adding docs subtree

docs/step2-05/exercise/src/reducers/pureFunctions.ts

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+import { Store, FilterTypes } from '../store';
+
+export function addTodo(state: Store['todos'], id: string, label: string): Store['todos'] {
+  // Write code to clone the state object while inserting a new TodoItem inside
+  // - the new object must be of the type TodoItem
+  // - the new state should be cloned using the spread syntax
+  // - return the new state
+
+  return state;
+}
+
+export function remove(state: Store['todos'], id: string) {
+  // Write code:
+  // - to clone the state object into new state object
+  // - remove and item from the new state by using the "delete" keyword
+  // - return the new state
+
+  return state;
+}
+
+export function complete(state: Store['todos'], id: string) {
+  // Write code:
+  // - to clone the state object into new state object
+  // - create a clone of the state[id] into a new item object
+  // - modify new state and set the id key to the value of the new item object
+
+  return state;
+}
+
+export function clear(state: Store['todos']) {
+  // Write code:
+  // - to clone the state object into new state object
+  // - loop through the keys of the new state object
+  // - remove those items inside that new state if the item is completed using the "delete" keyword
+  // - return the new state
+
+  return state;
+}