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Angular HTTP GET Example

By Arvind Rai, August 10, 2021
On this page we will provide Angular HTTP GET example. The Angular HttpClient class performs HTTP requests. The HttpClient is available as an injectable class. It has methods to perform HTTP requests. Each method has multiple signatures and its return type varies based on the signature. The HttpClient methods are get(), post(), put(), delete(), request(), head(), jsonp(), options(), patch(). They return Observable instance.
The Observable can be converted into Promise using RxJS Observable.toPromise() method. The HttpClient.get() communicates to server only when we fetch values from Observable or Promise.
On this page we will create a book service in which we will access books from a URL in JSON format. We will iterate the book using Observable instance and Promise instance. Find the complete example step by step.



Technologies Used

Find the technologies being used in our example.
1. Angular 12.1.0
2. Node.js 12.14.1
3. NPM 7.20.3

HttpClient.get()

The syntax of HttpClient.get() is as following.
get(url, options): Observable<any> 
url : String type that assigns endpoint URL.
options: This is an object type. This parameter is optional, if not passed, the default values are assigned.
Find the complete attributes of options parameter in get() method.
options: {
    headers?: HttpHeaders|{[header: string]: string | string[]},
    context?: HttpContext,
    observe?: 'body'|'events'|'response',
    params?: HttpParams|
          {[param: string]: string | number | boolean | ReadonlyArray<string|number|boolean>},
    reportProgress?: boolean,
    responseType?: 'arraybuffer'|'blob'|'json'|'text',
    withCredentials?: boolean
  } 

The return type Observable<any> depends on the responseType. If responseType?: "json" then it will return Observable<HttpEvent<Object>>. The responseType can be json, text, blob and arraybuffer. To use HttpClient.get(), find the steps.
Step-1: We need to import HttpClientModule in our application module.
import { HttpClientModule } from '@angular/common/http';
@NgModule({
  imports: [     
     HttpClientModule,
      ------
  ],
  ---------
}) 
export class AppModule { } 
Step-2: We should perform server communication in a service class and not in component. This approach is preferable by design. In a service class we will use dependency injection to get the instance of HttpClient as given below.
constructor(private http: HttpClient) { } 
Step-3: Pass URL to use get() method.
return this.http.get(this.url); 
In the above code, the http.get() returns instance of Observable that can be later subscribed to get result. We can also fetch Observable directly in HTML template using async pipe.


Observable and Promise

We can use either Observable or Promise to handle the HTTP response.
Observable: This is a RxJS API. Observable is a representation of any set of values over any amount of time. All Angular HttpClient methods return Observable response type. The Observable provides pipe method to execute RxJS operators such as map(), catchError() etc. The map() applies a function to each value emitted by source Observable and returns an instance of Observable. The catchError() is called when an error is occurred. The catchError() also returns Observable.

Promise: This is a JavaScript class. Promise is used for asynchronous computation. A Promise represents value which may be available now, in the future or never. The Promise is a proxy for a value which is not known when the promise is created. The Promise has methods such as then(), catch() etc. The HttpClient.get() returns Observable and to convert it into Promise we need to call RxJS toPromise() operator on the instance of Observable. The then() method of Promise returns a Promise and that can be chained later. The catch() method of Promise is executed when an error is occurred and it also returns Promise.

HttpClient.get() with Observable

By default HttpClient.get() returns Observable of JSON response type.
getBooksWithObservable(): Observable<Book[]> {
	return this.http.get(this.url).pipe(
			map(this.extractData),
			catchError(this.handleErrorObservable)
	)
} 
If we want to filter data by passing parameters, we can pass options object parameter containing params attribute as following.
getBooksWithObservable(category: string, writer: string): Observable<Book[]> {
       let httpParams = new HttpParams()
                        .set('category', category)
	                .set('writer', writer);

	return this.http.get(this.url, { params: httpParams }).pipe(
			map(this.extractData),
			catchError(this.handleErrorObservable)
	)
} 
Now find the extractData() method. The RxJS map() is used to map response object into other object if needed.
private extractData(res: any) {
	let body = res;
	return body;
} 
To handle error, RxJS provides catchError() operator. Find our handleErrorObservable() method.
private handleErrorObservable(error: any) {
	console.error(error.message || error);
	return throwError(error);
} 
The throwError is RxJS operator to throw error.
To display Observable in our HTML template we can go by two way.
1. Angular structural directive can directly use Observable with async pipe. Find the code snippet of component.
this.observableBooks = this.bookService.getBooksWithObservable(); 
Now we can iterate observableBooks with ngFor as following.
<ul>
  <li *ngFor="let book of observableBooks | async" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul> 
The async pipe will mark observableBooks to listen for any changes.
2. Subscribe to the Observable and fetch values. Find the code snippet of component.
this.observableBooks = this.bookService.getBooksWithObservable();
this.observableBooks.subscribe(
      books => this.books = books,
      error =>  this.errorMessage = <any>error); 
Now we can iterate books with ngFor in HTML template as following.
<ul>
  <li *ngFor="let book of books" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul> 

