Actions and reducers pattern in Angular

Introduction

The actions and reducers pattern helps manage app data in a clear way. It keeps track of changes step-by-step so your app stays organized and easy to fix.

When you want to keep track of user actions like clicks or form submissions.

When your app has data that changes over time and you want to update the screen smoothly.

When you want to separate how data changes from how the app shows data.

When you want to make your app easier to test and debug by following clear rules.

Syntax

Angular

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

export const actionName = createAction(
  '[Source] Event',
  props<{ payloadProperty: any }>()
);

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

export const initialState = { /* initial state object */ };

export const reducerName = createReducer(
  initialState,
  on(actionName, (state, { payloadProperty }) => ({
    ...state,
    /* new state changes */
  }))
);

Actions describe what happened, carrying any needed data.

Reducers decide how the state changes based on actions.

Examples

This action has no extra data. It just signals to increase a counter.

Angular

import { createAction } from '@ngrx/store';

export const increment = createAction('[Counter] Increment');

This action carries a string item to add to a list.

Angular

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

export const addItem = createAction(
  '[Todo] Add Item',
  props<{ item: string }>()
);

This reducer increases the count by 1 when the increment action happens.

Angular

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

export const initialState = { count: 0 };

export const counterReducer = createReducer(
  initialState,
  on(increment, state => ({ ...state, count: state.count + 1 }))
);

Sample Program

This Angular component shows a number and a button. When you click the button, it sends an increment action. The reducer updates the count by 1. The screen updates to show the new count.

This pattern keeps the data changes clear and separate from the display.

Angular

import { Component } from '@angular/core';
import { Store, createAction, createReducer, on, props } from '@ngrx/store';
import { Observable } from 'rxjs';

// Define action
export const increment = createAction('[Counter] Increment');

// Define state interface
interface CounterState {
  count: number;
}

// Initial state
const initialState: CounterState = { count: 0 };

// Reducer function
export const counterReducer = createReducer(
  initialState,
  on(increment, state => ({ ...state, count: state.count + 1 }))
);

@Component({
  selector: 'app-counter',
  template: `
    <main>
      <h1>Counter: {{ count$ | async }}</h1>
      <button (click)="incrementCount()" aria-label="Increment counter">Increment</button>
    </main>
  `,
  standalone: true
})
export class CounterComponent {
  count$: Observable<number>;

  constructor(private store: Store<{ counter: CounterState }>) {
    this.count$ = store.select(state => state.counter.count);
  }

  incrementCount() {
    this.store.dispatch(increment());
  }
}

Important Notes

Always keep actions simple and focused on one event.

Reducers must not change the old state directly; always return a new state object.

Use descriptive action names to understand what happened easily.

Summary

Actions tell what happened, carrying needed data.

Reducers decide how state changes based on actions.

This pattern helps keep app data organized and easy to manage.

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