Actions and reducers pattern in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an action with a type.

Angular

export const loadItems = createAction('[1]');

Drag options to blanks, or click blank then click option'

A'[Items] Load Items'

B'loadItems'

C'Items Load'

D'Load Items Action'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a simple string without brackets or source prefix.

Using camelCase instead of a descriptive string.

The action type should be a descriptive string usually in the format '[Source] Event'.

2fill in blank

medium

Complete the reducer to handle the loadItems action.

Angular

const itemsReducer = createReducer(initialState, on(loadItems, (state) => [1]));

Drag options to blanks, or click blank then click option'

A{ ...state, loading: true }

B{ loading: false }

Cstate.loading = true

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Mutating the state directly instead of returning a new object.

Returning incomplete state without copying existing properties.

Reducers return a new state object; here we set loading to true while keeping other state properties.

3fill in blank

hard

Fix the error in the reducer to update items on success action.

Angular

on(loadItemsSuccess, (state, [1]) => ({ ...state, items: payload.items, loading: false }))

Drag options to blanks, or click blank then click option'

Apayload

B{ payload }

C{ items }

Daction

Attempts:

3 left

💡 Hint

Common Mistakes

Using a single variable without destructuring.

Using incorrect variable names that don't match the action payload.

The second argument is destructured to get the payload object containing items.

4fill in blank

hard

Fill both blanks to create a selector for items and loading state.

Angular

export const selectItems = createSelector([1], (state) => state.[2]);

Drag options to blanks, or click blank then click option'

AselectFeature

Bitems

Cloading

DselectState

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect selector names.

Selecting a wrong property from the state.

The selector uses selectFeature to get the feature state and then selects the items property.

5fill in blank

hard

Fill all three blanks to define an effect that loads items on loadItems action.

Angular

loadItemsEffect = createEffect(() => this.actions$.pipe(ofType([1]), switchMap(() => this.service.[2]().pipe(map(items => loadItemsSuccess({ [3]: items }))))));

Drag options to blanks, or click blank then click option'

AloadItems

BfetchItems

Citems

DloadItemsSuccess

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong action or service method names.

Incorrect payload property name in loadItemsSuccess.

The effect listens for loadItems action, calls fetchItems service method, and dispatches loadItemsSuccess with items.

Practice

1. In Angular's actions and reducers pattern, what is the main role of an action?

easy

A. To describe what happened and carry data about the event

B. To directly update the UI components

C. To store the entire application state

D. To fetch data from the server

Solution

1. Step 1: Understand the purpose of actions

   Actions are simple objects that describe an event that happened in the app and carry any necessary data.

2. Step 2: Differentiate from other parts

   Reducers handle state changes, not actions. UI updates and data fetching are separate concerns.

3. Final Answer:

   To describe what happened and carry data about the event -> Option A

4. Quick Check:

   Action = event description + data [OK]

Hint: Actions describe events, reducers change state [OK]

Common Mistakes:

• Confusing actions with reducers
• Thinking actions update UI directly
• Assuming actions hold the whole state

2. Which of the following is the correct syntax to define an action using Angular's createAction function?

easy

A. const loadItems = createAction('Load Items', payload);

B. const loadItems = createAction = 'Load Items';

C. const loadItems = actionCreate('Load Items');

D. const loadItems = createAction('Load Items');

Solution

1. Step 1: Recall createAction syntax

   The correct syntax is calling createAction with a string describing the action type.

2. Step 2: Check each option

   const loadItems = createAction('Load Items'); matches the correct syntax. const loadItems = createAction = 'Load Items'; uses wrong assignment. const loadItems = actionCreate('Load Items'); uses wrong function name. const loadItems = createAction('Load Items', payload); incorrectly adds a second argument without proper structure.

3. Final Answer:

   const loadItems = createAction('Load Items'); -> Option D

4. Quick Check:

   createAction('type') is correct [OK]

Hint: createAction takes a single string type [OK]

Common Mistakes:

• Using wrong function names
• Assigning createAction instead of calling it
• Passing payload directly as second argument

3. Given this reducer snippet, what will be the new state after dispatching { type: 'increment' } if the initial state is { count: 0 }?

function counterReducer(state = { count: 0 }, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    default:
      return state;
  }
}

medium

A. { count: 1 }

B. { count: 0 }

C. { count: -1 }

D. undefined

Solution

1. Step 1: Identify the action type and initial state

   The action type is 'increment' and initial state has count 0.

2. Step 2: Follow reducer logic for 'increment'

   The reducer returns a new state with count increased by 1, so count becomes 1.

3. Final Answer:

   { count: 1 } -> Option A

4. Quick Check:

   increment adds 1 to count [OK]

Hint: Reducer returns new state based on action type [OK]

Common Mistakes:

• Returning old state instead of updated
• Confusing increment with decrement
• Expecting mutation instead of new object

4. What is wrong with this reducer code snippet?

function todoReducer(state = [], action) {
  if (action.type = 'add') {
    return [...state, action.payload];
  }
  return state;
}

medium

A. State should be an object, not an array

B. Missing default case in the reducer

C. Using assignment (=) instead of comparison (===) in the if condition

D. Reducer should not return a new array

Solution

1. Step 1: Check the if condition syntax

   The condition uses single equals (=) which assigns instead of compares. This causes a bug.

2. Step 2: Verify other parts

   Default case is handled by returning state. State as array is valid for todo list. Returning new array is correct for immutability.

3. Final Answer:

   Using assignment (=) instead of comparison (===) in the if condition -> Option C

4. Quick Check:

   Use '===' for comparison in conditions [OK]

Hint: Use '===' for comparisons, not '=' [OK]

Common Mistakes:

• Confusing '=' with '===' in conditions
• Thinking default case is missing
• Believing state must be an object

5. You want to add a new feature to reset the counter state to zero using actions and reducers. Which of these is the best way to implement the reset action and update the reducer accordingly?

hard

A. Add case 'reset': state.count = 0; return state; directly in reducer without action

B. Define const reset = createAction('reset'); and add case 'reset': return { count: 0 }; in reducer

C. Define const reset = createAction('reset', () => 0); and return 0 in reducer

D. Use dispatch({ type: 'reset', count: 0 }) and ignore reducer changes

Solution

1. Step 1: Define the reset action properly

   Use createAction with a string type 'reset' to define the action.

2. Step 2: Update reducer to handle reset

   Add a case for 'reset' that returns a new state object with count set to 0, ensuring immutability.

3. Final Answer:

   Define const reset = createAction('reset'); and add case 'reset': return { count: 0 }; in reducer -> Option B

4. Quick Check:

   Action + reducer case resets state immutably [OK]

Hint: Create action and return new state in reducer [OK]

Common Mistakes:

• Mutating state directly in reducer
• Ignoring reducer update for new action
• Misusing createAction with payload function