That catches undefined values (most likely good) but also catches 0 and "0" (most likely bad). Checking for truth had occured to me, but I decided to be consistent with the other solutions, and "0" counting as empty was too big a question mark. Checking for truth is 22% faster than checking the length, though:

          Rate      OP   other fishbot   Joost ikegami   truth
OP       193/s      --    -32%    -67%    -88%    -89%    -91%
other    284/s     47%      --    -52%    -82%    -84%    -87%
fishbot  592/s    207%    109%      --    -62%    -67%    -73%
Joost   1578/s    719%    456%    166%      --    -13%    -29%
ikegami 1818/s    843%    540%    207%     15%      --    -18%
truth   2221/s   1052%    682%    275%     41%     22%      --

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In reply to Re^6: Count non-empty hash values by ikegami
in thread Count non-empty hash values by pboin

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