[Gvozd]

array = [1,2,3,4,5,6];
length = 2;
arr = array.reduce(function(arr, el, index, array){
    var count = array.length/length;
    index++;
    if(index - count*~~(index/count) < 1) {
        arr.push(el);
    }
    return arr;
}, []);
console.log(arr);