Fixes #5244 (#5245)

compiler/jsgen.nim

@@ -1173,6 +1173,7 @@ proc genDeref(p: PProc, n: PNode, r: var TCompRes) =
   else:
     var a: TCompRes
     gen(p, n.sons[0], a)
+    r.kind = resExpr
     if a.typ == etyBaseIndex:
       r.res = "$1[$2]" % [a.address, a.res]
     elif n.sons[0].kind == nkCall:

tests/js/tbyvar.nim

@@ -49,3 +49,13 @@ block: # Test get addr of byvar return value
   let a = addr t["hi"]
   a[] = 10
   doAssert(t["hi"] == 10)
+
+block: # Test var arg inside case expression. #5244
+  proc foo(a: var string) =
+    a = case a
+    of "a": "error"
+    of "b": "error"
+    else: a
+  var a = "ok"
+  foo(a)
+  doAssert(a == "ok")