Effects for side effects in Angular - Mini Project: Build & Apply

Effects for Side Effects in Angular

📖 Scenario: You are building a simple Angular app that fetches a list of books from a server. You want to handle the side effect of loading books using Angular Effects.

🎯 Goal: Create an Angular effect to load books from a mock API and dispatch success or failure actions accordingly.

📋 What You'll Learn

Create an action to start loading books

Create success and failure actions for loading books

Create an effect that listens for the load action and calls a mock service

Dispatch success or failure actions based on the service response

💡 Why This Matters

🌍 Real World

Managing side effects like API calls in Angular apps is essential for clean, maintainable code and better user experience.

💼 Career

Understanding Angular Effects is important for frontend developers working with NgRx to handle asynchronous operations and state management.

1

Create the load books action

Create an action called loadBooks using createAction from '@ngrx/store' with the type '[Books] Load Books'.

Angular

// Your code here

Hint

Use createAction to define a simple action without payload.

2

Create success and failure actions

Create two actions called loadBooksSuccess and loadBooksFailure using createAction. The success action should have a payload property books of type string[]. The failure action should have a payload property error of type string. Use types '[Books] Load Books Success' and '[Books] Load Books Failure' respectively.

Angular

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

export const loadBooks = createAction('[Books] Load Books');

// Your code here

Hint

Use props to define the payload shape for success and failure actions.

3

Create the books effect

Create an Angular injectable class called BooksEffects. Inject Actions and a mock service called BooksService in the constructor. Create an effect called loadBooks$ using createEffect that listens for loadBooks action, calls BooksService.getBooks(), and maps the result to loadBooksSuccess action or catches errors and maps to loadBooksFailure action.

Angular

import { Injectable } from '@angular/core';
import { Actions, createEffect, ofType } from '@ngrx/effects';
import { catchError, map, mergeMap, of } from 'rxjs';

// Your code here

Hint

Use ofType to filter actions, mergeMap to call the service, and catchError to handle errors.

4

Register the effect in the module

In your Angular module, import EffectsModule from '@ngrx/effects' and register BooksEffects using EffectsModule.forRoot([BooksEffects]) in the imports array.

Angular

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { EffectsModule } from '@ngrx/effects';
import { BooksEffects } from './books.effects';

@NgModule({
  declarations: [],
  imports: [
    BrowserModule,
    // Your code here
  ],
  providers: [],
  bootstrap: []
})
export class AppModule {}

Hint

Use EffectsModule.forRoot to register your effects in the root module.

Practice

1. What is the main purpose of Effects in Angular applications?

easy

A. To define routes in the application

B. To style components dynamically

C. To handle side tasks like data fetching or logging outside components

D. To manage component templates

Solution

1. Step 1: Understand the role of Effects

   Effects are designed to handle side effects such as data fetching or logging, which are tasks outside the component's direct responsibilities.

2. Step 2: Compare with other options

   Styling, template management, and routing are handled by other Angular features, not Effects.

3. Final Answer:

   To handle side tasks like data fetching or logging outside components -> Option C

4. Quick Check:

   Effects = side tasks handler [OK]

Hint: Effects manage side tasks outside components [OK]

Common Mistakes:

• Confusing Effects with component styling
• Thinking Effects manage routing
• Assuming Effects handle templates

2. Which of the following is the correct way to create an effect using Angular's createEffect function?

easy

A. const loadData$ = createEffect(() => { this.actions$.pipe(ofType(load), switchMap(() => fetchData())); });

B. const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), switchMap(() => fetchData())));

C. const loadData$ = createEffect(this.actions$.pipe(ofType(load), switchMap(() => fetchData())));

D. const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), map(() => fetchData())));

Solution

1. Step 1: Check the syntax of createEffect

   The createEffect function expects a function returning an observable, so it should be () => this.actions$.pipe(...).

