Component interaction

Pass data from parent to child with input binding
Test it for Pass data from parent to child with input binding
Intercept input property changes with a setter
Test it for Intercept input property changes with a setter
Intercept input property changes with ngOnChanges()
Test it for Intercept input property changes with ngOnChanges()
Parent listens for child event
Test it for Parent listens for child event
Parent interacts with child using local variable
Test it for Parent interacts with child using local variable
Parent calls an @ViewChild()
Test it for Parent calls an @ViewChild()
Parent and children communicate using a service
Test it for Parent and children communicate using a service

This cookbook contains recipes for common component communication scenarios in which two or more components share information.

See the live example / download example .

Pass data from parent to child with input binding

HeroChildComponent has two input properties , typically adorned with @Input() decorator.

component-interaction/src/app/hero-child.component.ts

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

import { Hero } from './hero';

@Component({
  selector: 'app-hero-child',
  template: `
    <h3>{{hero.name}} says:</h3>
    <p>I, {{hero.name}}, am at your service, {{masterName}}.</p>
  `
})
export class HeroChildComponent {
  @Input() hero!: Hero;
  @Input('master') masterName = '';
}

The second @Input aliases the child component property name masterName as 'master' .

The HeroParentComponent nests the child HeroChildComponent inside an *ngFor repeater, binding its master string property to the child's master alias, and each iteration's hero instance to the child's hero property.

component-interaction/src/app/hero-parent.component.ts

      
      import { Component } from '@angular/core';

import { HEROES } from './hero';

@Component({
  selector: 'app-hero-parent',
  template: `
    <h2>{{master}} controls {{heroes.length}} heroes</h2>

    <app-hero-child
      *ngFor="let hero of heroes"
      [hero]="hero"
      [master]="master">
    </app-hero-child>
  `
})
export class HeroParentComponent {
  heroes = HEROES;
  master = 'Master';
}

The running application displays three heroes:

Test it for Pass data from parent to child with input binding

E2E test that all children were instantiated and displayed as expected:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
const heroNames = ['Dr. IQ', 'Magneta', 'Bombasto'];
const masterName = 'Master';

it('should pass properties to children properly', async () => {
  const parent = element(by.tagName('app-hero-parent'));
  const heroes = parent.all(by.tagName('app-hero-child'));

  for (let i = 0; i < heroNames.length; i++) {
    const childTitle = await heroes.get(i).element(by.tagName('h3')).getText();
    const childDetail = await heroes.get(i).element(by.tagName('p')).getText();
    expect(childTitle).toEqual(heroNames[i] + ' says:');
    expect(childDetail).toContain(masterName);
  }
});
// ...

Intercept input property changes with a setter

Use an input property setter to intercept and act upon a value from the parent.

The setter of the name input property in the child NameChildComponent trims the whitespace from a name and replaces an empty value with default text.

component-interaction/src/app/name-child.component.ts

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

@Component({
  selector: 'app-name-child',
  template: '<h3>"{{name}}"</h3>'
})
export class NameChildComponent {
  @Input()
  get name(): string { return this._name; }
  set name(name: string) {
    this._name = (name && name.trim()) || '<no name set>';
  }
  private _name = '';
}

Here's the NameParentComponent demonstrating name variations including a name with all spaces:

component-interaction/src/app/name-parent.component.ts

      
      import { Component } from '@angular/core';

@Component({
  selector: 'app-name-parent',
  template: `
    <h2>Master controls {{names.length}} names</h2>

    <app-name-child *ngFor="let name of names" [name]="name"></app-name-child>
  `
})
export class NameParentComponent {
  // Displays 'Dr. IQ', '<no name set>', 'Bombasto'
  names = ['Dr. IQ', '   ', '  Bombasto  '];
}

