If duplicates are encountered, splice them out of arr. After
splice is removed, the next item in the array is still the current index, so you need to first j--; and then j++ in the loop to keep the index correct.

For example, in the second outer loop, i = 1, j = 2, then arr[1] is 2, arr[2] is also 2, arr[2] will be splice out, and the array becomes [1,2, 4,2], the next element 4 is still the 2nd item, and will be missed if j-- is not done first.

What you said above is correct. To add, it can be achieved directly by using filter.

var arr = [ 1,2,2,4,2 ];

arr.filter(function (e, i) {
    return arr.indexOf(e) === i;
})