Angular 8 - Http Client Programming

Http client programming is a must needed feature in every modern web application. Nowadays, lot of application exposes their functionality through REST API (functionality over HTTP protocol). With this in mind, Angular Team provides extensive support to access HTTP server. Angular provides a separate module, HttpClientModule and a service, HttpClient to do HTTP programming.
Let us learn how to how to use HttpClient service in this chapter. Developer should have a basic knowledge in Http programming to understand this chapter.
The prerequisite to do Http programming is the basic knowledge of Http protocol and REST API technique. Http programming involves two part, server and client. Angular provides support to create client side application. Express a popular web framework provides support to create server side application.
Let us create an Expense Rest API using express framework and then access it from our ExpenseManager application using Angular HttpClient service.
Open a command prompt and create a new folder, express-rest-api.
cd /go/to/workspace 
mkdir express-rest-api 
cd expense-rest-api

Initialise a new node application using below command −
npm init

npm init will ask some basic questions like project name (express-rest-api), entry point (server.js), etc., as mentioned below −
This utility will walk you through creating a package.json file. 
It only covers the most common items, and tries to guess sensible defaults. 
See `npm help json` for definitive documentation on these fields and exactly what they do. 
Use `npm install <pkg>` afterwards to install a package and save it as a dependency in the package.json file. 
Press ^C at any time to quit. 
package name: (expense-rest-api) 
version: (1.0.0) 
description: Rest api for Expense Application 
entry point: (index.js) server.js 
test command:
git repository: 
keywords: 
author: 
license: (ISC) 
About to write to \path\to\workspace\expense-rest-api\package.json: { 
   "name": "expense-rest-api", 
   "version": "1.0.0", 
   "description": "Rest api for Expense Application", 
   "main": "server.js", 
   "scripts": { 
      "test": "echo \"Error: no test specified\" && exit 1" 
   }, 
   "author": "", 
   "license": "ISC" 
} 
Is this OK? (yes) yes
Install express, sqlite and cors modules using below command −
npm install express sqlite3 cors

Create a new file sqlitedb.js and place below code −
var sqlite3 = require('sqlite3').verbose()
const DBSOURCE = "expensedb.sqlite"

let db = new sqlite3.Database(DBSOURCE, (err) => {
   if (err) {
      console.error(err.message)
      throw err
   }else{
      console.log('Connected to the SQLite database.')
      db.run(`CREATE TABLE expense (
         id INTEGER PRIMARY KEY AUTOINCREMENT,
         item text, 
         amount real, 
         category text, 
         location text, 
         spendOn text, 
         createdOn text 
         )`,
            (err) => {
               if (err) {
                  console.log(err);
               }else{
                  var insert = 'INSERT INTO expense (item, amount, category, location, spendOn, createdOn) VALUES (?,?,?,?,?,?)'

                  db.run(insert, ['Pizza', 10, 'Food', 'KFC', '2020-05-26 10:10', '2020-05-26 10:10'])
                  db.run(insert, ['Pizza', 9, 'Food', 'Mcdonald', '2020-05-28 11:10', '2020-05-28 11:10'])
                  db.run(insert, ['Pizza', 12, 'Food', 'Mcdonald', '2020-05-29 09:22', '2020-05-29 09:22'])
                  db.run(insert, ['Pizza', 15, 'Food', 'KFC', '2020-06-06 16:18', '2020-06-06 16:18'])
                  db.run(insert, ['Pizza', 14, 'Food', 'Mcdonald', '2020-06-01 18:14', '2020-05-01 18:14'])
               }
            }
      );  
   }
});

module.exports = db
Here, we are creating a new sqlite database and load some sample data.
Open server.js and place below code −
var express = require("express")
var cors = require('cors')
var db = require("./sqlitedb.js")

var app = express()
app.use(cors());

var bodyParser = require("body-parser");
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

var HTTP_PORT = 8000 
app.listen(HTTP_PORT, () => {
   console.log("Server running on port %PORT%".replace("%PORT%",HTTP_PORT))
});

app.get("/", (req, res, next) => {
    res.json({"message":"Ok"})
});

app.get("/api/expense", (req, res, next) => {
   var sql = "select * from expense"
   var params = []
   db.all(sql, params, (err, rows) => {
      if (err) {
        res.status(400).json({"error":err.message});
        return;
      }
      res.json(rows)
     });