Test it for Intercept input property changes with a setter

E2E tests of input property setter with empty and non-empty names:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
it('should display trimmed, non-empty names', async () => {
  const nonEmptyNameIndex = 0;
  const nonEmptyName = '"Dr. IQ"';
  const parent = element(by.tagName('app-name-parent'));
  const hero = parent.all(by.tagName('app-name-child')).get(nonEmptyNameIndex);

  const displayName = await hero.element(by.tagName('h3')).getText();
  expect(displayName).toEqual(nonEmptyName);
});

it('should replace empty name with default name', async () => {
  const emptyNameIndex = 1;
  const defaultName = '"<no name set>"';
  const parent = element(by.tagName('app-name-parent'));
  const hero = parent.all(by.tagName('app-name-child')).get(emptyNameIndex);

  const displayName = await hero.element(by.tagName('h3')).getText();
  expect(displayName).toEqual(defaultName);
});
// ...

Intercept input property changes with `ngOnChanges()`

Detect and act upon changes to input property values with the ngOnChanges() method of the OnChanges lifecycle hook interface.

You might prefer this approach to the property setter when watching multiple, interacting input properties.

Learn about ngOnChanges() in the Lifecycle Hooks chapter.

This VersionChildComponent detects changes to the major and minor input properties and composes a log message reporting these changes:

component-interaction/src/app/version-child.component.ts

      
      import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-version-child',
  template: `
    <h3>Version {{major}}.{{minor}}</h3>
    <h4>Change log:</h4>
    <ul>
      <li *ngFor="let change of changeLog">{{change}}</li>
    </ul>
  `
})
export class VersionChildComponent implements OnChanges {
  @Input() major = 0;
  @Input() minor = 0;
  changeLog: string[] = [];

  ngOnChanges(changes: SimpleChanges) {
    const log: string[] = [];
    for (const propName in changes) {
      const changedProp = changes[propName];
      const to = JSON.stringify(changedProp.currentValue);
      if (changedProp.isFirstChange()) {
        log.push(`Initial value of ${propName} set to ${to}`);
      } else {
        const from = JSON.stringify(changedProp.previousValue);
        log.push(`${propName} changed from ${from} to ${to}`);
      }
    }
    this.changeLog.push(log.join(', '));
  }
}

The VersionParentComponent supplies the minor and major values and binds buttons to methods that change them.

component-interaction/src/app/version-parent.component.ts

      
      import { Component } from '@angular/core';

@Component({
  selector: 'app-version-parent',
  template: `
    <h2>Source code version</h2>
    <button type="button" (click)="newMinor()">New minor version</button>
    <button type="button" (click)="newMajor()">New major version</button>
    <app-version-child [major]="major" [minor]="minor"></app-version-child>
  `
})
export class VersionParentComponent {
  major = 1;
  minor = 23;

  newMinor() {
    this.minor++;
  }

  newMajor() {
    this.major++;
    this.minor = 0;
  }
}

Here's the output of a button-pushing sequence:

Test it for Intercept input property changes with `ngOnChanges()`

Test that both input properties are set initially and that button clicks trigger the expected ngOnChanges calls and values:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
// Test must all execute in this exact order
it('should set expected initial values', async () => {
  const actual = await getActual();

  const initialLabel = 'Version 1.23';
  const initialLog = 'Initial value of major set to 1, Initial value of minor set to 23';

  expect(actual.label).toBe(initialLabel);
  expect(actual.count).toBe(1);
  expect(await actual.logs.get(0).getText()).toBe(initialLog);
});

it("should set expected values after clicking 'Minor' twice", async () => {
  const repoTag = element(by.tagName('app-version-parent'));
  const newMinorButton = repoTag.all(by.tagName('button')).get(0);

  await newMinorButton.click();
  await newMinorButton.click();

  const actual = await getActual();

  const labelAfter2Minor = 'Version 1.25';
  const logAfter2Minor = 'minor changed from 24 to 25';

  expect(actual.label).toBe(labelAfter2Minor);
  expect(actual.count).toBe(3);
  expect(await actual.logs.get(2).getText()).toBe(logAfter2Minor);
});

