scope
Local Scope or Function Scope
Block Scope

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Question 2

What Do You Mean by “this“?

Answer 2

‘this’ is always a reference to an object

Question 3

What is “Anonymous Function” in JS?

Answer 3

Anonymous Function is a function that does not have any name associated with it.

Question 4

What Do You Know About BOM? (Browser Object Model)

Answer 4

The Browser Object Model (BOM) allows JavaScript to “talk to” the browser.

Question 5

What is DOM?

Answer 5

When a web page is loaded, the browser creates a “Document Object Model” of the page.

The HTML DOM model is constructed as a tree of Objects.

Question 6

How To Return a Character from a Specific Index?

Answer 6

charAt() string method

Question 7

What are the Different Data Types in JS?

Answer 7

Primitive: String, Number, Boolean, Null, Undefined, Symbol, BigInt
Composite: Object and Array

Question 8

What is “Prototype Property” in JS?

Answer 8

The prototype property allows you to add properties and methods to an object.

Question 9

What is “Closure” in JS?

Answer 9

A closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

Question 10

Asynchronous Programming and Its Importance

Answer 10

Accommodates for the lag between when a function is called to when the value of that function is returned.

allows user to go about their business in an application, while processes run in the background.

Question 11

Why Debugging is Required in JS?

Answer 11

scenarios where the script does not show any error in the browser.

the output is not similar to what is expected.

best option to find out the error is by debugging. This can be done by either console.log() or by using the debugger keyword.

Question 12

What is “Function Hoisting” in JS?

Answer 12

Function hoisting is a JavaScript mechanism where function declarations are moved to the top of their scope before code execution. (Excludes function expressions)

Question 13

Naming Conventions for Variables in JS

Answer 13

Don’t use reserves key words ie. Boolean

Don’t start the variable name with a number.

Variables are case sensitive.

Question 14

What is a “Callback” in JS?

Answer 14

A ‘callback’ function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

Question 15

What is “Global Scope”?

Answer 15

Global scope is the entire Javascript execution environment. Any variables assigned without an explicit scope automatically become global variables. The same is true for functions.

Question 16

What is “Local Scope” in JS?

Answer 16

Variables declared within a JavaScript function are in local scope. They can only be accessed from within that function.

Question 17

What will this “==” comparison operator return in JS?

Answer 17

It returns true if the operand on the left of it is (loosely) equal to the operand on the right of it.

Question 18

What will this “===” comparison operator return in JS?

Answer 18

It returns true if the operand on the left of it is (strictly) equal to the operand on the right of it.

Question 19

What is an “Object” in JS?

Answer 19

Objects are data structures that contain data. They can be a simple key/value pair, or they can be a complex system of nested key/value pairs representing a user and all of their posts, likes, friends, comments, personal data, etc.

Question 20

What is ‘Undefined(undefined)’ used for?

Answer 20

Used for unintentionally missing values.

Question 21

What is ‘Null (null)’ used for?

Answer 21

Used for intentionally missing values.

Question 22

What is ‘Booleans (true and false)’ used for?

Answer 22

Used for logical operations.

Question 23

What are ‘Numbers (-100, 3.14)’ used for?

Answer 23

Used for math calculations.

Question 24

What are ‘Strings (“hello”, “abracadabra”)’ used for?

Answer 24

Used for text.

Question 25

What are ‘Symbols (uncommon)’ used for?

Answer 25

Used to hide implementation details.

Question 26

What are ‘BigInts (uncommon and new)’ used for?

Answer 26

Used for math on big numbers.

Question 27

What is Object.is()?

Answer 27

Object.is() is a method that determines whether two values are the same value. Returns Boolean

Question 28

What is Object.assign()?

Answer 28

Object.assign() method is used to copy all enumerable own properties from one or more source objects to a target object.

Question 29

What is Object.entries()?

Answer 29

Object.entries() method returns an array of a given object’s key value pairs.

Question 30

What is Object.values()?

Answer 30

Object.values() method returns an array of a given object’s property values, in the same order as that provided by a for…in loop

Question 31

What is “The Window Object” is JS?

Answer 31

The window object is supported by all browsers. It represents the browser’s window.

Question 32

What automatically becomes members of “The Window Object”?

Answer 32

All global JavaScript objects, functions, and variables.

Question 33

What are the properties of the “The Window Object”?

Answer 33

Global variables.

Question 34

What are the methods of the “The Window Object”?

Answer 34

Global functions.

Question 35

What is Client-Side JavaScript (CSJS)?

Answer 35

Enables the enhancement and manipulation of web pages and client browsers.

Question 36

What is Server-Side JavaScript (SSJS)?

Answer 36

Enables back-end access to databases, file systems, and servers.

Question 37

What are the rules of declaring a “constant” in JS?

Answer 37

A constant cannot change value through assignment or be re-declared while the script is running.

const only prevents re-assignments but doesn’t prevent mutations.

Question 38

What is variable hoisting?

Answer 38

(var) declared variables are hoisted, you can refer to the variable anywhere in its scope, even if its declaration isn’t reached yet.

The value is always undefined, only declaration is hoisted, not its initialization.

Question 39

What is a “Template Literal” in JS?

Answer 39

A way to concatenate strings while allowing embedded expressions and improving readability.

Question 40

What is an “expression” in JS

Answer 40

A piece of code that produces a value.

Question 41

What does Object.create() method do?

Answer 41

Creates a new object, using an existing object as the prototype of the newly created object.

Question 42

What does the ‘new’ operator do?

Answer 42

Creates an instance of a constructor function, and binds the newly created object to the ‘this’ keyword for the duration of the function call.