what is the difference between passing array to a function with the name only and passing it by accessing one of its indexes?

Question

In this block of code the result will be "unll" for the first console and an empty array for the second console

function fn(o) {
o = null;
console.log(o); //console o
}
var x = [];
fn(x);
console.log(x); //console x

but in this block of code

function fn(o) {
o[0] = null;
console.log(o); //console o
}
var x = [];
fn(x);
console.log(x); //console x

when i access the first element of the array the result of both consoles is an array with null value in the first element.

And this is really confusing for me, i understand that in the first block of code the array was passed by value, so it didn't affect the x array.

but why in the second block it was passed with reference!!

So does anyone has an explanation please?