@Output decorator with EventEmitter in Angular - Performance & Optimization

Performance: @Output decorator with EventEmitter

MEDIUM IMPACT

This affects interaction responsiveness and event handling efficiency between Angular components.

Communicating data from child to parent component using @Output and EventEmitter

Angular

import { Component, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'child-comp',
  template: `<button (click)="emitEvent()">Click</button>`
})
export class ChildComponent {
  @Output() dataChange = new EventEmitter<number>();
  private lastEmittedTime = 0;

  emitEvent() {
    const now = Date.now();
    if (now - this.lastEmittedTime > 1000) { // throttle events
      this.dataChange.emit(now);
      this.lastEmittedTime = now;
    }
  }
}

Throttling event emissions reduces unnecessary change detection and event handling, improving responsiveness.

📈 Performance GainReduces event emissions by limiting frequency, lowering CPU usage and improving INP.

Communicating data from child to parent component using @Output and EventEmitter

Angular

import { Component, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'child-comp',
  template: `<button (click)="emitEvent()">Click</button>`
})
export class ChildComponent {
  @Output() dataChange = new EventEmitter<any>();

  emitEvent() {
    // Emitting event with new object every click
    this.dataChange.emit({ time: Date.now() });
  }
}

Emitting a new object on every click causes Angular to run change detection and update parent unnecessarily, increasing CPU usage.

📉 Performance CostTriggers multiple change detection cycles and event handler executions per click, increasing INP latency.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Emit event every click with new object	Multiple parent updates	Multiple reflows per event	High paint cost due to frequent updates	[X] Bad
Throttle event emissions to reduce frequency	Fewer parent updates	Single or fewer reflows	Lower paint cost	[OK] Good

Rendering Pipeline

When @Output emits an event, Angular triggers change detection in parent components, which may cause DOM updates and re-rendering if data changes.

→Change Detection

→Layout

→Paint

⚠️ BottleneckChange Detection stage is most expensive due to event-triggered checks and potential DOM updates.

Core Web Vital Affected

INP

This affects interaction responsiveness and event handling efficiency between Angular components.

Optimization Tips

1Avoid emitting new object references on every event to prevent unnecessary change detection.

2Throttle or debounce event emissions to improve interaction responsiveness.

3Use Angular's OnPush change detection strategy in parent components to reduce re-rendering.

Performance Quiz - 3 Questions

What is a common performance issue when using @Output with EventEmitter in Angular?

A Emitting events too frequently causing excessive change detection

B Not using @Input to pass data to child components

C Using EventEmitter without importing it

D Declaring @Output without a selector

DevTools: Performance

How to check: Record a performance profile while interacting with the child component button. Look for frequent change detection cycles and event handler calls.

What to look for: High number of change detection invocations and long scripting times indicate inefficient event emissions.

Practice

1. What is the main purpose of the @Output decorator in Angular?

easy

A. To allow a child component to send events to its parent component

B. To receive data from a parent component

C. To style the component's template

D. To import external modules into the component

Solution

1. Step 1: Understand the role of @Output

   The @Output decorator is used in Angular to create a custom event that a child component can emit to notify its parent component.

2. Step 2: Differentiate from other decorators

   Unlike @Input which receives data, @Output sends events upward. It does not style or import modules.

3. Final Answer:

   To allow a child component to send events to its parent component -> Option A

4. Quick Check:

   @Output sends events from child to parent [OK]

Hint: Remember: @Output sends events up from child to parent [OK]

Common Mistakes:

• Confusing @Output with @Input
• Thinking @Output styles components
• Assuming @Output imports modules

2. Which of the following is the correct syntax to declare an @Output property named notify using EventEmitter in Angular?

easy

A. @Output() notify = new EventEmitter;

B. @Output notify = EventEmitter<string>();

C. @Output() notify = new EventEmitter<string>();

D. @Output() notify: EventEmitter<string> = EventEmitter();

Solution

1. Step 1: Recall correct EventEmitter syntax

   The correct way is to use @Output() decorator with a property initialized as new EventEmitter<type>().

2. Step 2: Check each option

   @Output() notify = new EventEmitter<string>(); correctly uses parentheses after @Output and calls new EventEmitter<string>(). Others miss parentheses or use wrong initialization.

3. Final Answer:

   @Output() notify = new EventEmitter<string>(); -> Option C

4. Quick Check:

   Use @Output() with new EventEmitter() [OK]

Hint: Always use parentheses: @Output() and new EventEmitter() [OK]

Common Mistakes:

• Omitting parentheses after @Output
• Not using 'new' keyword with EventEmitter
• Incorrect property type declaration

3. Given this child component code:

import { Component, Output, EventEmitter } from '@angular/core';

@Component({ selector: 'child-comp', template: `Click` })
export class ChildComponent {
  @Output() alertEvent = new EventEmitter<string>();

  sendAlert() {
    this.alertEvent.emit('Hello from child');
  }
}

What happens when the button is clicked in the parent template listening as <child-comp (alertEvent)="showMessage($event)"></child-comp>?

medium

A. The child component reloads

B. Nothing happens because alertEvent is private

C. An error occurs because emit is not a function

D. The parent method showMessage is called with 'Hello from child' as argument

Solution

1. Step 1: Understand EventEmitter usage

   The child component emits an event named alertEvent with the string 'Hello from child'.

