Variables declared with var keyword are function scoped.
What this means that variables can be accessed across that function even if we declare that variable inside a block.

function giveMeX(showX) {
  if (showX) {
    var x = 5;
  }
  return x;
}

console.log(giveMeX(false));
console.log(giveMeX(true));

The first console.log statement logs undefined
and the second 5. We can access the x variable due
to the reason that it gets hoisted at the top of the function scope. So our function code is intepreted like this.

function giveMeX(showX) {
  var x; // has a default value of undefined
  if (showX) {
    x = 5;
  }
  return x;
}

If you are wondering why it logs undefined in the first console.log statement remember variables declared without an initial value has a default value of undefined.

Variables declared with let and const keyword are block scoped. What this means that variable can only be accessed on that block {} on where we declare it.

function giveMeX(showX) {
  if (showX) {
    let x = 5;
  }
  return x;
}


function giveMeY(showY) {
  if (showY) {
    let y = 5;
  }
  return y;
}

If we call this functions with an argument of false it throws a Reference Error because we can't access the x and y variables outside that block and those variables are not hoisted.

There is also a difference between let and const we can assign new values using let but we can't in const but const are mutable meaning. What this means is if the value that we assign to a const is an object we can change the values of those properties but can't reassign a new value to that variable.