面试题与答案

Angular is a TypeScript-based open-source front-end platform that makes it easy to build web, mobile and desktop applications. The major features of this framework include declarative templates, dependency injection, end to end tooling which ease application development.

Angular is a completely revived component-based framework in which an application is a tree of individual components. For tabular view check here:

http://withoutbook.com/DifferenceBetweenSubjects.php?subId1=79&subId2=115&d=Difference%20between%20AngularJS%20and%20Angular

TypeScript is a strongly typed superset of JavaScript created by Microsoft that 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. You can install TypeScript globally as:

npm install -g typescript

Let's see a simple example of TypeScript usage:-

function greeter(person: string) {

  return "Hello, " + person;

}

let user = "Arindam";

document.body.innerHTML = greeter(user);

The greeter method allows only string type as argument.

Angular has the key components below,

1. 1. Component: These are the basic building blocks of an Angular application to control HTML views.
   2. 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.
   3. Templates: These represent the views of an Angular application.
   4. Services: Are used to create components which can be shared across the entire application.
   5. Metadata: This can be used to add more data to an Angular class.

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>

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:

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';

}

In short, A component(@component) is a directive-with-a-template.

Some of the major differences are mentioned in a tabular form

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,

Using separate template file such as app.component.html

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

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
4. The providers option is used to configure set of injectable objects that are available in the injector of this module.
5. The entryComponents option is a set of components dynamically loaded into the view.

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

   <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">

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

   <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

   <input type="email" [(ngModel)]="user.email">

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

   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

   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

   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, 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

     }

   }

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,

Below are the list of few commands, which will come handy while creating angular projects

1. Creating New Project: ng new

2. 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

3. Running the Project: ng serve

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.

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,

The above service uses Http service as a dependency.

We use Angular ngFor directive in the template to display each item in the list. For example, here we iterate over list of users,

The user variable in the ngFor double-quoted instruction is a template input variable

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,

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.

A pipe takes in data as input and transforms it to a desired output. For example, let us take a pipe to transform a component's birthday property into a human-friendly date using date pipe.