Ensure param callbacks initialized (#1439)

* Implement Param.select

* Implement Param.select to ensure JS callbacks are processed

panel/param.py

@@ -532,6 +532,23 @@ def widget_type(cls, pobj):
                     return cls._mapping[t](pobj)
                 return cls._mapping[t]
 
+    def select(self, selector=None):
+        """
+        Iterates over the Viewable and any potential children in the
+        applying the Selector.
+
+        Arguments
+        ---------
+        selector: type or callable or None
+          The selector allows selecting a subset of Viewables by
+          declaring a type or callable function to filter by.
+
+        Returns
+        -------
+        viewables: list(Viewable)
+        """
+        return super().select(selector) + self.layout.select(selector)
+
     def get_root(self, doc=None, comm=None):
         """
         Returns the root model and applies pre-processing hooks

panel/tests/test_param.py

@@ -832,6 +832,24 @@ class Test(param.Parameterized):
     assert len(model.tabs) == 1
 
 
+def test_param_js_callbacks(document, comm):
+    class JsButton(param.Parameterized):
+        param_btn = param.Action(lambda self: print('Action Python Response'), label='Action')
+
+    param_button = Param(JsButton())
+    code = "console.log('Action button clicked')"
+    param_button[1].js_on_click(code=code)
+
+    model = param_button.get_root(document, comm=comm)
+
+    button = model.children[1]
+    assert len(button.js_event_callbacks) == 1
+    callbacks = button.js_event_callbacks
+    assert 'button_click' in callbacks
+    assert len(callbacks['button_click']) == 1
+    assert code in callbacks['button_click'][0].code
+
+
 class View(param.Parameterized):
 
     a = param.Integer(default=0)