});

app.get("/api/expense/:id", (req, res, next) => {
   var sql = "select * from expense where id = ?"
   var params = [req.params.id]
   db.get(sql, params, (err, row) => {
      if (err) {
         res.status(400).json({"error":err.message});
         return;
      }
      res.json(row)
   });
});

app.post("/api/expense/", (req, res, next) => {
   var errors=[]
   if (!req.body.item){
      errors.push("No item specified");
   }
   var data = {
      item : req.body.item,
      amount: req.body.amount,
      category: req.body.category,
      location : req.body.location,
      spendOn: req.body.spendOn,
      createdOn: req.body.createdOn,
   }
   var sql = 'INSERT INTO expense (item, amount, category, location, spendOn, createdOn) VALUES (?,?,?,?,?,?)'
   var params =[data.item, data.amount, data.category, data.location, data.spendOn, data.createdOn]
   db.run(sql, params, function (err, result) {
      if (err){
         res.status(400).json({"error": err.message})
         return;
      }
      data.id = this.lastID;
      res.json(data);
   });
})

app.put("/api/expense/:id", (req, res, next) => {
   var data = {
      item : req.body.item,
      amount: req.body.amount,
      category: req.body.category,
      location : req.body.location,
      spendOn: req.body.spendOn
   }
   db.run(
      `UPDATE expense SET
         item = ?, 

         amount = ?,
         category = ?, 
         location = ?, 

         spendOn = ? 
         WHERE id = ?`,
            [data.item, data.amount, data.category, data.location,data.spendOn, req.params.id],
      function (err, result) {
         if (err){
            console.log(err);
            res.status(400).json({"error": res.message})
            return;
         }
         res.json(data)
   });
})

app.delete("/api/expense/:id", (req, res, next) => {
   db.run(
      'DELETE FROM expense WHERE id = ?',
      req.params.id,
      function (err, result) {
         if (err){
            res.status(400).json({"error": res.message})
            return;
         }
         res.json({"message":"deleted", changes: this.changes})
   });
})

app.use(function(req, res){
   res.status(404);
});
Here, we create a basic CURD rest api to select, insert, update and delete expense entry.
Run the application using below command −
npm run start

Open a browser, enter http://localhost:8000/ and press enter. You will see below response −
{ 
   "message": "Ok" 
}

It confirms our application is working fine.
Change the url to http://localhost:8000/api/expense and you will see all the expense entries in JSON format.
[
   {
      "id": 1,

      "item": "Pizza",
      "amount": 10,
      "category": "Food",
      "location": "KFC",
      "spendOn": "2020-05-26 10:10",
      "createdOn": "2020-05-26 10:10"
   },
   {
      "id": 2,
      "item": "Pizza",
      "amount": 14,
      "category": "Food",
      "location": "Mcdonald",
      "spendOn": "2020-06-01 18:14",
      "createdOn": "2020-05-01 18:14"
   },
   {
      "id": 3,
      "item": "Pizza",
      "amount": 15,
      "category": "Food",
      "location": "KFC",
      "spendOn": "2020-06-06 16:18",
      "createdOn": "2020-06-06 16:18"
   },
   {
      "id": 4,
      "item": "Pizza",
      "amount": 9,
      "category": "Food",
      "location": "Mcdonald",
      "spendOn": "2020-05-28 11:10",
      "createdOn": "2020-05-28 11:10"
   },
   {
      "id": 5,
      "item": "Pizza",
      "amount": 12,
      "category": "Food",
      "location": "Mcdonald",
      "spendOn": "2020-05-29 09:22",
      "createdOn": "2020-05-29 09:22"
   }
]
Finally, we created a simple CURD REST API for expense entry and we can access the REST API from our Angular application to learn HttpClient module.
Let us learn how to configure HttpClient service in this chapter.
HttpClient service is available inside the HttpClientModule module, which is available inside the @angular/common/http package.
To register HttpClientModule module −
Import the HttpClientModule in AppComponent
import { HttpClientModule } from '@angular/common/http';