it("should set expected values after clicking 'Major' once", async () => {
  const repoTag = element(by.tagName('app-version-parent'));
  const newMajorButton = repoTag.all(by.tagName('button')).get(1);

  await newMajorButton.click();
  const actual = await getActual();

  const labelAfterMajor = 'Version 2.0';
  const logAfterMajor = 'major changed from 1 to 2, minor changed from 23 to 0';

  expect(actual.label).toBe(labelAfterMajor);
  expect(actual.count).toBe(2);
  expect(await actual.logs.get(1).getText()).toBe(logAfterMajor);
});

async function getActual() {
  const versionTag = element(by.tagName('app-version-child'));
  const label = await versionTag.element(by.tagName('h3')).getText();
  const ul = versionTag.element((by.tagName('ul')));
  const logs = ul.all(by.tagName('li'));

  return {
    label,
    logs,
    count: await logs.count(),
  };
}
// ...

Parent listens for child event

The child component exposes an EventEmitter property with which it emits events when something happens. The parent binds to that event property and reacts to those events.

The child's EventEmitter property is an output property , typically adorned with an @Output() decorator as seen in this VoterComponent :

component-interaction/src/app/voter.component.ts

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

@Component({
  selector: 'app-voter',
  template: `
    <h4>{{name}}</h4>
    <button type="button" (click)="vote(true)"  [disabled]="didVote">Agree</button>
    <button type="button" (click)="vote(false)" [disabled]="didVote">Disagree</button>
  `
})
export class VoterComponent {
  @Input()  name = '';
  @Output() voted = new EventEmitter<boolean>();
  didVote = false;

  vote(agreed: boolean) {
    this.voted.emit(agreed);
    this.didVote = true;
  }
}

Clicking a button triggers emission of a true or false , the boolean payload .

The parent VoteTakerComponent binds an event handler called onVoted() that responds to the child event payload $event and updates a counter.

component-interaction/src/app/votetaker.component.ts

      
      import { Component } from '@angular/core';

@Component({
  selector: 'app-vote-taker',
  template: `
    <h2>Should mankind colonize the Universe?</h2>
    <h3>Agree: {{agreed}}, Disagree: {{disagreed}}</h3>

    <app-voter
      *ngFor="let voter of voters"
      [name]="voter"
      (voted)="onVoted($event)">
    </app-voter>
  `
})
export class VoteTakerComponent {
  agreed = 0;
  disagreed = 0;
  voters = ['Dr. IQ', 'Celeritas', 'Bombasto'];

  onVoted(agreed: boolean) {
    if (agreed) {
      this.agreed++;
    } else {
      this.disagreed++;
    }
  }
}

The framework passes the event argument —represented by $event — to the handler method, and the method processes it:

Test it for Parent listens for child event

Test that clicking the Agree and Disagree buttons update the appropriate counters:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
it('should not emit the event initially', async () => {
  const voteLabel = element(by.tagName('app-vote-taker')).element(by.tagName('h3'));
  expect(await voteLabel.getText()).toBe('Agree: 0, Disagree: 0');
});

it('should process Agree vote', async () => {
  const voteLabel = element(by.tagName('app-vote-taker')).element(by.tagName('h3'));
  const agreeButton1 = element.all(by.tagName('app-voter')).get(0)
    .all(by.tagName('button')).get(0);

  await agreeButton1.click();

  expect(await voteLabel.getText()).toBe('Agree: 1, Disagree: 0');
});

it('should process Disagree vote', async () => {
  const voteLabel = element(by.tagName('app-vote-taker')).element(by.tagName('h3'));
  const agreeButton1 = element.all(by.tagName('app-voter')).get(1)
    .all(by.tagName('button')).get(1);

  await agreeButton1.click();

  expect(await voteLabel.getText()).toBe('Agree: 0, Disagree: 1');
});
// ...

