Not a bug but a feature?

[vlad-km]

; SLIME 2.24
CL-USER> (setq a #(1 2 3 4))
; in: SETQ A
;     (SETQ A #(1 2 3 4))
; 
; caught WARNING:
;   undefined variable: COMMON-LISP-USER::A
; 
; compilation unit finished
;   Undefined variable:
;     A
;   caught 1 WARNING condition
#(1 2 3 4)
CL-USER> (aref a nil)
; Evaluation aborted on #<SB-INT:INVALID-ARRAY-INDEX-ERROR expected-type: (INTEGER 0 (4)) datum: NIL>.
CL-USER>

JSCL

Welcome to JSCL (version 0.9.0-alpha.0 built on 17 February 2026)

JSCL is a Common Lisp implementation on Javascript.
For more information, visit the project page at https://github.com/jscl-project/jscl.

CL-USER> (aref #(1 2 3) nil)

#j:undefined
CL-USER>

[davazp]

We should fix it.
If we want the js equivalent without checks we can export it from the FFI package.