Get all the combinations of N elements of multidimensional array

ChrisB solution had a mistake, he wasn’t caching the length of the loop before the arr.shift, and it was not returning the last combination, I think this will do the job:

function getCombinations (arr, n) {
    var i, j, k, elem, l = arr.length, childperm, ret = [];
    if (n == 1){
        for (i = 0; i < arr.length; i++) {
            for (j = 0; j < arr[i].length; j++) {
                ret.push([arr[i][j]]);
            }
        }
        return ret;
    }
    else {
        for (i = 0; i < l; i++) {
            elem = arr.shift();
            for (j = 0; j < elem.length; j++) {
                childperm = getCombinations(arr.slice(), n-1);
                for (k = 0; k < childperm.length; k++) {
                    ret.push([elem[j]].concat(childperm[k]));
                }
            }
        }
        return ret;
    }
}


getCombinations([["A"],["B"],["C","D"]], 2);

// [["A", "B"], ["A", "C"], ["A", "D"], ["B", "C"], ["B", "D"]]

getCombinations([["A"],["B"],["C"],["D"]], 2);

// [["A", "B"], ["A", "C"], ["A", "D"], ["B", "C"], ["B", "D"], ["C", "D"]]