Angular test questions Flashcards
What is Angular Framework?
-TypeScript-based open-source front-end platform
-For web, mobile and desktop applications.
-The major features of this framework include declarative templates, dependency injection, end to end tooling which ease application development.
What is TypeScript?
TypeScript is a strongly typed overlay of JavaScript
-adds optional types, classes, async/await and many other features, and compiles to plain JavaScript.
-Angular is written entirely in TypeScript as a primary language.
Let’s see a simple example of TypeScript usage:
function greeter(person: string) {
return "Hello, " + person;
}
let user = "Sudheer";
document.body.innerHTML = greeter(user);
The greeter method allows only string type as argument.
Write a pictorial diagram of Angular architecture?
The main building blocks of an Angular application are shown in the diagram below:- ScreenShot
What are the key components of Angular?
CMMTS
Component: These are the basic building blocks of an Angular application to control HTML views.
Modules: An Angular module is a set of angular basic building blocks like components, directives, services etc. An application is divided into logical pieces and each piece of code is called as “module” which perform a single task.
Templates: These represent the views of an Angular application.
Services: Are used to create components which can be shared across the entire application.
Metadata: This can be used to add more data to an Angular class.
What are directives?
Directives add behaviour to an existing DOM element or an existing component instance.
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
<p myHighlight>Highlight me!</p>
What are components?
Components are the most basic UI building block of an Angular app, which form a tree of Angular components. These components are a subset of directives.
Unlike directives, components always have a template, and only one component can be instantiated per element in a template.
simple example of Angular component
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';
}
What are the differences between Component and Directive?
In a short note, A component(@component) is a directive-with-a-template.
- To register a component we use @Component meta-data annotation To register a directive we use @Directive meta-data annotation
- Components are typically used to create UI widgets
Directives are used to add behavior to an existing DOM element - Component is used to break down the application into smaller components
Directive is used 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
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, 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,
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
import { Component } from '@angular/core';
@Component ({
selector: 'my-app',
templateUrl: 'app/app.component.html'
})
export class AppComponent {
title: string = 'Hello World';
}
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,
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AppComponent } from './app.component';
@NgModule ({
imports: [ BrowserModule ],
declarations: [ AppComponent ],
bootstrap: [ AppComponent ],
providers: []
})
export class AppModule { }
The NgModule decorator has five important (among all) options:
The imports option is used to import other dependent modules. The BrowserModule is required by default for any web based angular application.
The declarations option is used to define components in the respective module.
The bootstrap option tells Angular which Component to bootstrap in the application.
The providers option is used to configure a set of injectable objects that are available in the injector of this module.
The entryComponents option is a set of components dynamically loaded into the view.
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,
ngOnChanges: When the value of a data bound property changes, then this method is called.
ngOnInit: This is called whenever the initialization of the directive/component after Angular first displays the data-bound properties happens.
ngDoCheck: This is for the detection and to act on changes that Angular can’t or won’t detect on its own.
ngAfterContentInit: This is called in response after Angular projects external content into the component’s view.
ngAfterContentChecked: This is called in response after Angular checks the content projected into the component
ngAfterViewInit: This is called in response after Angular initializes the component’s views and child views.
ngAfterViewChecked: This is called in response after Angular checks the component’s views and child views.
ngOnDestroy: This is the cleanup phase just before Angular destroys the directive/component.
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.
From the Component to the DOM:
Interpolation: {{ value }}: Adds the value of a property from the component
<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
<input type="email" [value]="user.email">
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 <button (click)="logout()"></button>
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
<input type="email" [(ngModel)]="user.email">
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 Class decorators, e.g. @Component and @NgModule
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!');
}
}
Property decorators Used for properties inside classes, e.g. @Input and @Output
import { Component, Input } from '@angular/core';
@Component({
selector: 'my-component',
template: '<div>Property decorator</div>'
})
export class MyComponent {
@Input()
title: string;
}
Method decorators Used for methods inside classes, e.g. @HostListener
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
}
}
Parameter decorators Used for parameters inside class constructors, e.g. @Inject, @Optional
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
}
}
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.
npm install @angular/cli@latest
Below are the list of few commands, which will come handy while creating angular projects
* Creating New Project: ng new
- Generating Components, Directives & Services: ng generate/g 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
- Running the Project: ng serve
What is the difference between constructor and ngOnInit?
