diff --git a/metakernel/_metakernel.py b/metakernel/_metakernel.py
index b5b3490d..d0bc6a8f 100644
--- a/metakernel/_metakernel.py
+++ b/metakernel/_metakernel.py
@@ -805,12 +805,16 @@ def initialize(self, argv=None):
                 self.argv = argv
 
             def start(self):
-                kernel_spec = self.kernel_class().kernel_json
+                instance = self.kernel_class()
+                kernel_spec = instance.kernel_json
                 with TemporaryDirectory() as td:
                     dirname = os.path.join(td, kernel_spec['name'])
                     os.mkdir(dirname)
                     with open(os.path.join(dirname, 'kernel.json'), 'w') as f:
                         json.dump(kernel_spec, f, sort_keys=True)
+                    if hasattr(instance, 'kernel_javascript') and instance.kernel_javascript.strip():
+                        with open(os.path.join(dirname, 'kernel.js'), 'w') as f:
+                            f.write(instance.kernel_javascript)
                     filenames = ['logo-64x64.png', 'logo-32x32.png']
                     name = self.kernel_class.__module__
                     for filename in filenames: