JavaScriptのリスト内包表記が括弧悪い

quicksort.js

// for firefox31
const quicksort = (array) =>
{
    if (array.length == 0) return [];
     
    const head = array[0];
    const rest = array.slice(1);
    const small = [i for each (i in rest) if (head >= i)]; //rest.filter(a => head >= a);
    const large = [i for each (i in rest) if (head < i)];  //rest.filter(a => head < a);
     
    return [].concat(quicksort(small), head, quicksort(large));
};

quicksort([2,5,3,2,-1]); // Array [ -1, 2, 2, 3, 5 ]