js generates spurious >>> on shr (#17767)

* js generates spurious >>> on shr
* Add shr arithmetic test
* fix variables from const to let during testing

compiler/jsgen.nim

@@ -683,8 +683,7 @@ proc arith(p: PProc, n: PNode, r: var TCompRes, op: TMagic) =
     var x, y: TCompRes
     gen(p, n[1], x)
     gen(p, n[2], y)
-    let trimmer = unsignedTrimmer(n[1].typ.skipTypes(abstractRange).size)
+    r.res = "($1 >>> $2)" % [x.rdLoc, y.rdLoc]
-    r.res = "(($1 $2) >>> $3)" % [x.rdLoc, trimmer, y.rdLoc]
   of mCharToStr, mBoolToStr, mIntToStr, mInt64ToStr, mFloatToStr,
       mCStrToStr, mStrToStr, mEnumToStr:
     arithAux(p, n, r, op)

tests/stdlib/tarithmetics.nim

@@ -0,0 +1,49 @@
+discard """
+  targets: "c cpp js"
+"""
+
+# TODO: in future work move existing arithmetic tests (tests/arithm/*) into this file
+# FYI https://github.com/nim-lang/Nim/pull/17767
+
+template main =
+  # put all arithmetic tests
+
+  block tshr:
+    block: # Signed types
+      let
+        a1 = -3
+        a2 = -2
+        b1 = -4'i8
+        b2 = 1'i8
+        c1 = -5'i16
+        c2 = 1'i16
+        d1 = -7i32
+        d2 = 1'i32
+        e1 = -9'i64
+        e2 = 1'i64
+      doAssert a1 shr a2 == -1
+      doAssert b1 shr b2 == -2
+      doAssert c1 shr c2 == -3
+      doAssert d1 shr d2 == -4
+      doAssert e1 shr e2 == -5
+
+    block: # Unsigned types
+      let
+        a1 = 3'u
+        a2 = 2'u
+        b1 = 2'u8
+        b2 = 1'u8
+        c1 = 5'u16
+        c2 = 1'u16
+        d1 = 6'u32
+        d2 = 1'u32
+        e1 = 8'u64
+        e2 = 1'u64
+      doAssert a1 shr a2 == 0
+      doAssert b1 shr b2 == 1
+      doAssert c1 shr c2 == 2
+      doAssert d1 shr d2 == 3
+      doAssert e1 shr e2 == 4
+
+static: main()
+main()