Include HttpClientModule in imports meta data of AppComponent.
@NgModule({ 
   imports: [ 
      BrowserModule, 
      // import HttpClientModule after BrowserModule. 
      HttpClientModule, 
   ] 
}) 
export class AppModule {}

Let us create a new service ExpenseEntryService in our ExpenseManager application to interact with Expense REST API. ExpenseEntryService will get the latest expense entries, insert new expense entries, modify existing expense entries and delete the unwanted expense entries.
Open command prompt and go to project root folder.
cd /go/to/expense-manager

Start the application.
ng serve

Run the below command to generate an Angular service, ExpenseService.
ng generate service ExpenseEntry

This will create two Typescript files (expense entry service & its test) as specified below −
CREATE src/app/expense-entry.service.spec.ts (364 bytes) 
CREATE src/app/expense-entry.service.ts (141 bytes)

Open ExpenseEntryService (src/app/expense-entry.service.ts) and import ExpenseEntry, throwError and catchError from rxjs library and import HttpClient, HttpHeaders and HttpErrorResponse from @angular/common/http package.
import { Injectable } from '@angular/core'; 
import { ExpenseEntry } from './expense-entry'; import { throwError } from 'rxjs';
import { catchError } from 'rxjs/operators'; 
import { HttpClient, HttpHeaders, HttpErrorResponse } from 
'@angular/common/http';

Inject the HttpClient service into our service.
constructor(private httpClient : HttpClient) { }

Create a variable, expenseRestUrl to specify the Expense Rest API endpoints.
private expenseRestUrl = 'http://localhost:8000/api/expense';

Create a variable, httpOptions to set the Http Header option. This will be used during the Http Rest API call by Angular HttpClient service.
private httpOptions = { 
   headers: new HttpHeaders( { 'Content-Type': 'application/json' }) 
};

The complete code is as follows −
import { Injectable } from '@angular/core';
import { ExpenseEntry } from './expense-entry';
import { Observable, throwError } from 'rxjs';
import { catchError, retry } from 'rxjs/operators';
import { HttpClient, HttpHeaders, HttpErrorResponse } from '@angular/common/http';

@Injectable({
   providedIn: 'root'
})
export class ExpenseEntryService {
   private expenseRestUrl = 'api/expense';
   private httpOptions = {
      headers: new HttpHeaders( { 'Content-Type': 'application/json' })
   };

   constructor(
      private httpClient : HttpClient) { }
}

HttpClient provides get() method to fetch data from a web page. The main argument is the target web url. Another optional argument is the option object with below format −
{
   headers?: HttpHeaders | {[header: string]: string | string[]},
   observe?: 'body' | 'events' | 'response',

   params?: HttpParams|{[param: string]: string | string[]},
   reportProgress?: boolean,
   responseType?: 'arraybuffer'|'blob'|'json'|'text',
   withCredentials?: boolean,
}

Here,
headers − HTTP Headers of the request, either as string, array of string or array of HttpHeaders.
headers − HTTP Headers of the request, either as string, array of string or array of HttpHeaders.
observe − Process the response and return the specific content of the response. Possible values are body, response and events. The default option of observer is body.
observe − Process the response and return the specific content of the response. Possible values are body, response and events. The default option of observer is body.
params − HTTP parameters of the request, either as string, array of string or array of HttpParams.
params − HTTP parameters of the request, either as string, array of string or array of HttpParams.
reportProgress − Whether to report the progress of the process or not (true or false).
reportProgress − Whether to report the progress of the process or not (true or false).
responseType − Refers the format of the response. Possible values are arraybuffer, blob, json and text.
responseType − Refers the format of the response. Possible values are arraybuffer, blob, json and text.
withCredentials − Whether the request has credentials or not (true or false).
withCredentials − Whether the request has credentials or not (true or false).
All options are optional.
get() method returns the response of the request as Observable. The returned Observable emit the data when the response is received from the server.
The sample code to use get() method is as follows −
httpClient.get(url, options) 
.subscribe( (data) => console.log(data) );

get() method has an option to return observables, which emits typed response as well. The sample code to get typed response (ExpenseEntry) is as follows:
httpClient.get<T>(url, options) .subscribe( (data: T) => console.log(data) );

