function combinations(arr, k){
    var i,
    subI,
    ret = [],
    sub,
    next;
    for(i = 0; i < arr.length; i++){
        if(k === 1){
            ret.push( [ arr[i] ] );
        }else{
            sub = combinations(arr.slice(i+1, arr.length), k-1);
            for(subI = 0; subI < sub.length; subI++ ){
                next = sub[subI];
                next.unshift(arr[i]);
                ret.push( next );
            }
        }
    }
    return ret;
}
combinations([0,1,2,3,4], 3);
// produces: [[0, 1, 2], [0, 1, 3], [0, 1, 4], [0, 2, 3], [0, 2, 4], [0, 3, 4], [1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4]]

combinations(["Crosby", "Stills", "Nash", "Young"], 3);
// produces: [["Crosby", "Stills", "Nash"], ["Crosby", "Stills", "Young"], ["Crosby", "Nash", "Young"], ["Stills", "Nash", "Young"]]