fix #17275 (#17276)

lib/pure/json.nim

@@ -909,7 +909,7 @@ proc parseJson*(s: Stream, filename: string = ""; rawIntegers = false, rawFloats
 when defined(js):
   from std/math import `mod`
   from std/jsffi import JSObject, `[]`, to
-  from std/private/jsutils import getProtoName
+  from std/private/jsutils import getProtoName, isInteger, isSafeInteger
 
   proc parseNativeJson(x: cstring): JSObject {.importjs: "JSON.parse(#)".}
 
@@ -919,8 +919,11 @@ when defined(js):
     of "[object Array]": return JArray
     of "[object Object]": return JObject
     of "[object Number]":
-      if cast[float](x) mod 1.0 == 0:
-        return JInt
+      if isInteger(x):
+        if isSafeInteger(x):
+          return JInt
+        else:
+          return JString
       else:
         return JFloat
     of "[object Boolean]": return JBool

lib/std/private/jsutils.nim

@@ -37,3 +37,7 @@ when defined(js):
     asm """`result` = typeof BigUint64Array != 'undefined'"""
 
   proc getProtoName*[T](a: T): cstring {.importjs: "Object.prototype.toString.call(#)".}
+
+  proc isInteger*[T](x: T): bool {.importjs: "Number.isInteger(#)".}
+
+  proc isSafeInteger*[T](x: T): bool {.importjs: "Number.isSafeInteger(#)".}

tests/stdlib/tjson.nim

@@ -240,10 +240,7 @@ doAssert isRefSkipDistinct(MyOtherDistinct)
 
 let x = parseJson("9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999")
 
-when defined(js): # xxx fixme
-  doAssert x.kind == JInt
-else:
-  doAssert x.kind == JString
+doAssert x.kind == JString
 
 block: # bug #15835
   type