Parent interacts with child using local variable

A parent component cannot use data binding to read child properties or invoke child methods. Do both by creating a template reference variable for the child element and then reference that variable within the parent template as seen in the following example.

The following is a child CountdownTimerComponent that repeatedly counts down to zero and launches a rocket. The start and stop methods control the clock and a countdown status message displays in its own template.

component-interaction/src/app/countdown-timer.component.ts

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

@Component({
  selector: 'app-countdown-timer',
  template: '<p>{{message}}</p>'
})
export class CountdownTimerComponent implements OnDestroy {

  intervalId = 0;
  message = '';
  seconds = 11;

  ngOnDestroy() { this.clearTimer(); }

  start() { this.countDown(); }
  stop()  {
    this.clearTimer();
    this.message = `Holding at T-${this.seconds} seconds`;
  }

  private clearTimer() { clearInterval(this.intervalId); }

  private countDown() {
    this.clearTimer();
    this.intervalId = window.setInterval(() => {
      this.seconds -= 1;
      if (this.seconds === 0) {
        this.message = 'Blast off!';
      } else {
        if (this.seconds < 0) { this.seconds = 10; } // reset
        this.message = `T-${this.seconds} seconds and counting`;
      }
    }, 1000);
  }
}

The CountdownLocalVarParentComponent that hosts the timer component is as follows:

component-interaction/src/app/countdown-parent.component.ts

      
      import { Component } from '@angular/core';
import { CountdownTimerComponent } from './countdown-timer.component';

@Component({
  selector: 'app-countdown-parent-lv',
  template: `
    <h3>Countdown to Liftoff (via local variable)</h3>
    <button type="button" (click)="timer.start()">Start</button>
    <button type="button" (click)="timer.stop()">Stop</button>
    <div class="seconds">{{timer.seconds}}</div>
    <app-countdown-timer #timer></app-countdown-timer>
  `,
  styleUrls: ['../assets/demo.css']
})
export class CountdownLocalVarParentComponent { }

The parent component cannot data bind to the child's start and stop methods nor to its seconds property.

Place a local variable, #timer , on the tag <app-countdown-timer> representing the child component. That gives you a reference to the child component and the ability to access any of its properties or methods from within the parent template.

This example wires parent buttons to the child's start and stop and uses interpolation to display the child's seconds property.

Here, the parent and child are working together.

Test it for Parent interacts with child using local variable

Test that the seconds displayed in the parent template match the seconds displayed in the child's status message. Test also that clicking the Stop button pauses the countdown timer:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
// The tests trigger periodic asynchronous operations (via `setInterval()`), which will prevent
// the app from stabilizing. See https://angular.io/api/core/ApplicationRef#is-stable-examples
// for more details.
// To allow the tests to complete, we will disable automatically waiting for the Angular app to
// stabilize.
beforeEach(() => browser.waitForAngularEnabled(false));
afterEach(() => browser.waitForAngularEnabled(true));

it('timer and parent seconds should match', async () => {
  const parent = element(by.tagName(parentTag));
  const startButton = parent.element(by.buttonText('Start'));
  const seconds = parent.element(by.className('seconds'));
  const timer = parent.element(by.tagName('app-countdown-timer'));

  await startButton.click();

  // Wait for `<app-countdown-timer>` to be populated with any text.
  await browser.wait(() => timer.getText(), 2000);

  expect(await timer.getText()).toContain(await seconds.getText());
});

it('should stop the countdown', async () => {
  const parent = element(by.tagName(parentTag));
  const startButton = parent.element(by.buttonText('Start'));
  const stopButton = parent.element(by.buttonText('Stop'));
  const timer = parent.element(by.tagName('app-countdown-timer'));

  await startButton.click();
  expect(await timer.getText()).not.toContain('Holding');

  await stopButton.click();
  expect(await timer.getText()).toContain('Holding');
});
// ...

Parent calls an `@ViewChild()`

The local variable approach is straightforward. But it is limited because the parent-child wiring must be done entirely within the parent template. The parent component itself has no access to the child.

