To create a component we add @Component decorator in a class passing some parameters:

• providers: Resources that will be injected into the component constructor
• selector: The query selector that will find the element in the HTML and replace by the component
• styles: Inline styles. NOTE: DO NOT use this parameter with require, it works on development but when you build the application in production all your styles are lost
• styleUrls: Array of path to style files
• template: String that contains your HTML
• templateUrl: Path to a HTML file

There are other parameters you can configure, but the listed ones are what you will use the most.

A simple example:

import { Component } from '@angular/core';
 
@Component({
  selector: 'app-required',
  styleUrls: ['required.component.scss'],
  // template: `This field is required.`,
  templateUrl: 'required.component.html',
})
export class RequiredComponent { }

Templates are HTML files that may contain logic.

You can specify a template in two ways:

Passing template as a file path

@Component({
  templateUrl: 'hero.component.html',
})

Passing a template as an inline code

@Component({
  template: `<div>My template here</div>`,
})

Templates may contain styles. The styles declared in @Component are different from your application style file, anything applied in the component will be restricted to this scope. For example, say you add:

div { background: red; }

All divs inside the component will be red, but if you have other components, other divs in your HTML they will not be changed at all.

The generated code will look like this:

You can add styles to a component in two ways:

Passing an array of file paths

@Component({
  styleUrls: ['hero.component.css'],
})

Passing an array of inline codes

styles: [ `div { background: lime; }` ]

You shouldn't use styles with require as it will not work when you build your application to production.

hero.component.html

<form (ngSubmit)="submit($event)" [formGroup]="form" novalidate>
  <input type="text" formControlName="name" />
  <button type="submit">Show hero name</button>
</form>

hero.component.ts

import { FormControl, FormGroup, Validators } from '@angular/forms';

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

@Component({
  selector: 'app-hero',
  templateUrl: 'hero.component.html',
})
export class HeroComponent {
  public form = new FormGroup({
    name: new FormControl('', Validators.required),
  });

  submit(event) {
    console.log(event);
    console.log(this.form.controls.name.value);
  }
}

hero.component.spec.ts

import { ComponentFixture, TestBed, async } from '@angular/core/testing';

import { HeroComponent } from './hero.component';
import { ReactiveFormsModule } from '@angular/forms';

describe('HeroComponent', () => {
  let component: HeroComponent;
  let fixture: ComponentFixture<HeroComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [HeroComponent],
      imports: [ReactiveFormsModule],
    }).compileComponents();

    fixture = TestBed.createComponent(HeroComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  }));

  it('should be created', () => {
    expect(component).toBeTruthy();
  });

  it('should log hero name in the console when user submit form', async(() => {
    const heroName = 'Saitama';
    const element = <HTMLFormElement>fixture.debugElement.nativeElement.querySelector('form');

    spyOn(console, 'log').and.callThrough();

    component.form.controls['name'].setValue(heroName);

    element.querySelector('button').click();

    fixture.whenStable().then(() => {
      fixture.detectChanges();
      expect(console.log).toHaveBeenCalledWith(heroName);
    });
  }));

  it('should validate name field as required', () => {
    component.form.controls['name'].setValue('');
    expect(component.form.invalid).toBeTruthy();
  });
});

Components will render in their respective selector, so you can use that to nest components.

If you have a component that shows a message:

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

@Component({
  selector: 'app-required',
  template: `{{name}} is required.`
})
export class RequiredComponent {
  @Input()
  public name: String = '';
}

You can use it inside another component using app-required (this component's selector):

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

@Component({
  selector: 'app-sample',
  template: `
    <input type="text" name="heroName" />
    <app-required name="Hero Name"></app-required>
  `
})
export class RequiredComponent {
  @Input()
  public name: String = '';
}