ROT13 rotates alphabetic characters by 13 positions, producing the same output for the same input string. That stable result makes it useful as a memoization example and as a small web trick for hiding puzzle answers or discount codes. section 7.3
The treatment focuses on why a repeatable function benefits from caching, then shows ROT13 as the concrete case. It stays at the level of usage and performance benefit rather than detailing the cipher rules. section 7.3
ROT13 discount code
A discount code like functional_js_50_off becomes shapgvbany_wf_50_bss after ROT13, showing the cipher on a real string.
