Count inversions in an array

countInversions.js

function countInversions(xs){
    var count = 0;

    function mergeSort(xs){
        var size = xs.length
        if (size < 2) return xs
 
        var mid = Math.floor(size / 2),
            left = xs.slice(0, mid),
            right = xs.slice(mid)

        return merge(mergeSort(left), mergeSort(right))
    }
 
    function merge(left, right){
        var result = [];

        while (left.length && right.length) {
            if(left[0] <= right[0]) {
                result.push(left.shift())
            } else {
                count++
                result.push(right.shift())
            }
        }

        while (left.length)
            result.push(left.shift())
        while (right.length)
            result.push(right.shift())

        return result;
    }
    
    mergeSort(xs)
    
    return count
}

var xs = [34, 203, 3, 746, 200, 984, 198, 764, 9]

countInversions(xs) //9

Bugged: [4,5,6,1,2,3]