Effect for side effects in Angular - Performance & Optimization

Performance: Effect for side effects

MEDIUM IMPACT

This affects how and when side effects run in Angular components, impacting interaction responsiveness and rendering stability.

Running side effects in Angular components

Angular

import { Component, effect, signal } from '@angular/core';
@Component({ selector: 'app-example', template: `<p>{{count()}}</p>` })
export class ExampleComponent {
  count = signal(0);
  constructor() {
    effect(() => {
      const current = this.count();
      document.body.style.backgroundColor = current % 2 === 0 ? 'lightblue' : 'lightgreen';
    });
    setInterval(() => this.count.update(c => c + 1), 1000);
  }
}

Using Angular's effect with signals schedules side effects reactively and avoids blocking rendering, improving responsiveness.

📈 Performance GainSingle reflow per update, non-blocking rendering, and smoother user interaction.

Running side effects in Angular components

Angular

import { Component, OnInit } from '@angular/core';
@Component({ selector: 'app-example', template: `<p>{{count}}</p>` })
export class ExampleComponent implements OnInit {
  count = 0;
  ngOnInit() {
    setInterval(() => {
      this.count++;
      // Direct DOM manipulation or heavy logic here
      document.body.style.backgroundColor = this.count % 2 === 0 ? 'lightblue' : 'lightgreen';
    }, 1000);
  }
}

Direct DOM manipulation inside ngOnInit causes layout thrashing and blocks Angular's rendering cycle, leading to jank and poor input responsiveness.

📉 Performance CostTriggers multiple reflows every second and blocks rendering during each update.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Direct DOM manipulation in ngOnInit	Multiple direct writes	Multiple reflows per interval	High paint cost due to style thrashing	[X] Bad
Angular effect with signals	Reactive updates only	Single reflow per update	Lower paint cost with batched updates	[OK] Good

Rendering Pipeline

Angular's effect runs side effects after reactive state changes, allowing the browser to batch style and layout updates efficiently.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckLayout stage due to style changes triggered by side effects

Core Web Vital Affected

INP

This affects how and when side effects run in Angular components, impacting interaction responsiveness and rendering stability.

Optimization Tips

1Use Angular's effect to run side effects reactively and avoid blocking rendering.

2Avoid direct DOM manipulation inside lifecycle hooks to prevent layout thrashing.

3Monitor layout recalculations in DevTools to detect costly side effects.

Performance Quiz - 3 Questions

What is the main performance benefit of using Angular's effect for side effects instead of direct DOM manipulation in lifecycle hooks?

A It increases bundle size but improves style calculation speed.

B It schedules side effects reactively, reducing blocking and improving input responsiveness.

C It delays all rendering until all effects complete.

D It eliminates the need for CSS styles.

DevTools: Performance

How to check: Record a performance profile while interacting with the component or waiting for updates. Look for long tasks and layout thrashing in the flame chart.

What to look for: Check for repeated layout recalculations and long scripting tasks indicating blocking side effects.

Practice

1. What is the main purpose of an Effect in Angular?

easy

A. To style components with CSS dynamically

B. To define the main UI layout of a component

C. To handle user input events directly in the template

D. To run side tasks like data loading or logging when app state changes

Solution

1. Step 1: Understand what side effects mean in Angular

   Side effects are extra tasks like fetching data or logging that happen outside the main app logic.

2. Step 2: Identify the role of Effects

   Effects run these side tasks automatically when app state changes, keeping main logic clean.

3. Final Answer:

   To run side tasks like data loading or logging when app state changes -> Option D

4. Quick Check:

   Effect = side tasks on state change [OK]

Hint: Effects run extra tasks when app data changes [OK]

Common Mistakes:

• Thinking Effects handle UI layout
• Confusing Effects with event handlers
• Believing Effects style components

2. Which of the following is the correct way to create an Effect in Angular using RxJS operators?

easy

A. createEffect(() => this.actions$.subscribe(action => console.log(action)))

B. createEffect(() => this.actions$.map(action => action.type))

C. createEffect(() => this.actions$.pipe(ofType(loadData), tap(() => console.log('Loading'))))

D. createEffect(() => this.actions$.filter(action => action.type === 'loadData'))

Solution

1. Step 1: Recall the correct RxJS operators for Effects

   Effects use pipe with operators like ofType to filter actions and tap for side effects.

2. Step 2: Check each option's syntax

   createEffect(() => this.actions$.pipe(ofType(loadData), tap(() => console.log('Loading')))) correctly uses pipe, ofType, and tap. Others misuse operators or subscribe directly, which is incorrect inside Effects.

