Crystal-Clear-Javascript

                                             Javascript

 First what do you mean 'Javascript'

 In simple words 'Every thing is javascript happen inside an Execution Context and it is   lightweight,interpreted(means "it will execute the code line wise ") programming language.'

  javascript is client slide scripted language

   javascript is most popular language now day,

Why Study Javascript?

• Javascript is  used in web-development and extend to mobile app development ,desktop app development  and game development
• Every thing in javascript  comes installed on every modern web browser('like :google chrome,safari) and not need  any environment setup 

Let the terms of javascript :

• Execution Context(the environment in which our code is executed and is evaluted)
• Execution  Stack

                                                         Execution Stack

Execution stack also known as 'calling stack'  is  a stack follow 'LIFO'(last in first out) Structure 

which is used to store all the execution context created during the code execution

  Variable object

For each function (function declaration ) a property is created in the varible object .which is pointing  to that function

for each varible (variable declaration ) a property  is  created in the variable object which is the set to undefined

Hoisting is a javascript mechanism where variable and function declarations are  moved to the top of

their  scope before the code execution

 Function Hoisting

Before the execution phase every variable and funtion moved to top of their global  scope  and set variable value to 'undefined ' and  in case of function it store the all function definition

CODE:

          console.log(fun())

           console.log(x)

           var x=10

         function fun(){

              console.log('printing function')

         }

OUT-PUT on Browser:

1.  undefined
2.   f fun(){`

              console.log('printing function')

         } 

Lexical Environment :

it's the internal js engine construct that holds identifier-variable mapping. (here identifier refers to the name of variables/functions, and variable is the reference to actual object [including function type object] or primitive value). A lexical environment also holds a reference to a parent lexical environment.

In simple words   (local memory + parent lexical Environment )  this both shows the lexical environment of  current execution context

And every lexical environment tracks its parent lexical environment (that of parent execution context)

 As a result, every function has a chain of lexical environments attached to it.

Scope : it's the language agnostic concept, to refer to the visibility of variables or functions to the executing code. 

In js a variable or function is visible to the executing code, if it is there in the current lexical environment or in the lexical-environment-chain of the enclosing function. In case of global code, the chain does not exist.

Regarding Let and Const

'Let' and  'Const' declarations are both block-scoped, which means they are only accessible within the { } surrounding them "var', on the other hand, doesn't have this restriction.

     TEMPORAL DEAD ZONE

This  term is describing  the state where variables are un-reachable. They are in scope, but they aren't declared.

Temporal dead zone : time between  the initiation of variable  and assigning value to variable is known as temporal dead zone

CODE:

1:{                         console.log(money)// This is the temporal dead zone for the money variable

                           let money= 25 ;  // now temporal dead zone no  more

                           console.log(money) 

}

2:{                        console.log(a)

                   let a=12;  /// we can't use same name of variable name with once it is defined to 'let '

                   var a=12; 

}

Here you can find Reference error   in both the cases