You can't use the local variable technique if the parent component's class relies on the child component's class . The parent-child relationship of the components is not established within each component's respective class with the local variable technique. Because the class instances are not connected to one another, the parent class cannot access the child class properties and methods.

When the parent component class requires that kind of access, inject the child component into the parent as a ViewChild .

The following example illustrates this technique with the same Countdown Timer example. Neither its appearance nor its behavior changes. The child CountdownTimerComponent is the same as well.

The switch from the local variable to the ViewChild technique is solely for the purpose of demonstration.

Here is the parent, CountdownViewChildParentComponent :

component-interaction/src/app/countdown-parent.component.ts

      
      import { AfterViewInit, ViewChild } from '@angular/core';
import { Component } from '@angular/core';
import { CountdownTimerComponent } from './countdown-timer.component';

@Component({
  selector: 'app-countdown-parent-vc',
  template: `
    <h3>Countdown to Liftoff (via ViewChild)</h3>
    <button type="button" (click)="start()">Start</button>
    <button type="button" (click)="stop()">Stop</button>
    <div class="seconds">{{ seconds() }}</div>
    <app-countdown-timer></app-countdown-timer>
  `,
  styleUrls: ['../assets/demo.css']
})
export class CountdownViewChildParentComponent implements AfterViewInit {

  @ViewChild(CountdownTimerComponent)
  private timerComponent!: CountdownTimerComponent;

  seconds() { return 0; }

  ngAfterViewInit() {
    // Redefine `seconds()` to get from the `CountdownTimerComponent.seconds` ...
    // but wait a tick first to avoid one-time devMode
    // unidirectional-data-flow-violation error
    setTimeout(() => this.seconds = () => this.timerComponent.seconds, 0);
  }

  start() { this.timerComponent.start(); }
  stop() { this.timerComponent.stop(); }
}

It takes a bit more work to get the child view into the parent component class .

First, you have to import references to the ViewChild decorator and the AfterViewInit lifecycle hook.

Next, inject the child CountdownTimerComponent into the private timerComponent property using the @ViewChild property decoration.

The #timer local variable is gone from the component metadata. Instead, bind the buttons to the parent component's own start and stop methods and present the ticking seconds in an interpolation around the parent component's seconds method.

These methods access the injected timer component directly.

The ngAfterViewInit() lifecycle hook is an important wrinkle. The timer component isn't available until after Angular displays the parent view. So it displays 0 seconds initially.

Then Angular calls the ngAfterViewInit lifecycle hook at which time it is too late to update the parent view's display of the countdown seconds. Angular's unidirectional data flow rule prevents updating the parent view's in the same cycle. The application must wait one turn before it can display the seconds.

Use setTimeout() to wait one tick and then revise the seconds() method so that it takes future values from the timer component.

Test it for Parent calls an `@ViewChild()`

Use the same countdown timer tests as before.

Parent and children communicate using a service

A parent component and its children share a service whose interface enables bidirectional communication within the family .

The scope of the service instance is the parent component and its children. Components outside this component subtree have no access to the service or their communications.

This MissionService connects the MissionControlComponent to multiple AstronautComponent children.

component-interaction/src/app/mission.service.ts

      
      import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';

@Injectable()
export class MissionService {

  // Observable string sources
  private missionAnnouncedSource = new Subject<string>();
  private missionConfirmedSource = new Subject<string>();

  // Observable string streams
  missionAnnounced$ = this.missionAnnouncedSource.asObservable();
  missionConfirmed$ = this.missionConfirmedSource.asObservable();

  // Service message commands
  announceMission(mission: string) {
    this.missionAnnouncedSource.next(mission);
  }

  confirmMission(astronaut: string) {
    this.missionConfirmedSource.next(astronaut);
  }
}