Error handling is one of the important aspect in the HTTP programming. Encountering error is one of the common scenario in HTTP programming.
Errors in HTTP Programming can be categories into two groups −
Client side issues can occur due to network failure, misconfiguration, etc., If client side error happens, then the get() method throws ErrorEvent object.
Client side issues can occur due to network failure, misconfiguration, etc., If client side error happens, then the get() method throws ErrorEvent object.
Server side issues can occur due to wrong url, server unavailability, server programming errors, etc.,
Server side issues can occur due to wrong url, server unavailability, server programming errors, etc.,
Let us write a simple error handling for our ExpenseEntryService service.
private httpErrorHandler (error: HttpErrorResponse) {
   if (error.error instanceof ErrorEvent) {
      console.error("A client side error occurs. The error message is " + error.message);
      } else {
         console.error(
            "An error happened in server. The HTTP status code is "  + error.status + " and the error returned is " + error.message);
      }

   return throwError("Error occurred. Pleas try again");
}

The error function can be called in get() as specified below −
httpClient.get(url, options)  
   .pipe(catchError(this.httpErrorHandler) 
   .subscribe( (data) => console.log(data) )

As we mentioned earlier, errors can happen and one way is to handle it. Another option is to try for certain number of times. If the request failed due to network issue or the HTTP server is temporarily offline, the next request may succeed.
We can use rxjs library’s retry operator in this scenario as specified below
httpClient.get(url, options) 
   .pipe( 
      retry(5), 
      catchError(this.httpErrorHandler)) 
   .subscribe( (data) => console.log(data) )

Let us do the actual coding to fetch the expenses from Expense Rest API in our ExpenseManager application.
Open command prompt and go to project root folder.
cd /go/to/expense-manager

Start the application.
ng serve

Add getExpenseEntries() and httpErrorHandler() method in ExpenseEntryService (src/app/expense-entry.service.ts) service.
getExpenseEntries() : Observable<ExpenseEntry[]> {
   return this.httpClient.get<ExpenseEntry[]>(this.expenseRestUrl, this.httpOptions)
   .pipe(retry(3),catchError(this.httpErrorHandler));
}

getExpenseEntry(id: number) : Observable<ExpenseEntry> {
   return this.httpClient.get<ExpenseEntry>(this.expenseRestUrl + "/" + id, this.httpOptions)
   .pipe(
      retry(3),
      catchError(this.httpErrorHandler)
   );
}

private httpErrorHandler (error: HttpErrorResponse) {
   if (error.error instanceof ErrorEvent) {
      console.error("A client side error occurs. The error message is " + error.message);
   } else {
      console.error(
         "An error happened in server. The HTTP status code is "  + error.status + " and the error returned is " + error.message);
   }

   return throwError("Error occurred. Pleas try again");
}
Here,
getExpenseEntries() calls the get() method using expense end point and also configures the error handler. Also, it configures httpClient to try for maximum of 3 times in case of failure. Finally, it returns the response from server as typed (ExpenseEntry[]) Observable object.
getExpenseEntries() calls the get() method using expense end point and also configures the error handler. Also, it configures httpClient to try for maximum of 3 times in case of failure. Finally, it returns the response from server as typed (ExpenseEntry[]) Observable object.
getExpenseEntry is similar to getExpenseEntries() except it passes the id of the ExpenseEntry object and gets ExpenseEntry Observable object.
getExpenseEntry is similar to getExpenseEntries() except it passes the id of the ExpenseEntry object and gets ExpenseEntry Observable object.
The complete coding of ExpenseEntryService is as follows −
import { Injectable } from '@angular/core';
import { ExpenseEntry } from './expense-entry';

import { Observable, throwError } from 'rxjs';
import { catchError, retry } from 'rxjs/operators';
import { HttpClient, HttpHeaders, HttpErrorResponse } from '@angular/common/http';

