Javascript var, let and const differences

var declarations are globally scoped or function scoped while let and const are block scoped. var variables can be updated and re-declared within its scope; let variables can be updated but not re-declared; const variables can neither be updated nor re-declared.

A var variable can be redeclared and updated.

A let variable be be updated but not redeclared.

The Scope of let

If I declare a let variable at the global scope, then redeclare it within a block (curly brackets) as in the example below, I will not get an error in the console, but it will not actually redeclare let.

let points = 50;

let winner = false;

if(points > 40) {

  let winner = true;

}

// If I call:

winner

// It returns the first value: 

false

Why is this happening?

Because let winner = false and let winner = true, are actually two separate variables because they are scoped differently, even though they have the same name. To clarify:

// This 'let' is scoped to the window (globally):

let winner = false;

if(points > 40) {

// This 'let' is scoped to the block (between the curly brackets):

  let winner = true;

}

In the above example if we change bothletvariables to var, then call winner in the console it returns true because it is not inside a function. (Remember: var is function scoped.) The var variable is being redeclared within its scope, which is the window in this case.

let points = 50;

var winner = false;

if(points > 40) {

  var winner = true;

}

// If I call:

winner

// It now returns: 

true

The Differences Between let and const

const variables cannot be updated. let variables are made to be updated.

// If I define the const variable:

const key = 'xyz123';

// Then try to redeclare it:

key = 'xyz1234'

// I get the following error:

Uncaught TypeError: Assignment to constant variable.

References:

https://www.freecodecamp.org/news/var-let-and-const-whats-the-difference/#:~:text=var%20declarations%20are%20globally%20scoped%20or%20function%20scoped%20while%20let,be%20updated%20nor%20re%2Ddeclared.