The MissionControlComponent both provides the instance of the service that it shares with its children (through the providers metadata array) and injects that instance into itself through its constructor:

component-interaction/src/app/missioncontrol.component.ts

      
      import { Component } from '@angular/core';

import { MissionService } from './mission.service';

@Component({
  selector: 'app-mission-control',
  template: `
    <h2>Mission Control</h2>
    <button type="button" (click)="announce()">Announce mission</button>

    <app-astronaut
      *ngFor="let astronaut of astronauts"
      [astronaut]="astronaut">
    </app-astronaut>

    <h3>History</h3>
    <ul>
      <li *ngFor="let event of history">{{event}}</li>
    </ul>
  `,
  providers: [MissionService]
})
export class MissionControlComponent {
  astronauts = ['Lovell', 'Swigert', 'Haise'];
  history: string[] = [];
  missions = ['Fly to the moon!',
              'Fly to mars!',
              'Fly to Vegas!'];
  nextMission = 0;

  constructor(private missionService: MissionService) {
    missionService.missionConfirmed$.subscribe(
      astronaut => {
        this.history.push(`${astronaut} confirmed the mission`);
      });
  }

  announce() {
    const mission = this.missions[this.nextMission++];
    this.missionService.announceMission(mission);
    this.history.push(`Mission "${mission}" announced`);
    if (this.nextMission >= this.missions.length) { this.nextMission = 0; }
  }
}

The AstronautComponent also injects the service in its constructor. Each AstronautComponent is a child of the MissionControlComponent and therefore receives its parent's service instance:

component-interaction/src/app/astronaut.component.ts

      
      import { Component, Input, OnDestroy } from '@angular/core';

import { MissionService } from './mission.service';
import { Subscription } from 'rxjs';

@Component({
  selector: 'app-astronaut',
  template: `
    <p>
      {{astronaut}}: <strong>{{mission}}</strong>
      <button
        type="button"
        (click)="confirm()"
        [disabled]="!announced || confirmed">
        Confirm
      </button>
    </p>
  `
})
export class AstronautComponent implements OnDestroy {
  @Input() astronaut = '';
  mission = '<no mission announced>';
  confirmed = false;
  announced = false;
  subscription: Subscription;

  constructor(private missionService: MissionService) {
    this.subscription = missionService.missionAnnounced$.subscribe(
      mission => {
        this.mission = mission;
        this.announced = true;
        this.confirmed = false;
    });
  }

  confirm() {
    this.confirmed = true;
    this.missionService.confirmMission(this.astronaut);
  }

  ngOnDestroy() {
    // prevent memory leak when component destroyed
    this.subscription.unsubscribe();
  }
}

Notice that this example captures the subscription and unsubscribe() when the AstronautComponent is destroyed. This is a memory-leak guard step. There is no actual risk in this application because the lifetime of a AstronautComponent is the same as the lifetime of the application itself. That would not always be true in a more complex application.

You don't add this guard to the MissionControlComponent because, as the parent, it controls the lifetime of the MissionService .

The History log demonstrates that messages travel in both directions between the parent MissionControlComponent and the AstronautComponent children, facilitated by the service:

Test it for Parent and children communicate using a service

Tests click buttons of both the parent MissionControlComponent and the AstronautComponent children and verify that the history meets expectations:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
it('should announce a mission', async () => {
  const missionControl = element(by.tagName('app-mission-control'));
  const announceButton = missionControl.all(by.tagName('button')).get(0);
  const history = missionControl.all(by.tagName('li'));

  await announceButton.click();

  expect(await history.count()).toBe(1);
  expect(await history.get(0).getText()).toMatch(/Mission.* announced/);
});

it('should confirm the mission by Lovell', async () => {
  await testConfirmMission(1, 'Lovell');
});

it('should confirm the mission by Haise', async () => {
  await testConfirmMission(3, 'Haise');
});

it('should confirm the mission by Swigert', async () => {
  await testConfirmMission(2, 'Swigert');
});

async function testConfirmMission(buttonIndex: number, astronaut: string) {
  const missionControl = element(by.tagName('app-mission-control'));
  const announceButton = missionControl.all(by.tagName('button')).get(0);
  const confirmButton = missionControl.all(by.tagName('button')).get(buttonIndex);
  const history = missionControl.all(by.tagName('li'));

  await announceButton.click();
  await confirmButton.click();

  expect(await history.count()).toBe(2);
  expect(await history.get(1).getText()).toBe(`${astronaut} confirmed the mission`);
}
// ...

