fix #16978 unittest js foreign exception (#16998)

* fix #16978 unittest js foreign exception

lib/pure/unittest.nim

@@ -515,7 +515,9 @@ template suite*(name, body) {.dirty.} =
     finally:
       suiteEnded()
 
-template exceptionTypeName(e: typed): string = $e.name
+proc exceptionTypeName(e: ref Exception): string {.inline.} =
+  if e == nil: "<foreign exception>"
+  else: $e.name
 
 template test*(name, body) {.dirty.} =
   ## Define a single test case identified by `name`.
@@ -552,8 +554,11 @@ template test*(name, body) {.dirty.} =
       let e = getCurrentException()
       let eTypeDesc = "[" & exceptionTypeName(e) & "]"
       checkpoint("Unhandled exception: " & getCurrentExceptionMsg() & " " & eTypeDesc)
-      var stackTrace {.inject.} = e.getStackTrace()
-      fail()
+      if e == nil: # foreign
+        fail()
+      else:
+        var stackTrace {.inject.} = e.getStackTrace()
+        fail()
 
     finally:
       if testStatusIMPL == TestStatus.FAILED:

tests/js/tunittest_error2.nim

@@ -0,0 +1,16 @@
+discard """
+  exitcode: 1
+  outputsub: '''
+Unhandled exception: Cannot read property 'charCodeAt' of null [<foreign exception>]
+[FAILED] Bad test
+  '''
+  matrix: "-d:nodejs"
+  targets: "js"
+  joinable: false
+"""
+
+# bug #16978
+import unittest
+test "Bad test":
+  var x: cstring = nil
+  let y = x[0]