fix nim-lang#17490 (nim-lang#17491)

lib/pure/json.nim

@@ -927,7 +927,7 @@ when defined(js):
 
   proc parseNativeJson(x: cstring): JSObject {.importjs: "JSON.parse(#)".}
 
-  proc getVarType(x: JSObject): JsonNodeKind =
+  proc getVarType(x: JSObject, isRawNumber: var bool): JsonNodeKind =
     result = JNull
     case $getProtoName(x) # TODO: Implicit returns fail here.
     of "[object Array]": return JArray
@@ -937,6 +937,7 @@ when defined(js):
         if isSafeInteger(x):
           return JInt
         else:
+          isRawNumber = true
           return JString
       else:
         return JFloat
@@ -946,13 +947,13 @@ when defined(js):
     else: assert false
 
   proc len(x: JSObject): int =
-    assert x.getVarType == JArray
     asm """
       `result` = `x`.length;
     """
 
   proc convertObject(x: JSObject): JsonNode =
-    case getVarType(x)
+    var isRawNumber = false
+    case getVarType(x, isRawNumber)
     of JArray:
       result = newJArray()
       for i in 0 ..< x.len:
@@ -973,7 +974,12 @@ when defined(js):
       result = newJFloat(x.to(float))
     of JString:
       # Dunno what to do with isUnquoted here
-      result = newJString($x.to(cstring))
+      if isRawNumber:
+        var value: cstring
+        {.emit: "`value` = `x`.toString();".}
+        result = newJRawNumber($value)
+      else:
+        result = newJString($x.to(cstring))
     of JBool:
       result = newJBool(x.to(bool))
     of JNull:

tests/stdlib/tjson.nim

@@ -300,3 +300,14 @@ block: # bug #17383
   when not defined(js):
     testRoundtrip(int64.high): "9223372036854775807"
     testRoundtrip(uint64.high): "18446744073709551615"
+
+
+block:
+  let a = "18446744073709551615"
+  let b = a.parseJson
+  doAssert b.kind == JString
+  let c = $b
+  when defined(js):
+    doAssert c == "18446744073709552000"
+  else:
+    doAssert c == "18446744073709551615"