Last reviewed on Mon Feb 28 2022

Stay Ahead in Today’s Competitive Market!
Unlock your company’s full potential with a Virtual Delivery Center (VDC). Gain specialized expertise, drive seamless operations, and scale effortlessly for long-term success.

Angular components overview

Prerequisites
Creating a component
Creating a component using the Angular CLI
Creating a component manually
Specifying a component's CSS selector
Defining a component's template
Declaring a component's styles
Next steps

Components are the main building block for Angular applications. Each component consists of:

An HTML template that declares what renders on the page
A TypeScript class that defines behavior
A CSS selector that defines how the component is used in a template
Optionally, CSS styles applied to the template

This topic describes how to create and configure an Angular component.

To view or download the example code used in this topic, see the live example / download example .

Prerequisites

To create a component, verify that you have met the following prerequisites:

Install the Angular CLI.
Create an Angular workspace with initial application. If you don't have a project, create one using ng new <project-name> , where <project-name> is the name of your Angular application.

Creating a component

The best way to create a component is with the Angular CLI. You can also create a component manually.

Creating a component using the Angular CLI

To create a component using the Angular CLI:

From a terminal window, navigate to the directory containing your application.
Run the ng generate component <component-name> command, where <component-name> is the name of your new component.

By default, this command creates the following:

A directory named after the component
A component file, <component-name>.component.ts
A template file, <component-name>.component.html
A CSS file, <component-name>.component.css
A testing specification file, <component-name>.component.spec.ts

Where <component-name> is the name of your component.

You can change how ng generate component creates new components. For more information, see ng generate component in the Angular CLI documentation.

Creating a component manually

Although the Angular CLI is the best way to create an Angular component, you can also create a component manually. This section describes how to create the core component file within an existing Angular project.

To create a new component manually:

Navigate to your Angular project directory.
Create a new file, <component-name>.component.ts .

At the top of the file, add the following import statement.

      
      import { Component } from '@angular/core';

After the import statement, add a @Component decorator.

      
      @Component({
})

Choose a CSS selector for the component.

      
      @Component({
  selector: 'app-component-overview',
})

For more information on choosing a selector, see Specifying a component's selector.

Define the HTML template that the component uses to display information. In most cases, this template is a separate HTML file.
```
      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
})
    
```
For more information on defining a component's template, see Defining a component's template.

Select the styles for the component's template. In most cases, you define the styles for your component's template in a separate file.

      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
  styleUrls: ['./component-overview.component.css']
})

Add a class statement that includes the code for the component.

      
      export class ComponentOverviewComponent {

}

Specifying a component's CSS selector

Every component requires a CSS selector . A selector instructs Angular to instantiate this component wherever it finds the corresponding tag in template HTML. For example, consider a component hello-world.component.ts that defines its selector as app-hello-world . This selector instructs Angular to instantiate this component any time the tag <app-hello-world> appears in a template.

Specify a component's selector by adding a selector statement to the @Component decorator.

      
      @Component({
  selector: 'app-component-overview',
})

Defining a component's template

A template is a block of HTML that tells Angular how to render the component in your application. Define a template for your component in one of two ways: by referencing an external file, or directly within the component.

To define a template as an external file, add a templateUrl property to the @Component decorator.

      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
})

To define a template within the component, add a template property to the @Component decorator that contains the HTML you want to use.

