difference between var const and let code example

Example 1: difference between var and let

var is function scoped and let is block scoped. Let's say you have:
function understanding_var() {
	if (1 == 1) {
    	var x = 5;
        console.log('the value of x inside the if statement is ' + x);
    }
    console.log(x);
} 
//output: the value of x inside the if statement is 5
		  5

function understanding_let() {
	if (1 == 1) {
    	let x = 5;
        console.log('the value of x inside the if statement is ' + x);
    }
    console.log(x);
} 
//output: the value of x inside the if statement is 5
		  Uncaught ReferenceError: x is not defined
          
var is defined throughout the entire function, even if it's inside the if 
statement, but the scope of let is always within the curly braces, not outside
it, even if the conditional statement is inside the function.

Example 2: var vs let vs const

var: 
	- hoisted (always declared at top of scope, global if none)
    - function scope
let:
    - block scope
    - not redeclarable
const: 
    - block scope
    - not reassignable
    - not redeclarable
    
Note: Although it may seem like these hold only semantic meaning, using the
appropriate keywords helps the JS engines' compiler to decide on what to optimize.

Example 3: let and var difference

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar);

  {
    let baz = "Bazz";
    console.log(baz);
  }

  console.log(baz); // ReferenceError
}

run();