Actions and reducers pattern in Angular - Mini Project: Build & Apply

Actions and reducers pattern

📖 Scenario: You are building a simple Angular app to manage a counter. The app uses the actions and reducers pattern to update the counter state.

🎯 Goal: Create an Angular standalone component that uses signals and the actions and reducers pattern to increase and decrease a counter value.

📋 What You'll Learn

Create an initial state object with a count property set to 0

Create an action type variable called INCREMENT with value 'increment'

Create a reducer function called counterReducer that takes state and action and returns updated state

Create a standalone Angular component called CounterComponent that uses signals and the reducer to update and display the count

💡 Why This Matters

🌍 Real World

Managing state in Angular apps using actions and reducers helps keep your app predictable and easier to maintain, especially as it grows.

💼 Career

Understanding actions and reducers is key for Angular developers working with state management libraries or building scalable apps with clear state flow.

1

Create the initial state object

Create a constant called initialState that is an object with a property count set to 0.

Angular

# Your code here

Hint

Use const initialState = { count: 0 }; to create the initial state.

2

Create the action type variable

Create a constant called INCREMENT and set it to the string 'increment'.

Angular

const initialState = { count: 0 };
# Your code here

Hint

Use const INCREMENT = 'increment'; to define the action type.

3

Create the reducer function

Create a function called counterReducer that takes parameters state and action. Inside, use a switch statement on action.type. If it matches INCREMENT, return a new state object with count increased by 1. Otherwise, return the original state.

Angular

const initialState = { count: 0 };
const INCREMENT = 'increment';
# Your code here

Hint

Use a switch on action.type and return updated state for INCREMENT.

4

Create the Angular component using signals and reducer

Create a standalone Angular component called CounterComponent. Inside, create a signal called state initialized with initialState. Create a function called dispatch that takes an action and updates state by calling counterReducer with current state() and the action. In the template, display the current count using {{ state().count }} and add a button with a click event that calls dispatch with an action of type INCREMENT. Use semantic HTML and add an aria-label to the button for accessibility.

Angular

const initialState = { count: 0 };
const INCREMENT = 'increment';

function counterReducer(state, action) {
    switch (action.type) {
        case INCREMENT:
            return { count: state.count + 1 };
        default:
            return state;
    }
}

// Your code here

Hint

Use Angular signals and update state with the reducer inside dispatch. Add a button with click event and aria-label.

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