2. Step 2: Verify operators used

   Using switchMap is correct for side effects that return new observables. const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), switchMap(() => fetchData()))); uses this correctly.

3. Final Answer:

   const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), switchMap(() => fetchData()))); -> Option B

4. Quick Check:

   createEffect needs a function returning observable [OK]

Hint: createEffect needs a function returning an observable [OK]

Common Mistakes:

• Passing observable directly instead of a function
• Using map instead of switchMap for async calls
• Not returning the observable inside createEffect

3. Given this effect code snippet:

loadData$ = createEffect(() => this.actions$.pipe(
  ofType(loadData),
  switchMap(() => this.api.getData()),
  map(data => loadDataSuccess({ data })),
  catchError(() => of(loadDataFailure()))
));

What happens when the loadData action is dispatched?

medium

A. The API call is ignored and no action is dispatched

B. The effect causes a syntax error and stops

C. Only loadDataFailure action is dispatched immediately

D. The API call is made, and on success, loadDataSuccess action is dispatched

Solution

1. Step 1: Understand the effect flow

   When loadData action is dispatched, the effect listens and triggers the API call via switchMap.

2. Step 2: Analyze success and error handling

   On success, map dispatches loadDataSuccess with data; on error, catchError dispatches loadDataFailure.

3. Final Answer:

   The API call is made, and on success, loadDataSuccess action is dispatched -> Option D

4. Quick Check:

   Effect triggers API and dispatches success or failure [OK]

Hint: Effects dispatch success or failure actions after API calls [OK]

Common Mistakes:

• Assuming no action is dispatched after API call
• Confusing map with catchError behavior
• Thinking effect causes syntax error

4. Identify the error in this effect code:

saveData$ = createEffect(() => this.actions$.pipe(
  ofType(saveData),
  switchMap(action => this.api.save(action.payload)),
  map(() => saveDataSuccess()),
  catchError(error => saveDataFailure({ error }))
));

medium

A. The catchError operator should return an observable

B. The switchMap should not use action parameter

C. The map operator must return the original action

D. The effect should not use createEffect function

Solution

1. Step 1: Check catchError usage

   The catchError operator must return an observable, but here it returns an action object directly.

2. Step 2: Correct catchError return

   Wrapping the action in of() makes it an observable, fixing the error.

3. Final Answer:

   The catchError operator should return an observable -> Option A

4. Quick Check:

   catchError must return observable [OK]

Hint: Always wrap catchError return in of() to return observable [OK]

Common Mistakes:

• Returning plain object instead of observable in catchError
• Misusing switchMap parameters
• Thinking map must return original action

5. You want to create an effect that listens for login actions, calls an API to authenticate, and then dispatches either loginSuccess or loginFailure. Additionally, you want to log every login attempt regardless of success or failure. Which approach correctly implements this using Angular Effects?

hard

A. Use two separate effects: one for API call with dispatch, another for logging with dispatch: false

B. Use one effect with switchMap for API call and tap for logging inside the same pipe

C. Use one effect with map for API call and catchError for logging

D. Use one effect with filter to block logging and API call

Solution

1. Step 1: Separate concerns for side effects

   Logging is a side effect that does not dispatch actions, so it should be in a separate effect with dispatch: false.

2. Step 2: API call effect dispatches success or failure

   The main effect handles the API call and dispatches loginSuccess or loginFailure accordingly.

3. Step 3: Final design

   Two effects keep code clean and responsibilities clear: one for API calls with dispatch, one for logging without dispatch.

4. Final Answer:

   Use two separate effects: one for API call with dispatch, another for logging with dispatch: false -> Option A

5. Quick Check:

   Separate effects for dispatching and logging [OK]

Hint: Use separate effects for dispatching and non-dispatching tasks [OK]

Common Mistakes:

• Combining logging and dispatching in one effect incorrectly
• Using map instead of switchMap for API calls
• Forgetting dispatch: false for logging effect