Core

Question 1

Where does JS execute?

Answer 1

Execution Context

Question 2

What does the parts of Execution Context?

Answer 2

• Memory or Variable Envoriment
• Code or Thread of Execution

Question 3

What does Memory or Variable Envoriment does?

Answer 3

Allocate memory for all variables and functions

Question 4

What is the initial value of memory for variables and functions?

Answer 4

Undefined for variables and the code inside the function for functions.

Question 5

What happens in the Code execution phase?

Answer 5

Variable assignation and create a new execution context for functions

Question 6

What happen with an execution context when a partner function hit return keyword?

Answer 6

It’s deleted

Question 7

How does arrow function behave in memory phase?

Answer 7

As a Variable.
= undefined

Question 8

What is Lexical Enviroment

Answer 8

Execution Context with a reference to Lexical Environment to it’s parent (think of it as a linked list node) Lexical it’s means herarky. Function C it’s Lexical setting inside B function.

Question 9

What is Hoisting?

Answer 9

It’s the process to allocated memory space for all variables and function before code execution.

Question 10

Are Let and Const Hoisted?

Answer 10

Yes, they are Hoisted but in other place that can’t be accessed before initialisation.

Question 11

What is a Block?

Answer 11

• Code inside curly braces is called block.
• Multiple statements are grouped inside a block, so it can be written where JS expects single statement like if, for, while, etc.
• Values are stored inside separate memory than global. They are stored in Blocks. (let and Const are block scopes)

Question 12

What is Closure

Answer 12

A combination of a function and it’s Lexical scope bundled together.
* Advantage data hidden and encapsulation.
* Disvantage may consume a lot of memory if garbage colecto didn’t work as expected

Const count = () => {
let counter = 0
return () => {
return counter++
}
}
const c = count()
c() //1
c() //2

Question 13

What is Garbage Collector

Answer 13

Javascript engine that frees memory when the variables or closure won’t be needed anymore

Question 14

What is a pure Function

Answer 14

Is a function that always return the same value if the same arguments are passed. It doesn’t depends of any state or data change

Question 15

What is the Event Loop

Answer 15

Event loop continuosly observes Call Stack and when it is empty it transfer tasks to Call Stack.

Question 16

What is the behavior of Call Back function?

Answer 16

Callback functions and event handers are first stored in Web API environment and then transfer to Callback Queue

Question 17

What is the behavior of Promises?

Answer 17

Promises are stored in API environment and then transfered to Microtask Queue (priority queue)

Question 18

What is Starvation?

Answer 18

Too many micro tasks in the queue are blocking to call back queue excecute

Question 19

What does Parsing do in Js?

Answer 19

Parsing breaks code into tokens and convert it into AST (Abstract Syntax Tree)

Question 20

Is Javascript an interpreter or a compiler language?

Answer 20

Both. Follows JIT compilation. It interpretes while optimizing as much as it can.

Question 21

Arrow Functions vs Regular Functions

Answer 21

• No argument object in arrow function
• Arrow functions do not create their own this binding
• Arrow functions cannot be used as constructors
• Arrow functions cannot be declared
• Arrow functions cannot be accessed before initialization
• Arrow functions when uses as a class methods bind to the class
• Arrow function resolve this lexicaly

Question 22

What is Binding

Answer 22

Binding is the process to assign an object to the ‘this’ keyword when a function is execute.

Question 23

What types of Binding exists?

Answer 23

• Default (direct invocation)
• Explicit (indirect invocation)
• Implicit (method invocation)
• New (object instantiation)
• Lexical (arrow function)

Question 24

What does call() do?

Answer 24

Call a function using another context (this)

Question 25

How do primitive values pass in Js?

Answer 25

By value: Primitive values: number, string, boolean, undefined, null, symbol

By reference: object, array, functions

Question 26

What is this?
Hello()

Answer 26

Function invocation expression

Question 27

What is this in a function invocation?

Answer 27

This is a reference to an Execution Context
This == window
This == undefined (strict mode)

This.method() == object

Question 28

Types of Functions

Answer 28

• Function Declaration or Function Statment
  (function with name and hoisted)
• Function Expression
  (const test = function () {}
  (Not Hoisted)
• Arrow Function

Question 29

Design patterns

Answer 29

• Singleton (allow only one instance of the object)
• Prototype
• Builder

Question 30

What is IIFE?

Answer 30

Inmeadiately Invoked Function Expression. It uses to create a private scoop for variables and not pulluting the global namespace

Question 31

What is functional programming in JavaScript

Answer 31

Pure functions, inmutability

Question 32

What is a promise?

Answer 32

An object or placeholder for a certain period of time until we receive the value from an asynchronous operation or fail.

Question 33

What does an async function return?

Answer 33

Always a promise.
* A promise inside an async function return the promise itself

Question 34

What is the difference between classic resolve promise and await?

Answer 34

• Classic doesn’t await inside the function.
• await awaits the promise resolve and continue the excecution

Question 35

What does fetch function returns?

Answer 35

A response with a body

Question 36

Where do setTimeout, events and promises and sync code are stored before go to the call stack?

Answer 36

SetTimeout: task queue or callback queue
Events: task queue or callback queue
Promises: micro task
Sync: call stack