adding docs subtree

docs/step2-05/demo/src/reducers/index.ts

@@ -0,0 +1,20 @@
+import { Store } from '../store';
+import { addTodo, remove, complete, clear } from './pureFunctions';
+
+export function reducer(state: Store['todos'], payload: any): Store['todos'] {
+  switch (payload.type) {
+    case 'addTodo':
+      return addTodo(state, payload.id, payload.label);
+
+    case 'remove':
+      return remove(state, payload.id);
+
+    case 'complete':
+      return complete(state, payload.id);
+
+    case 'clear':
+      return clear(state);
+  }
+
+  return state;
+}