@Output decorator with EventEmitter in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the EventEmitter class.

Angular

import { Component, [1] } from '@angular/core';

Drag options to blanks, or click blank then click option'

AInjectable

BInput

CNgModule

DEventEmitter

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Input instead of EventEmitter

Forgetting to import EventEmitter

Importing NgModule which is unrelated here

The EventEmitter class is imported from @angular/core to create custom events.

2fill in blank

medium

Complete the code to declare an output property named 'notify' using the @Output decorator.

Angular

@Output() [1] = new EventEmitter<string>();

Drag options to blanks, or click blank then click option'

Ainput

Bnotify

Cevent

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Input instead of @Output

Naming the property incorrectly

Not initializing with new EventEmitter()

The output property must be named notify to match the event name.

3fill in blank

hard

Fix the error in emitting the event with the message 'Hello'.

Angular

this.notify.[1]('Hello');

Drag options to blanks, or click blank then click option'

Atrigger

Bsend

Cemit

Ddispatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using send or trigger which are not EventEmitter methods

Forgetting parentheses after the method name

The emit method is used to send the event to the parent component.

4fill in blank

hard

Fill both blanks to declare an output property 'update' that emits numbers.

Angular

@Output() [1] = new [2]<number>();

Drag options to blanks, or click blank then click option'

Aupdate

Bnotify

CEventEmitter

DInput

Attempts:

3 left

💡 Hint

Common Mistakes

Using Input instead of EventEmitter

Naming the property incorrectly

Not specifying the generic type

The property name is update and it must be an instance of EventEmitter with a number type.

5fill in blank

hard

Fill all three blanks to emit a boolean value 'true' from the output property 'status'.

Angular

this.[1].[2]([3]);

Drag options to blanks, or click blank then click option'

Astatus

Bemit

Ctrue

Dnotify

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'notify' instead of 'status' for the property

Using 'send' or other methods instead of 'emit'

Not passing a boolean value

The output property is status, the method to send events is emit, and the value emitted is true.

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