What is the Temporal dead zone?

In case of let and const variables, Basically, Temporal Dead Zone is a zone "before your variable is declared",
i.e where you can not access the value of these variables, it will throw an error.

ex.

let sum = a + 5;        //---------
//some other code       //         | ------>  this is TDZ for variable a
                        //         |
console.log(sum)        //---------
let a = 5;

above code gives an error

the same code will not give an error when we use var for variable 'a',

ex.

var sum = a;                            
console.log(sum)     //prints undefined
var a = 5;

let and const have two broad differences from var:

1.They are block scoped.
2.Accessing a var before it is declared has the result undefined; accessing a let or const before it is declared throws ReferenceError:

console.log(aVar); // undefined
console.log(aLet); // Causes ReferenceError: Cannot access 'aLet' before initialization

var aVar = 1;
let aLet = 2;

It appears from these examples that let declarations (and const, which works the same way) may not be hoisted, since aLet does not appear to exist before it is assigned a value.

That is not the case, however—let and const are hoisted (like var, class and function), but there is a period between entering scope and being declared where they cannot be accessed. This period is the temporal dead zone (TDZ).

---

To Summarize

Hoisting:
let,const,var are all get hoisted process.
(whats mean they go upper and declare in the top of the scope.)

hoisting process: var, let, const
Initialisation process: var

---

Thanks, and see you next time!