6. What are directives?
Directives add behaviour to an existing DOM element or an existing component
instance.
```typescript
import { Directive, ElementRef, Input } from '@angular/core';
@Directive({ selector: '[myHighlight]' })
export class HighlightDirective {
constructor(el: ElementRef) {
el.nativeElement.style.backgroundColor = 'yellow';
}
}
```
Now this directive extends HTML element behavior with a yellow background
as below
```html
<p myHighlight>Highlight me!</p>
```
7. What are components?
Components are the most basic UI building block of an Angular app which formed
a tree of Angular components. These components are subset of directives. Unlike
directives, components always have a template and only one component can be
instantiated per an element in a template.
Let's see a simple example of Angular component
```typescript
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
template: ` <div>
<h1>{{title}}</h1>
<div>Learn Angular6 with examples</div>
</div> `,
})
export class AppComponent {
title: string = 'Welcome to Angular world';
}
```
8. What are the differences between Component and Directive?
In a short note, A component(@component) is a directive-with-a-template.
Some of the major differences are mentioned in a tabular form
| Component | Directive |
|---- | ---------
| To register a component we use @Component meta-data annotation | To register directives we use @Directive meta-data
annotation |
| Components are typically used to create UI widgets| Directive is used
to add behavior to an existing DOM element |
| Component is used to break up the application into smaller components|
Directive is use to design re-usable components|
| Only one component can be present per DOM element | Many directives can
be used per DOM element |
| @View decorator or templateurl/template are mandatory | Directive doesn't
use View|
9. What is a template?
A template is a HTML view where you can display data by binding controls to
properties of an Angular component. You can store your component's template in
one of two places. You can define it inline using the template property, or you
can define the template in a separate HTML file and link to it in the component
metadata using the @Component decorator's templateUrl property.
**Using inline template with template syntax,**
```typescript
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
template: '
<div>
<h1>{{title}}</h1>
<div>Learn Angular</div>
</div>
'
})
export class AppComponent {
title: string = 'Hello World';
}
```
**Using separate template file such as app.component.html**
```typescript
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
templateUrl: 'app/app.component.html'
})
export class AppComponent {
title: string = 'Hello World';
}
```
10. What is a module?
Modules are logical boundaries in your application and the application is
divided into separate modules to separate the functionality of your application.
Lets take an example of **app.module.ts** root module declared with
**@NgModule** decorator as below,
```typescript
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
@NgModule ({
imports: [ BrowserModule ],
declarations: [ AppComponent ],
bootstrap: [ AppComponent ]
})
export class AppModule { }
```
The NgModule decorator has three options
1. The imports option is used to import other dependent modules.
The BrowserModule is required by default for any web based angular application
2. The declarations option is used to define components in the respective
module
3. The bootstrap option tells Angular which Component to bootstrap in the
application
11. What are lifecycle hooks available?
Angular application goes through an entire set of processes or has a lifecycle right from its initiation to the end of the application.
The representation of lifecycle in pictorial representation as follows,
The description of each lifecycle method is as below,
1. **ngOnChanges:** When the value of a data bound property changes, then this method is called.
2. **ngOnInit:** This is called whenever the initialization of the directive/component after Angular first displays the data-bound properties happens.
3. **ngDoCheck:** This is for the detection and to act on changes that Angular can't or won't detect on its own.
4. **ngAfterContentInit:** This is called in response after Angular projects external content into the component's view.
5. **ngAfterContentChecked:** This is called in response after Angular checks the content projected into the component.
6. **ngAfterViewInit:** This is called in response after Angular initializes the component's views and child views.
7. **ngAfterViewChecked:** This is called in response after Angular checks the component's views and child views.
8. **ngOnDestroy:** This is the cleanup phase just before Angular destroys
the directive/component.
12. What is a data binding?
Data binding is a core concept in Angular and allows to define communication
between a component and the DOM, making it very easy to define interactive
applications without worrying about pushing and pulling data. There are four
forms of data binding(divided as 3 categories) which differ in the way the data is flowing.
1. **From the Component to the DOM:**
**Interpolation:** {{ value }}: Adds the value of a property from the
component
```html
<li>Name: {{ user.name }}</li>
<li>Address: {{ user.address }}</li>
```
**Property binding:** [property]=”value”: The value is passed from the
component to the specified property or simple HTML attribute
```html
<input type="email" [value]="user.email">
```
2. **From the DOM to the Component:**
**Event binding: (event)=”function”:** When a specific DOM event happens
(eg.: click, change, keyup), call the specified method in the component
```html
<button (click)="logout()"></button>
```
3. **Two-way binding:**
**Two-way data binding:** [(ngModel)]=”value”: Two-way data binding allows to have the data flow both ways. For example, in the
