bug#43037: Eval error in the ‘c-lang-defvar’ or ‘c-lang-setvar’

[AC]

On October 16, 2020 12:04:06 PM EDT, Alan Mackenzie <acm@muc.de> wrote:
>
>> Were all instances of 'js-mode' in the above-quoted message meant to
>be 'js' instead?
>
>Apologies.  js-mode should indeed have been js, as in js.el and js.elc.
>

Thank you!

I can confirm your suggestion works fine: simply touching lisp/progmodes/js.el 
to change its timestamp induced the build system to recompile it, and now the 
error's gone.