Actions and reducers pattern in Angular - Step-by-Step Execution

Concept Flow - Actions and reducers pattern

User triggers action

↓

Action dispatched

↓

Reducer receives action

↓

Reducer updates state

↓

New state emitted

↓

UI updates with new state

This flow shows how an action triggers a reducer to update the state, which then updates the UI.

Execution Sample

Angular

import { createAction, createReducer, on } from '@ngrx/store';

const increment = createAction('Increment');

const counterReducer = createReducer(
  0,
  on(increment, state => state + 1)
);

This code defines an increment action and a reducer that increases the state by 1 when the action is dispatched.

Execution Table

Step	Action Dispatched	State Before	Reducer Logic	State After	UI Update
1	increment	0	state + 1	1	Counter shows 1
2	increment	1	state + 1	2	Counter shows 2
3	increment	2	state + 1	3	Counter shows 3
4	No action	3	No change	3	Counter remains 3

💡 No action dispatched, so state remains unchanged and UI does not update.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
state	0	1	2	3	3

Key Moments - 2 Insights

Why does the state not change if no action is dispatched?

Because reducers only update state when they receive a matching action, as shown in step 4 of the execution_table where no action means no state change.

What happens inside the reducer when an action is dispatched?

The reducer runs its logic for that action, here adding 1 to the current state, as seen in steps 1-3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state after step 2?

A 1

B 3

C 2

D 0

💡 Hint

Check the 'State After' column at step 2 in the execution_table.

At which step does the UI show the counter as 3?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'UI Update' column in the execution_table.

If we dispatch the increment action twice more after step 3, what will the state be?

A 4

B 5

C 3

D 6

💡 Hint

Each increment adds 1 to the state; after 3 it is 3, so two more increments add 2.

Concept Snapshot

Actions and reducers pattern in Angular:
- Action: a plain event describing what happened.
- Reducer: a pure function updating state based on action.
- Dispatch action to trigger reducer.
- Reducer returns new state.
- UI updates automatically with new state.

Full Transcript

In Angular, the actions and reducers pattern helps manage state changes clearly. When a user triggers an action, it is dispatched to the reducer. The reducer receives the current state and the action, then returns a new state based on the action's logic. This new state updates the UI. For example, an increment action increases a counter state by one each time it is dispatched. If no action is dispatched, the state stays the same. This pattern keeps state predictable and easy to follow.

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