      
      @Component({
  selector: 'app-component-overview',
  template: '<h1>Hello World!</h1>',
})

If you want your template to span multiple lines, use backticks ( ` ). For example:

      
      @Component({
  selector: 'app-component-overview',
  template: `
    <h1>Hello World!</h1>
    <p>This template definition spans multiple lines.</p>
  `
})

An Angular component requires a template defined using template or templateUrl . You cannot have both statements in a component.

Declaring a component's styles

Declare component styles used for its template in one of two ways: By referencing an external file, or directly within the component.

To declare the styles for a component in a separate file, add a styleUrls property to the @Component decorator.

      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
  styleUrls: ['./component-overview.component.css']
})

To declare the styles within the component, add a styles property to the @Component decorator that contains the styles you want to use.

      
      @Component({
  selector: 'app-component-overview',
  template: '<h1>Hello World!</h1>',
  styles: ['h1 { font-weight: normal; }']
})

The styles property takes an array of strings that contain the CSS rule declarations.

Next steps

For an architectural overview of components, see Introduction to components and templates
For additional options to use when creating a component, see Component in the API Reference
For more information on styling components, see Component styles
For more information on templates, see Template syntax

Last reviewed on Mon Feb 28 2022

Welcome to Knowledge Base!

KB at your finger tips

Angular

Angular - Component interaction

Component interaction

Pass data from parent to child with input binding

Test it for Pass data from parent to child with input binding

Intercept input property changes with a setter

Test it for Intercept input property changes with a setter

Intercept input property changes with `ngOnChanges()`

Test it for Intercept input property changes with `ngOnChanges()`

Parent listens for child event

Test it for Parent listens for child event

Parent interacts with child using local variable

Test it for Parent interacts with child using local variable

Parent calls an `@ViewChild()`

Test it for Parent calls an `@ViewChild()`

Parent and children communicate using a service

Test it for Parent and children communicate using a service

Angular

Angular

Angular

Angular

Angular - Angular components overview

Angular components overview

Prerequisites

Creating a component

Creating a component using the Angular CLI

Creating a component manually

Specifying a component's CSS selector

Defining a component's template

Declaring a component's styles

Next steps

Thank you for subscribing to AiDOOS! Stay connected for updates!

Welcome to Knowledge Base!

KB at your finger tips

Angular

Angular - Component interaction

Component interaction link

Pass data from parent to child with input binding link

Test it for Pass data from parent to child with input binding link

Intercept input property changes with a setter link

Test it for Intercept input property changes with a setter link

Intercept input property changes with ngOnChanges() link

Test it for Intercept input property changes with ngOnChanges() link

Parent listens for child event link

Test it for Parent listens for child event link

Parent interacts with child using local variable link

Test it for Parent interacts with child using local variable link

Parent calls an @ViewChild() link

Test it for Parent calls an @ViewChild() link

Parent and children communicate using a service link

Test it for Parent and children communicate using a service link

Angular

link

Angular

link

Angular

link

Angular

link

Angular - Angular components overview

Angular components overview link

Prerequisites link

Creating a component link

Creating a component using the Angular CLI link

Creating a component manually link

Specifying a component's CSS selector link

Defining a component's template link

Declaring a component's styles link

Next steps link

Component interaction

Pass data from parent to child with input binding

Test it for Pass data from parent to child with input binding

Intercept input property changes with a setter

Test it for Intercept input property changes with a setter

Intercept input property changes with `ngOnChanges()`

Test it for Intercept input property changes with `ngOnChanges()`

Parent listens for child event

Test it for Parent listens for child event

Parent interacts with child using local variable

Test it for Parent interacts with child using local variable

Parent calls an `@ViewChild()`

Test it for Parent calls an `@ViewChild()`

Parent and children communicate using a service

Test it for Parent and children communicate using a service

Angular components overview

Prerequisites

Creating a component

Creating a component using the Angular CLI

Creating a component manually

Specifying a component's CSS selector

Defining a component's template

Declaring a component's styles

Next steps