let, var or const, what's the difference?

let, var , const? when do we use this variables really?
Back in 2018, I got the opportunity to play around a little bit with JavaScript. During that year, I was thrilled to learn how this language performs. The first thing I learned was to declare a variable. My first variable was var name = 'el marlo' after I so this variable be used within my function and console.log(name) // => el marlo. As a beginner programmer, I got excited. Fast forward to 2024, officially starting my journey into Software Engineering, getting my hands into JavaScript after so many years and a lot of this have changed. The first thing I noticed, there were more options to declare a variable? What is let and const, I was only familiar with var.

var

var is the oldest keyword for declaring variable. Therefore, let's address the difference against the other two: let and const to help us decide which one should go into our code.

I learned that var was a keyword that if you are planning to use, to be very careful or don't use it at all due to a lacking in block scope or in plain english, the code that goes inside the curly braces {}. In addition, it can create bugs in your code because var variables can be re-declared and updated:

var favHobby = "Eskate";
var favHobby = "Sleeping";
var favHobby = "Joking";

console.log(favHobby); // => Joking
console.log(favHobby); // => Joking
console.log(favHobby); // => Joking

let

letis the update version of var. This variable is blocked scoped, which means that unlike var, anything that we declare within {curly braces will only be available within this scope}:

let x = 1;

if (x === 1) {
  let x = 2;

  console.log(x);
  // Expected output: 2
}

console.log(x);
// Expected output: 1

Example from: mdn web docs
In addition, let can be updated but NOT re-declared.

const

const is the more reliable variable to used for the following reasons: const declarations are block scoped: which means it is only accessible {within the block}. Another strong reason is, const cannot be updated or re-declared unless it is an object. If const was an object, then properties could be added, removed or updated.

const number = 42;

try {
  number = 99;
} catch (err) {
  console.log(err);
  // Expected output: TypeError: invalid assignment to const 'number'
  // (Note: the exact output may be browser-dependent)
}

console.log(number);
// Expected output: 42

Example from: mdn web docs