The code was simple, concatenate a string to form a selector:
'#in-category-' . termId. Undefined. Always undefined.
I started dumping all sorts of things in the console to debug.
termId was defined. The code was passing through conditional statements correctly.
I was just about to send the diff to my work mate Dzikri with the note “this is the general idea but I’ve made a silly mistake” when I saw it.
I’ve noticed this immediately in people’s code. I’m sure you did too.