javascript function output math calculation

I am following a javascript video tutorial on pluralsight.com & they have a javascript function but I don't understand how they got this result:

function factorial(n){
    if (n === 0 || n === 1){
    return 1;
    }
return n * factorial(n-1);

}
console.log( factorial(5) ); // result is 120

Can anyone please explain me why this function is returning 120? I would expect it to return 5 * 4 = 20? >> JS Fiddle Link