Fix negative indexed arrays for JS, refs #13966 (#14152)

* Fix negative arrays for JS, refs #13966

* small extra fix: no need to .slice() cstring in JS

compiler/jsgen.nim

@@ -1161,9 +1161,9 @@ proc genArrayAddr(p: PProc, n: PNode, r: var TCompRes) =
     first = firstOrd(p.config, typ[0])
   if optBoundsCheck in p.options:
     useMagic(p, "chckIndx")
-    r.res = "chckIndx($1, $2, ($3 != null ? $3.length : 0)+$2-1)-$2" % [b.res, rope(first), tmp]
+    r.res = "chckIndx($1, $2, ($3 != null ? $3.length : 0)+$2-1)-($2)" % [b.res, rope(first), tmp]
   elif first != 0:
-    r.res = "($1)-$2" % [b.res, rope(first)]
+    r.res = "($1)-($2)" % [b.res, rope(first)]
   else:
     r.res = b.res
   r.kind = resExpr
@@ -1897,8 +1897,8 @@ proc genMagic(p: PProc, n: PNode, r: var TCompRes) =
     let rhsIsLit = n[2].kind in nkStrKinds
     let (a, tmp) = maybeMakeTemp(p, n[1], lhs)
     if skipTypes(n[1].typ, abstractVarRange).kind == tyCString:
-      r.res = "if ($1 != null) { $4 += $2; } else { $4 = $2$3; }" % [
-        a, rhs.rdLoc, if rhsIsLit: nil else: ~".slice()", tmp]
+      r.res = "if ($1 != null) { $3 += $2; } else { $3 = $2; }" % [
+        a, rhs.rdLoc, tmp]
     else:
       r.res = "if ($1 != null) { $4 = ($4).concat($2); } else { $4 = $2$3; }" % [
           lhs.rdLoc, rhs.rdLoc, if rhsIsLit: nil else: ~".slice()", tmp]

tests/js/tarrayboundscheck.nim

@@ -39,6 +39,12 @@ proc test_arrayboundscheck() =
         echo "month out of bounds: ", idx
     except:
       echo "idx out of bounds: ", i
+  
+  # #13966
+  var negativeIndexed: array[-2..2, int] = [0, 1, 2, 3, 4]
+  negativeIndexed[-1] = 2
+  negativeIndexed[1] = 2
+  doAssert negativeIndexed == [0, 2, 2, 2, 4]
 
 test_arrayboundscheck()
 {.pop.}