Random Reducer

console.log(
  [1, "2", 3, 4, "5", true].reduce((p, c, i, a) => {
    if (p === null) {
      p = Math.floor(Math.random() * a.length);
    }

    if (typeof p !== "object") {
      if (i === p) {
        p = [c];
      }
    }

    return p;
  }, null)
);