recursiveReduce.js

[0, ['a', 'b', 'c'], 2, 'd', 4, 5, 'f', 7, ['g', 'h'], 9].reduce(function rec (prev, curr) {
    if (/array/i.test(curr.constructor)) {
      return curr.reduce(rec, prev);
    } else if (/string/i.test(curr.constructor)) {
      prev.push(curr);
    }

    return prev;
  }, []);

//["a", "b", "c", "d", "f", "g", "h"]