1) Scope:

Scope is that area in which values , expressions are visible , if u try to access any values , expressions out of it you wont be able to access it and will get reference error.

JavaScript has the following kinds of scopes:

Global scope

The default scope for all code running in script mode.

Module scope

The scope for code running in module mode.

Function scope

The scope created with a function.

In addition, variables declared with let or const can belong to an additional scope:

Block scope

The scope created with a pair of curly braces (a block).

function myFunction() {
  let a = 2;  // a can only be used in myFunction
  console.log(a);
}
console.log(a);  // Causes error

In the above example, we are not able to access variable a as it's declared using let which is a block-level scope and can only be accessed within the curly braces.

const a = 2;
exampleFunction();

function exampleFunction() {
  console.log("Inside function");
  console.log(a);
}

console.log("Outside function");
console.log(a);

In the above example, we are able to access variable a as its declare outside function and can be accessed anywhere throughout the program.

2) Single Thread

Javascript is a synchronous , single-threaded language which means it executes one task at a time and do it line by line.

A thread in computer science is just like executing multiple tasks or programs known as a thread.

function Outer(){

  function Inner(){

  }
  Inner();
}
Outer();

In the above example execution of call stack is explained which shows how JS works single threadedly.

3) Call Stack

Call Stack is just a mechanism in Javascript in which it keeps track of multiple functions running in JS code.

Let's take the example and nail down the concept.

function Outer(){

  function Inner(){

  }
  Inner();
}
Outer();

Steps how functions are pushed in call stack...

1st Step(Global Execution Context is created):

2nd step(Pushed Outer Function):

3rd step(Pushed Inner Function and popped Outer Function ):

Final step(Finally Call stack is empty):

4) Hositing

It is a concept in Javascript in which you try to access the variables and functions before they are initialized.

Steps How Hoisting Works :

The whole JS code is scanned and a global execution context is created in which variables , functions of code are made undefined, available respectively is known as Hoisting.

console.log(x);
getName();
var x = 10;

function getName() {
    console.log("Get name");
}

As, soon as we execute the above program , the whole JS code is scanned , global execution context is created and variables(i.e x) is assigned undefined till the time any value is assigned to it and function(i.e getName()) is made available.

Here, x = undefined

Here function getName is made available:

So, Overall we have to remember 2 golden rules in Hoisting:

1) Variables declarations are scanned and made undefined

2) Function declarations are scanned and made available