minor refactoring

Author: b-yogesh

chartlets.js/src/lib/hooks.ts

@@ -48,6 +48,13 @@ export function useSignalListeners(
   id: string | undefined,
   onChange: ComponentChangeHandler,
 ): { [key: string]: SignalHandler } {
+  /*
+       This is a partial representation of the parameter type from
+       SelectionParameter type from the `vega-lite` module. Since we are
+       only interested in extracting the handlers, the following
+       properties are required.
+     */
+  type SelectionParameter = { name: string; select: { on: string } };
   /*
      Here, we create map of signals which will be then used to create the
      map of signal-listeners because not all params are event-listeners, and we
@@ -60,7 +67,7 @@ export function useSignalListeners(
     if (!chart.params) return {};
     return chart.params
       .filter(
-        (param): param is { name: string; select: { on: string } } =>
+        (param): param is SelectionParameter =>
           isObject(param) &&
           param !== null &&
           "name" in param &&