Need explanation in this code's line of a for loop: arr.reverse([i]); Why this output?

Question

const num = [1, 2, 3, 4, 4, 3, 2, 1];
var len = num.length;
var arr = new Array();
for(let i = 0; i < len; i++) {
if(arr.indexOf(num[i]) == -1) {
arr.push(num[i]);
arr.reverse([i]);
}
}
console.log(arr);

//Output
[4, 2, 1, 3]

Answer 1

First iteration
1

2nd iteration
1,2
reverse it
2,1

3rd iteration
2,1,3
reverse it
3,1,2

4rd iteration
3,1,2,4
reverse it
[4,2,1,3]

The rest of iterations would be ignored, due to if condition is false, elements exist.

please note reverse function does not has any argument.

Answer 2

Thanks Calviղ .

Answer 3

Nice explanation