fixes #6532 array bounds check for non zero based arrays on the js backend (#6550)

compiler/jsgen.nim

@@ -1067,7 +1067,7 @@ proc genArrayAddr(p: PProc, n: PNode, r: var TCompRes) =
       else:
         r.res = "chckIndx($1, $2, strlen($3))-$2" % [b.res, rope(first), a.res]
     else:
-      r.res = "chckIndx($1, $2, $3.length-1)-$2" % [b.res, rope(first), a.res]
+      r.res = "chckIndx($1, $2, $3.length+$2-1)-$2" % [b.res, rope(first), a.res]
   elif first != 0:
     r.res = "($1)-$2" % [b.res, rope(first)]
   else:

tests/js/tarrayboundscheck.nim

@@ -0,0 +1,44 @@
+discard """
+  output: '''idx out of bounds: -1
+month out of bounds: 0
+Jan
+Feb
+Mar
+Apr
+May
+Jun
+Jul
+Aug
+Sep
+Oct
+Nov
+Dec
+month out of bounds: 13
+idx out of bounds: 14
+'''
+"""
+
+{.push boundChecks:on.}
+
+# see issue #6532:
+# js backend 0.17.3: array bounds check for non zero based arrays is buggy
+
+proc test_arrayboundscheck() =
+  var months: array[1..12, string] =
+    ["Jan", "Feb", "Mar", "Apr", "May", "Jun",
+     "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
+
+  var indices = [0,1,2,3,4,5,6,7,8,9,10,11,12,13]
+
+  for i in -1 .. 14:
+    try:
+      let idx = indices[i]
+      try:
+        echo months[idx]
+      except:
+        echo "month out of bounds: ", idx
+    except:
+      echo "idx out of bounds: ", i
+
+test_arrayboundscheck()
+{.pop.}