code: function keysIndex(a) {
      var keys = [],
          l,
          k,
          i;
  
      for (i = 0, l = a.length; i < l; i++)
        for (k in a[i])
          if (!~keys.indexOf(k))
            keys.push(k);
  
      return keys;
    }
docs: Retrieve an index of keys present in an array of objects