fixes #4703

compiler/jsgen.nim

@@ -1985,10 +1985,19 @@ proc genObjConstr(p: PProc, n: PNode, r: var TCompRes) =
     if i > 1: add(initList, ", ")
     var it = n.sons[i]
     internalAssert it.kind == nkExprColonExpr
-    gen(p, it.sons[1], a)
+    let val = it.sons[1]
+    gen(p, val, a)
     var f = it.sons[0].sym
     if f.loc.r == nil: f.loc.r = mangleName(f, p.target)
     fieldIDs.incl(f.id)
+
+    let typ = val.typ.skipTypes(abstractInst)
+    if (typ.kind in IntegralTypes+{tyCstring, tyRef, tyPtr} and
+          mapType(p, typ) != etyBaseIndex) or needsNoCopy(p, it.sons[1]):
+      discard
+    else:
+      useMagic(p, "nimCopy")
+      a.res = "nimCopy(null, $1, $2)" % [a.rdLoc, genTypeInfo(p, typ)]
     addf(initList, "$#: $#" | "'$#' => $#" , [f.loc.r, a.res])
   let t = skipTypes(n.typ, abstractInst + skipPtrs)
   createObjInitList(p, t, fieldIDs, initList)

tests/js/tcopying.nim

@@ -1,5 +1,7 @@
 discard """
   output: '''123
+2 9
+2 9
 '''
 """
 
@@ -11,3 +13,25 @@ proc changeArray(a: var MyArray) =
 var a : MyArray
 changeArray(a)
 echo a[0]
+
+# bug #4703
+# Test 1
+block:
+    let ary1 = [1, 2, 3]
+    var ary2 = ary1
+
+    ary2[1] = 9
+
+    echo ary1[1], " ", ary2[1]
+
+# Test 2
+block:
+    type TestObj = ref object of RootObj
+        ary2: array[3, int]
+
+    let ary1 = [1, 2, 3]
+    var obj = TestObj(ary2:ary1)
+
+    obj.ary2[1] = 9
+
+    echo ary1[1], " ", obj.ary2[1]