I did it as a test task. Given a set of integers S and a positive integer L, create a function that generates all the possible subsets of S of length L. Example: if S={1, 2, 3} and L=2, then subsets(S, L) is {1, 2}, {1, 3}, {2, 3}.

combinations.js

function range(n) {
    list = [];
    for (var i = 0; i < n; i++) {
        list[i] = i;
    }
    return list;
}
 
 
function show_items_by_indices(source_set, indices) {
    for (var i = 0; i < indices.length; i++) {
        document.write(source_set[indices[i]]);
    }
    document.write('<br/>');
}
 
 
function combinations(source_set, len_subseq) {
    var total = source_set.length;
    if (len_subseq > total) {
        return;
    }
    var indices = range(len_subseq);
 
    // First len_subseq elements from source_set.
    show_items_by_indices(source_set, indices);
 
    while (true) {
        var was_break = false;
        for (var i = 0; i < len_subseq; i++) {
            // It is a reversed index. "-1" is a zero numbering correction.
            var ri = len_subseq - i - 1;
 
            if (indices[ri] != (ri + total - len_subseq)) {
                was_break = true;
                break;
            }
        }
        if ( ! was_break) {
            return;
        }
 
        indices[ri]++;
 
        for (var i = ri + 1; i < len_subseq; i++) {
            indices[i] = indices[i - 1] + 1
        }
 
        show_items_by_indices(source_set, indices);
    }
}
 
combinations([1, 2, 3], 2);