Observe the warning triggered here:
$ clj -m cljs.main -co '{:infer-externs true}' -r
ClojureScript 1.10.339
cljs.user=> (defn foo? [x y] (< x (or y (.-MAX_VALUE js/Number))))
WARNING: cljs.core/<, all arguments must be numbers, got [nil #{nil js/Number}] instead at line 1 <cljs repl>
This warning doesn't appear if you don't enable {{:infer-externs}} (in which case {{(.-MAX_VALUE js/Number)}} inferred to be of type {{js}}).