combinations.js

function combinations(arr1, arr2) {
    return arr1.reduce(function (prev, curr) {
        return prev.concat(arr2.map(function (item) {
            return [curr, item];
        }));
    }, []);
}

> combinations([1,2,3], ['a', 'b', 'c', 'd']);
[ [ 1, 'a' ],
  [ 1, 'b' ],
  [ 1, 'c' ],
  [ 1, 'd' ],
  [ 2, 'a' ],
  [ 2, 'b' ],
  [ 2, 'c' ],
  [ 2, 'd' ],
  [ 3, 'a' ],
  [ 3, 'b' ],
  [ 3, 'c' ],
  [ 3, 'd' ] ]