HttpClient.get() with Promise

Let us understand to use Promise with HttpClient.get.
getBooksWithPromise(): Promise<Book[]> {
     return this.http.get(this.url).toPromise()
	    .then(this.extractData)
	    .catch(this.handleErrorPromise);
} 
In the above code, we are passing our HTTP URL to get() method that returns Observable. On this value we call RxJS toPromise() operator that will convert it into Promise. We can use then() to change the response object. If there is any error, the catch() method will execute. Find the handleErrorPromise() method used in catch().
private handleErrorPromise (error: Response | any) {
	console.error(error.message || error);
	return Promise.reject(error.message || error);
}
To display Promise in our HTML template we can go by two way.
1. Get instance of Promise and use it in HTML template with async pipe. Find the code snippet of component.
this.promiseBooks = this.bookService.getBooksWithPromise(); 
Now we will iterate promiseBooks in our HTML template using ngFor with async pipe as following.
<ul>
  <li *ngFor="let book of promiseBooks | async" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul> 
2. We can call then() method and assign value to our component variable as following.
this.promiseBooks = this.bookService.getBooksWithPromise();
this.promiseBooks.then(
        books => this.books = books,
        error =>  this.errorMessage = <any>error); 
Now iterate books in HTML template using ngFor as following.
<ul>
  <li *ngFor="let book of books" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul> 

Complete Example

Find the complete example.
book.service.ts
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, throwError } from 'rxjs';
import { map, catchError } from 'rxjs/operators';
import { Book } from './book';

@Injectable({
    providedIn: 'root'
})
export class BookService {
    url = "http://localhost:4200/assets/data/books.json";
    constructor(private http: HttpClient) { }
    getBooksWithObservable(): Observable {
        return this.http.get(this.url).pipe(
            map(this.extractData),
            catchError(this.handleErrorObservable)
        )
    }
    getBooksWithPromise(): Promise {
        return this.http.get(this.url).toPromise()
            .then(this.extractData)
            .catch(this.handleErrorPromise);
    }
    private extractData(res: any) {
        let body = res;
        return body;
    }
    private handleErrorObservable(error: any) {
        console.error(error.message || error);
        return throwError(error);
    }
    private handleErrorPromise(error: Response | any) {
        console.error(error.message || error);
        return Promise.reject(error.message || error);
    }
} 
book.ts
export interface Book {
   id: number;
   name: string;
} 
books.json
[
  {"id": 1, "name": "Core Java"},
  {"id": 2, "name": "Angular"},
  {"id": 3, "name": "Hibernate"}
] 
observable.component.ts
import { Component } from '@angular/core';
import { Observable } from 'rxjs';

import { BookService } from './book.service';
import { Book } from './book';

@Component({
   selector: 'app-observable',
   templateUrl: './observable.component.html'
})
export class ObservableComponent {
   observableBooks: Observable<Book[]>;
   books: Book[] = [];
   errorMessage = '';
   constructor(private bookService: BookService) {
      this.observableBooks = this.bookService.getBooksWithObservable();
      this.observableBooks.subscribe(
         books => this.books = books,
         error => this.errorMessage = error);
   }
} 
observable.component.html
<h3>Book Details with Observable</h3>
<ul>
  <li *ngFor="let book of observableBooks | async" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul>

<h3>Book Details with "subscribe"</h3>
<ul>
  <li *ngFor="let book of books" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul>

<div *ngIf="errorMessage"> {{errorMessage}} </div> 
promise.component.ts
import { Component } from '@angular/core';

import { BookService } from './book.service';
import { Book } from './book';

@Component({
   selector: 'app-promise',
   templateUrl: './promise.component.html'
})
export class PromiseComponent {
   promiseBooks: Promise<Book[]>;
   books: Book[] = [];
   errorMessage = '';
   constructor(private bookService: BookService) {
      this.promiseBooks = this.bookService.getBooksWithPromise();
      this.promiseBooks.then(
         books => this.books = books,
         error => this.errorMessage = error);
   }
} 
promise.component.html
<h3>Book Details with Promise</h3>
<ul>
  <li *ngFor="let book of promiseBooks | async" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul>

<h3>Book Details with "then"</h3>
<ul>
  <li *ngFor="let book of books" >
    Id: {{book.id}}, Name: {{book.name}}
  </li>
</ul>

<div *ngIf="errorMessage"> {{errorMessage}} </div> 
app.component.ts
import { Component } from '@angular/core';

@Component({
   selector: 'app-root',
   template: `
		<app-observable></app-observable>
		<app-promise></app-promise>
             `
})
export class AppComponent { 
} 
app.module.ts
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { HttpClientModule } from '@angular/common/http';
import { AppComponent } from './app.component';
import { ObservableComponent } from './observable.component';
import { PromiseComponent } from './promise.component';

@NgModule({
  imports: [
    BrowserModule,
    HttpClientModule
  ],
  declarations: [
    AppComponent,
    ObservableComponent,
    PromiseComponent
  ],
  providers: [
  ],
  bootstrap: [
    AppComponent
  ]
})
export class AppModule { } 

Run Application

To run the application, find the steps.
1. Download source code using download link given below on this page.
2. Use downloaded src in your Angular CLI application. To install Angular CLI, find the link.
3. Inside src/assets/data folder we have placed books.json file. We can access JSON data using following URL.
http://localhost:4200/assets/data/books.json 
4. Run ng serve using command prompt.
5. Access the URL http://localhost:4200
Find the print screen of the output.
Angular HTTP GET Example

Reference

Communicating with backend services

Download Source Code

POSTED BY
ARVIND RAI
ARVIND RAI











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