Effects for side effects in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an effect dispatches an action?

Consider an Angular effect that listens for an action and dispatches another action. What is the result in the application?

Angular

import { createEffect, ofType, Actions } from '@ngrx/effects';
import { Injectable } from '@angular/core';
import { map } from 'rxjs/operators';
import * as MyActions from './actions';

@Injectable()
export class MyEffects {
  effect$ = createEffect(() => this.actions$.pipe(
    ofType(MyActions.loadData),
    map(() => MyActions.loadDataSuccess({ data: [1, 2, 3] }))
  ));

  constructor(private actions$: Actions) {}
}

A The effect listens for 'loadData' action and dispatches 'loadDataSuccess' action, updating the store accordingly.

B The effect listens for 'loadData' but does not dispatch any action, so the store remains unchanged.

C The effect causes an infinite loop because it dispatches the same action it listens for.

D The effect runs only once and then stops listening for actions.

💡 Hint

Think about what happens when an effect returns a new action.

Effects listen for specific actions and can dispatch new actions to update the store or trigger other effects. Here, the effect listens for 'loadData' and dispatches 'loadDataSuccess' with data.

📝 Syntax

intermediate

Identify the syntax error in this effect definition

Which option shows the correct syntax for creating an effect that does NOT dispatch an action?

Angular

import { createEffect, ofType, Actions } from '@ngrx/effects';
import { Injectable } from '@angular/core';
import { tap } from 'rxjs/operators';
import * as MyActions from './actions';

@Injectable()
export class MyEffects {
  logEffect$ = createEffect(() => this.actions$.pipe(
    ofType(MyActions.loadData),
    tap(() => console.log('Load data action received'))
  ), { dispatch: false });

  constructor(private actions$: Actions) {}
}

A The effect is missing a return statement inside createEffect.

B The effect should use map instead of tap to dispatch an action.

C The effect must include { dispatch: false } to avoid dispatching an action.

D The effect should not use ofType operator.

💡 Hint

Think about how to create effects that only perform side effects without dispatching new actions.

When an effect does not dispatch an action, you must set { dispatch: false } in createEffect options. Using tap is correct for side effects like logging.

🔧 Debug

advanced

Why does this effect cause an infinite loop?

Examine the effect below. Why does it cause an infinite loop?

Angular

import { createEffect, ofType, Actions } from '@ngrx/effects';
import { Injectable } from '@angular/core';
import { map } from 'rxjs/operators';
import * as MyActions from './actions';

@Injectable()
export class MyEffects {
  loopEffect$ = createEffect(() => this.actions$.pipe(
    ofType(MyActions.loadData),
    map(() => MyActions.loadData())
  ));

  constructor(private actions$: Actions) {}
}

A Because the effect uses map instead of tap, causing a syntax error.

B Because the effect listens for 'loadData' and dispatches the same 'loadData' action, triggering itself repeatedly.

C Because the effect does not unsubscribe from the actions stream.

D Because the effect is missing { dispatch: false } option.

💡 Hint

Think about what happens when an effect dispatches the same action it listens for.

The effect listens for 'loadData' and dispatches 'loadData' again, causing an endless cycle of the same action triggering the effect repeatedly.

❓ state_output

advanced

What is the state after this effect runs?

Given the reducer and effect below, what will be the state after dispatching 'loadData'?

Angular

import { createReducer, on } from '@ngrx/store';
import * as MyActions from './actions';

export interface State {
  data: number[];
  loading: boolean;
}

export const initialState: State = {
  data: [],
  loading: false
};

export const myReducer = createReducer(
  initialState,
  on(MyActions.loadData, state => ({ ...state, loading: true })),
  on(MyActions.loadDataSuccess, (state, { data }) => ({ ...state, data, loading: false }))
);

// Effect
import { createEffect, ofType, Actions } from '@ngrx/effects';
import { Injectable } from '@angular/core';
import { map, delay } from 'rxjs/operators';

@Injectable()
export class MyEffects {
  loadData$ = createEffect(() => this.actions$.pipe(
    ofType(MyActions.loadData),
    delay(1000),
    map(() => MyActions.loadDataSuccess({ data: [10, 20, 30] }))
  ));

  constructor(private actions$: Actions) {}
}

A { data: [], loading: false } immediately after dispatch

B { data: [], loading: true } immediately after dispatch

C { data: [10, 20, 30], loading: true } immediately after dispatch

D { data: [10, 20, 30], loading: false } after 1 second delay

💡 Hint

Consider the timing of the effect and reducer updates.

Initially, 'loadData' sets loading to true. After 1 second, the effect dispatches 'loadDataSuccess' which updates data and sets loading to false.

🧠 Conceptual

expert

Why use effects instead of components for side effects?

In Angular with NgRx, why is it better to handle side effects like HTTP calls inside effects rather than inside components?

A Effects centralize side effects, keep components simple, and allow better testing and state management.

B Components cannot perform HTTP calls directly, so effects are required.

C Effects run on the server only, improving performance.

D Using effects avoids the need for reducers in state management.

💡 Hint

Think about separation of concerns and testability.

Effects help keep components focused on UI, centralize side effects, and improve maintainability and testing.

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