3. Final Answer:

   createEffect(() => this.actions$.pipe(ofType(loadData), tap(() => console.log('Loading')))) -> Option C

4. Quick Check:

   Effect uses pipe + ofType + tap [OK]

Hint: Use pipe with ofType and tap inside createEffect [OK]

Common Mistakes:

• Using subscribe inside createEffect
• Using map instead of tap for side effects
• Using filter without ofType

3. Given this Effect code snippet:

loadData$ = createEffect(() => this.actions$.pipe(
  ofType('LOAD_DATA'),
  tap(() => console.log('Data loading started'))
), { dispatch: false });

What will happen when the 'LOAD_DATA' action is dispatched?

medium

A. The message 'Data loading started' is logged, and no new action is dispatched

B. The message is logged and a new action is dispatched automatically

C. Nothing happens because dispatch is false

D. An error occurs because tap cannot be used here

Solution

1. Step 1: Understand the effect's dispatch option

   Setting dispatch: false means this Effect does not send out new actions after running.

2. Step 2: Analyze the tap operator

   The tap operator runs side code like logging but does not change or dispatch actions.

3. Final Answer:

   The message 'Data loading started' is logged, and no new action is dispatched -> Option A

4. Quick Check:

   dispatch false means no new action, tap logs side effect [OK]

Hint: dispatch: false means no new action dispatched [OK]

Common Mistakes:

• Assuming tap dispatches actions
• Thinking dispatch: false disables effect
• Confusing tap with map or switchMap

4. Identify the error in this Effect code:

saveData$ = createEffect(() => this.actions$.pipe(
  ofType('SAVE_DATA'),
  map(() => this.api.save()),
  tap(() => console.log('Save triggered'))
));

medium

A. Using map without returning an action causes an error

B. tap cannot be used after map

C. ofType should be replaced with filter

D. createEffect must not use arrow functions

Solution

1. Step 1: Check the map operator usage

   map must return a new action object for dispatching, but this.api.save() likely returns a Promise or void, not an action.

2. Step 2: Understand effect dispatch requirements

   Effects expect actions to be returned for dispatch unless dispatch: false is set, which is missing here.

3. Final Answer:

   Using map without returning an action causes an error -> Option A

4. Quick Check:

   map must return action for dispatch [OK]

Hint: map must return an action unless dispatch: false [OK]

Common Mistakes:

• Ignoring missing action return in map
• Thinking tap cannot follow map
• Confusing ofType with filter

5. You want to create an Effect that listens for a 'LOGIN' action, calls an async login API, and then dispatches either 'LOGIN_SUCCESS' or 'LOGIN_FAILURE' based on the result. Which code snippet correctly implements this?

hard

A. login$ = createEffect(() => this.actions$.pipe(
ofType('LOGIN'),
tap(() => this.authService.login()),
map(() => ({ type: 'LOGIN_SUCCESS' }))
));

B. login$ = createEffect(() => this.actions$.pipe(
ofType('LOGIN'),
switchMap(action => this.authService.login(action.credentials).pipe(
map(user => ({ type: 'LOGIN_SUCCESS', user })),
catchError(() => of({ type: 'LOGIN_FAILURE' }))
))
));

C. login$ = createEffect(() => this.actions$.pipe(
filter(action => action.type === 'LOGIN'),
map(() => this.authService.login()),
map(user => ({ type: 'LOGIN_SUCCESS', user }))
));

D. login$ = createEffect(() => this.actions$.pipe(
ofType('LOGIN'),
map(action => this.authService.login(action.credentials)),
map(user => ({ type: 'LOGIN_SUCCESS', user }))
));

Solution

1. Step 1: Identify async handling with switchMap

   Using switchMap allows calling the async login API and switching to its result stream.

2. Step 2: Handle success and error properly

   Inside switchMap, map creates a success action, and catchError returns a failure action wrapped in of to keep the stream alive.

3. Final Answer:

   login$ = createEffect(() => this.actions$.pipe( ofType('LOGIN'), switchMap(action => this.authService.login(action.credentials).pipe( map(user => ({ type: 'LOGIN_SUCCESS', user })), catchError(() => of({ type: 'LOGIN_FAILURE' })) )) )); -> Option B

4. Quick Check:

   Use switchMap + map + catchError for async effects [OK]

Hint: Use switchMap with map and catchError for async API calls [OK]

Common Mistakes:

• Using tap instead of switchMap for async calls
• Not handling errors with catchError
• Returning promises instead of observables