2. Step 2: Parent listens to event

   The parent template listens to alertEvent and calls showMessage passing the emitted string as $event.

3. Final Answer:

   The parent method showMessage is called with 'Hello from child' as argument -> Option D

4. Quick Check:

   Child emits event, parent handles it [OK]

Hint: Child emits event, parent listens with (eventName) [OK]

Common Mistakes:

• Thinking emit is private or inaccessible
• Assuming child reloads on emit
• Confusing emit with method call error

4. Identify the error in this child component code:

import { Component, Output, EventEmitter } from '@angular/core';

@Component({ selector: 'child-comp', template: `Notify` })
export class ChildComponent {
  @Output notify = new EventEmitter<string>();

  notifyParent() {
    this.notify.emit('Data sent');
  }
}

medium

A. EventEmitter should not have a type parameter

B. Missing parentheses after @Output decorator

C. Method notifyParent should return a value

D. Template syntax for click event is incorrect

Solution

1. Step 1: Check @Output decorator syntax

   The @Output decorator must have parentheses: @Output(). Missing parentheses causes a syntax error.

2. Step 2: Verify other parts

   EventEmitter can have a type parameter, method does not need to return, and template click syntax is correct.

3. Final Answer:

   Missing parentheses after @Output decorator -> Option B

4. Quick Check:

   @Output() needs parentheses [OK]

Hint: Always write @Output() with parentheses [OK]

Common Mistakes:

• Omitting parentheses after @Output
• Thinking EventEmitter cannot have type
• Expecting method to return value unnecessarily

5. You want a child component to emit an event only when a number input is positive. Which code correctly implements this using @Output and EventEmitter?

hard

A. import { Component, Output, EventEmitter } from '@angular/core';
@Component({ selector: 'child-comp', template: `Send` })
export class ChildComponent {
@Output() positiveNumber = new EventEmitter<number>();
numberInput = 0;

checkAndEmit() {
if (this.numberInput > 0) {
this.positiveNumber.emit(this.numberInput);
}
}
}

B. import { Component, Output, EventEmitter } from '@angular/core';
@Component({ selector: 'child-comp', template: `Send` })
export class ChildComponent {
@Output() positiveNumber = new EventEmitter<number>();
numberInput = 0;

emitNumber() {
this.positiveNumber.emit(this.numberInput);
}
}

C. import { Component, Output, EventEmitter } from '@angular/core';
@Component({ selector: 'child-comp', template: `Send` })
export class ChildComponent {
@Output() positiveNumber = new EventEmitter<number>();
numberInput = 0;

send() {
if (this.numberInput >= 0) {
this.positiveNumber.emit(this.numberInput);
}
}
}

D. import { Component, Output, EventEmitter } from '@angular/core';
@Component({ selector: 'child-comp', template: `Send` })
export class ChildComponent {
@Output() positiveNumber = new EventEmitter<number>();
numberInput = 0;

sendPositive() {
if (this.numberInput < 0) {
this.positiveNumber.emit(this.numberInput);
}
}
}

Solution

1. Step 1: Understand the condition for emitting

   The event should emit only if the number input is positive, meaning greater than zero.

2. Step 2: Check each option's condition

   import { Component, Output, EventEmitter } from '@angular/core';
   @Component({ selector: 'child-comp', template: `Send` })
   export class ChildComponent {
     @Output() positiveNumber = new EventEmitter<number>();
     numberInput = 0;
   
     checkAndEmit() {
       if (this.numberInput > 0) {
         this.positiveNumber.emit(this.numberInput);
       }
     }
   }

   emits only if numberInput > 0, which matches the requirement.

   import { Component, Output, EventEmitter } from '@angular/core';
   @Component({ selector: 'child-comp', template: `Send` })
   export class ChildComponent {
     @Output() positiveNumber = new EventEmitter<number>();
     numberInput = 0;
   
     emitNumber() {
       this.positiveNumber.emit(this.numberInput);
     }
   }

   emits unconditionally.

   import { Component, Output, EventEmitter } from '@angular/core';
   @Component({ selector: 'child-comp', template: `Send` })
   export class ChildComponent {
     @Output() positiveNumber = new EventEmitter<number>();
     numberInput = 0;
   
     send() {
       if (this.numberInput >= 0) {
         this.positiveNumber.emit(this.numberInput);
       }
     }
   }

   emits if number is zero or positive (>=0), which is incorrect.

   import { Component, Output, EventEmitter } from '@angular/core';
   @Component({ selector: 'child-comp', template: `Send` })
   export class ChildComponent {
     @Output() positiveNumber = new EventEmitter<number>();
     numberInput = 0;
   
     sendPositive() {
       if (this.numberInput < 0) {
         this.positiveNumber.emit(this.numberInput);
       }
     }
   }

   emits if number is negative (<0), which is wrong.

3. Final Answer:

   Option A correctly emits only when numberInput is positive -> Option A

4. Quick Check:

   Emit only if numberInput > 0 [OK]

Hint: Emit event only inside if (numberInput > 0) block [OK]

Common Mistakes:

• Emitting event without condition
• Using >= 0 instead of > 0
• Emitting on negative numbers