The Constructor is a default method of the class that is executed when the class is instantiated and ensures proper initialisation of fields in the class and its subclasses. Angular, or better Dependency Injector (DI), analyses the constructor parameters and when it creates a new instance by calling new MyClass() it tries to find providers that match the types of the constructor parameters, resolves them and passes them to the constructor.
ngOnInit is a life cycle hook called by Angular to indicate that Angular is done creating the component.
Mostly we use ngOnInit for all the initialization/declaration and avoid stuff to work in the constructor. The constructor should only be used to initialize class members but shouldn’t do actual “work”. So you should use constructor() to setup Dependency Injection and not much else. ngOnInit() is better place to “start” - it’s where/when components’ bindings are resolved.
export class App implements OnInit{
constructor(private myService: MyService){
//called first time before the ngOnInit()
}
ngOnInit(){
//called after the constructor and called after the first ngOnChanges()
//e.g. http call...
}
}
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,
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.
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.
How is Dependency Hierarchy formed?
Injectors in Angular have rules that can be leveraged to achieve the desired visibility of injectables in your applications. By understanding these rules, you can determine in which NgModule, Component, or Directive you should declare a provider.
Angular has two injector hierarchies:
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.
@Component({
selector: ‘async-observable-pipe’,
template: <div><code>observable|async</code>:
Time: {{ time | async }}</div>
})
export class AsyncObservablePipeComponent {
time: Observable<string>;
constructor() {
this.time = new Observable((observer) => {
setInterval(() => {
observer.next(new Date().toString());
}, 2000);
});
}
}</string>
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
What is the purpose of *ngFor directive?
We use Angular *ngFor directive in the template to display each item in the list. For example, here we can iterate over a list of users:
<li *ngFor=”let user of users”>
{{ user }}
</li>
The user variable in the *ngFor double-quoted instruction is a template input variable.
What is the purpose of *ngIf directive?
Sometimes an app needs to display a view or a portion of a view only under specific circumstances. The Angular *ngIf directive inserts or removes an element based on a truthy/falsy condition. Let’s take an example to display a message if the user age is more than 18:
<p *ngIf=”user.age > 18”>You are not eligible for student pass!</p>
Note: Angular isn’t showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in the larger projects with many data bindings.
What happens if you use script tag inside template?
Angular recognizes the value as unsafe and automatically sanitizes it, which removes the script tag but keeps safe content such as the text content of the script tag. This way it eliminates the risk of script injection attacks. If you still use it then it will be ignored and a warning appears in the browser console.
Let’s take an example of innerHtml property binding which causes XSS vulnerability,
export class InnerHtmlBindingComponent {
// For example, a user/attacker-controlled value from a URL.
htmlSnippet = 'Template <script>alert("0wned")</script> <b>Syntax</b>';
}
What is interpolation?
Interpolation is a special syntax that Angular converts into property binding. It’s a convenient alternative to property binding. It is represented by double curly braces({{}}). The text between the braces is often the name of a component property. Angular replaces that name with the string value of the corresponding component property.
Let’s take an example,
<h3>
{{title}}
<img src="{{url}}" style="height:30px">
</h3>
In the example above, Angular evaluates the title and url properties and fills in the blanks, first displaying a bold application title and then a URL.
What are template expressions?
A template expression produces a value similar to any Javascript expression. Angular executes the expression and assigns it to a property of a binding target; the target might be an HTML element, a component, or a directive. In the property binding, a template expression appears in quotes to the right of the = symbol as in [property]=”expression”. In interpolation syntax, the template expression is surrounded by double curly braces. For example, in the below interpolation, the template expression is {{username}},
<h3>{{username}}, welcome to Angular</h3>
The below javascript expressions are prohibited in template expression
assignments (=, +=, -=, …)
new
chaining expressions with ; or ,
increment and decrement operators (++ and –)
Birthday is {{ birthday | date }}
` }) export class BirthdayComponent { birthday = new Date(1987, 6, 18); // June 18, 1987 }Birthday is {{ birthday | date:'dd/MM/yyyy'}}
` // 18/06/1987 }) export class BirthdayComponent { birthday = new Date(1987, 6, 18); } Note: The parameter value can be any valid template expression, such as a string literal or a component property.Birthday is {{ birthday | date:'fullDate' | uppercase}}
` // THURSDAY, JUNE 18, 1987 }) export class BirthdayComponent { birthday = new Date(1987, 6, 18); }