below code snippet, both the email DOM input and component email property
are in sync
```html
<input type="email" [(ngModel)]="user.email">
```
13. What is metadata?
Metadata is used to decorate a class so that it can configure the expected behavior of the class. The metadata is represented by decorators
1. **Class decorators**, e.g. @Component and @NgModule
```typescript
import { NgModule, Component } from '@angular/core';
@Component({
selector: 'my-component',
template: '<div>Class decorator</div>',
})
export class MyComponent {
constructor() {
console.log('Hey I am a component!');
}
}
@NgModule({
imports: [],
declarations: [],
})
export class MyModule {
constructor() {
console.log('Hey I am a module!');
}
}
```
2. **Property decorators** Used for properties inside classes, e.g. @Input and @Output
```typescript
import { Component, Input } from '@angular/core';
@Component({
selector: 'my-component',
template: '<div>Property decorator</div>'
})
export class MyComponent {
@Input()
title: string;
}
```
3. **Method decorators** Used for methods inside classes, e.g. @HostListener
```typescript
import { Component, HostListener } from '@angular/core';
@Component({
selector: 'my-component',
template: '<div>Method decorator</div>'
})
export class MyComponent {
@HostListener('click', ['$event'])
onHostClick(event: Event) {
// clicked, `event` available
}
}
```
4. **Parameter decorators** Used for parameters inside class constructors,
e.g. @Inject
```typescript
import { Component, Inject } from '@angular/core';
import { MyService } from './my-service';
@Component({
selector: 'my-component',
template: '<div>Parameter decorator</div>'
})
export class MyComponent {
constructor(@Inject(MyService) myService) {
console.log(myService); // MyService
}
}
```
14. What is angular CLI?
Angular CLI(**Command Line Interface**) is a command line interface to
scaffold and build angular apps using nodejs style (commonJs) modules.
You need to install using below npm command,
```
npm install @angular/cli@latest
```
Below are the list of few commands, which will come handy while creating
angular projects
1. **Creating New Project:** ng new <project-name>
2. **Generating Components, Directives & Services:** ng generate/g
<feature-name>
The different types of commands would be,
* ng generate class my-new-class: add a class to your application
* ng generate component my-new-component: add a component to your
application
* ng generate directive my-new-directive: add a directive to your
application
* ng generate enum my-new-enum: add an enum to your application
* ng generate module my-new-module: add a module to your application
* ng generate pipe my-new-pipe: add a pipe to your application
* ng generate service my-new-service: add a service to your application
3. **Running the Project:** ng serve
15. What is the difference between constructor and ngOnInit?
TypeScript classes has a default method called constructor which is
normally used for the initialization purpose. Whereas ngOnInit method is
specific to Angular, especially used to define Angular bindings. Even though
constructor getting called first, it is preferred to move all of your Angular
bindings to ngOnInit method.
In order to use ngOnInit, you need to implement OnInit interface as below,
```typescript
export class App implements OnInit{
constructor(){
//called first time before the ngOnInit()
}
ngOnInit(){
//called after the constructor and called after the first ngOnChanges()
}
}
```
16. What is a service?
A service is used when a common functionality needs to be provided to
various modules. Services allow for greater separation of concerns for your
application and better modularity by allowing you to extract common
functionality out of components.
Let's create a repoService which can be used across components,
```typescript
import { Injectable } from '@angular/core';
import { Http } from '@angular/http';
@Injectable({ // The Injectable decorator is required for dependency
injection to work
// providedIn option registers the service with a specific NgModule
providedIn: 'root', // This declares the service with the root app
(AppModule)
})
export class RepoService{
constructor(private http: Http){
}
fetchAll(){
return this.http.get('https://api.github.com/repositories');
}
}
```
The above service uses Http service as a dependency.
17. What is dependency injection in Angular?
Dependency injection (DI), is an important application design pattern in
which a class asks for dependencies from external sources rather than creating
them itself. Angular comes with its own dependency injection framework for
resolving dependencies( services or objects that a class needs to perform its
function).So you can have your services depend on other services throughout your
application.
18. How is Dependency Hierarchy formed?
19. What is the purpose of async pipe?
The AsyncPipe subscribes to an observable or promise and returns the latest
value it has emitted. When a new value is emitted, the pipe marks the component
to be checked for changes.
Let's take a time observable which continuously updates the view for every 2
seconds with the current time.
```typescript
@Component({
selector: 'async-observable-pipe',
template: `<div><code>observable|async</code>:
Time: {{ time | async }}</div>`
})
export class AsyncObservablePipeComponent {
time = new Observable(observer =>
setInterval(() => observer.next(new Date().toString()), 2000)
);
}
```
20. What is the option to choose between inline and external template file?
You can store your component's template in one of two places. You can define
it inline using the **template** property, or you can define the template in a
separate HTML file and link to it in the component metadata using the
**@Component** decorator's **templateUrl** property.
The choice between inline and separate HTML is a matter of taste,
circumstances, and organization policy. But normally we use inline template
for small portion of code and external template file for bigger views. By
default, the Angular CLI generates components with a template file. But you
can override that with the below command,
```
ng generate component hero -it
```
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