Simplified Caesars Cipher
I don't know about the performance, but readability-wise, cramming that double-nested ternary on a single line of code isn't, well, ideal.

Give it some [vertical] air:

const rot13 = (str) =&gt; str 
   ? (code(str) &lt; 65 || code(str) &gt; 90) 
       ? head(str).concat(rot13(tail(str))) 
       : String.fromCharCode((((code(str) - 65)) + 13) % 26 + 65).concat(rot13(tail(str))) 
   : '';


Ah, already better.

I'm no javascript expert, but being all inline, it probably performs better than if you further improved readability by introducing a function for each of the two outcomes of the inner condition... on the other hand, that could just as well be premature optimization - so I'd still go for readability and extract the logic into its own function.