[May I have your attention please]

Before we start to talk (or write) about the difference between Var, Let and Const, we need to know two main concepts called Scope and Block, and to explain this I need your full atention please.

So, what is Scope ?

Scope means where a declared variable is available for use.

There it is! I told ya, very complicated isn’t !? ;)

What about Block ?

A block is chunk of code bounded by curly braces

Easy like that!

So, let’s talk (or write) about declaration of variables

VAR

[This one is not about the Video Assistant Referee]

So, what is the scope of var ?

var can be Globally Scoped or Function Scoped.

Globally Scoped when declared outside of a function, i.e., we can access this variable in the window. (don’t worry, we gonna see it later)

Funcion Scoped when declared inside a function, i.e., we can access this variable only inside the function

Lets take a look at the code bellow

var name = "lucas"

function foo() {
	var hey = "hey"
}

console.log(window.name) // "Lucas"
console.log(window.hey) // undefined

as I said before, the variable declared as name, is Globally Scoped and can be seen through the window as you can see in the console.log

LET

let is block scoped, i.e. lives inside curly braces. So a variable declared using let, inside a block with the let is only available for use inside that block.

Let’s take a look at the code bellow

let name = "lucas";

if (true) {
	let hey = "hey"
	console.log(hey) // "hey"
}
console.log(hey) // hey is not defined

We see that using hey outside its block (the curly braces where it was defined) returns an error. This is because let variables are block scoped.

CONST

const are block scoped and cant be updated

When you set a value to a variable declared with const, it can’t be updated, and will remain the same inside its scope.

const hey = "Hey you!"
hey = "ops I cant do that" // error : Assignment to constant variable.

But, how it works when the const variable has an object ?

Well, the const itself can’t be updated, but the properties of this object can!

Let’s see it

const user = {
	name : "lucas",
	city: "Porto"
}

user.name = "lucas germano";

So, any questions, opinions, insults !? ;)

Last modified on 2020-04-29