quicksort: find pivot then sort

var quicksort=function(arr){

    if(arr.length<=1) return arr;

    var pivotIndex = Math.floor(arr.length/2);

    var pivot = arr.slice(pivotIndex,1)[0];

    var left = [], right = [];

    for(var i =0;i<arr.length;i++){

        arr[i]<pivot?left.push(arr[i]):right.push(arr[j]);

    }

    return quicksort(left).concat([pivot],quicksort(right));

}