Convert store selectors to Typescript

frontend/src/Store/Selectors/createExistingIndexerSelector.ts

@@ -0,0 +1,17 @@
+import { some } from 'lodash';
+import { createSelector } from 'reselect';
+import AppState from 'App/State/AppState';
+import createAllIndexersSelector from './createAllIndexersSelector';
+
+function createExistingIndexerSelector() {
+  return createSelector(
+    (_: AppState, { definitionName }: { definitionName: string }) =>
+      definitionName,
+    createAllIndexersSelector(),
+    (definitionName, indexers) => {
+      return some(indexers, { definitionName });
+    }
+  );
+}
+
+export default createExistingIndexerSelector;