@Injectable({

   providedIn: 'root'
})
export class ExpenseEntryService {
   private expenseRestUrl = 'http://localhost:8000/api/expense';
   private httpOptions = {
      headers: new HttpHeaders( { 'Content-Type': 'application/json' })
   };

   constructor(private httpClient : HttpClient) { } 

   getExpenseEntries() : Observable {
      return this.httpClient.get(this.expenseRestUrl, this.httpOptions)
      .pipe(
         retry(3),
         catchError(this.httpErrorHandler)
      );
   }

   getExpenseEntry(id: number) : Observable {
      return this.httpClient.get(this.expenseRestUrl + "/" + id, this.httpOptions)
      .pipe(
         retry(3),
         catchError(this.httpErrorHandler)
      );
   }

   private httpErrorHandler (error: HttpErrorResponse) {
      if (error.error instanceof ErrorEvent) {
         console.error("A client side error occurs. The error message is " + error.message);
      } else {
         console.error(
            "An error happened in server. The HTTP status code is "  + error.status + " and the error returned is " + error.message);
      }

      return throwError("Error occurred. Pleas try again");
   }
}
Open ExpenseEntryListComponent (src-entry-list-entry-list.component.ts) and inject ExpenseEntryService through constructor as specified below:
constructor(private debugService: DebugService, private restService : 
ExpenseEntryService ) { }

Change the getExpenseEntries() function. Call getExpenseEntries() method from ExpenseEntryService instead of returning the mock items.
getExpenseItems() {  
   this.restService.getExpenseEntries() 
      .subscribe( data =− this.expenseEntries = data ); 
}

The complete ExpenseEntryListComponent coding is as follows −
import { Component, OnInit } from '@angular/core';
import { ExpenseEntry } from '../expense-entry';
import { DebugService } from '../debug.service';
import { ExpenseEntryService } from '../expense-entry.service';

@Component({
   selector: 'app-expense-entry-list',
   templateUrl: './expense-entry-list.component.html',
   styleUrls: ['./expense-entry-list.component.css'],
   providers: [DebugService]
})
export class ExpenseEntryListComponent implements OnInit {
   title: string;
   expenseEntries: ExpenseEntry[];
   constructor(private debugService: DebugService, private restService : ExpenseEntryService ) { }

   ngOnInit() {
      this.debugService.info("Expense Entry List component initialized");
      this.title = "Expense Entry List";

      this.getExpenseItems();
   }

   getExpenseItems() {
      this.restService.getExpenseEntries()
      .subscribe( data => this.expenseEntries = data );
   }
}
Finally, check the application and you will see the below response.
HTTP POST is similar to HTTP GET except that the post request will send the necessary data as posted content along with the request. HTTP POST is used to insert new record into the system.
HttpClient provides post() method, which is similar to get() except it support extra argument to send the data to the server.
Let us add a new method, addExpenseEntry() in our ExpenseEntryService to add new expense entry as mentioned below −
addExpenseEntry(expenseEntry: ExpenseEntry): Observable<ExpenseEntry> {
   return this.httpClient.post<ExpenseEntry>(this.expenseRestUrl, expenseEntry, this.httpOptions)
   .pipe(
      retry(3),
      catchError(this.httpErrorHandler)
   );
}
HTTP PUT is similar to HTTP POST request. HTTP PUT is used to update existing record in the system.
httpClient provides put() method, which is similar to post().
Let us add a new method, updateExpenseEntry() in our ExpenseEntryService to update existing expense entry as mentioned below:
updateExpenseEntry(expenseEntry: ExpenseEntry): Observable<ExpenseEntry> {
   return this.httpClient.put<ExpenseEntry>(this.expenseRestUrl + "/" + expenseEntry.id, expenseEntry, this.httpOptions)
   .pipe(
      retry(3),
      catchError(this.httpErrorHandler)
   );
}
HTTP DELETE is similar to http GET request. HTTP DELETE is used to delete entries in the system.
httpclient provides delete() method, which is similar to get().
Let us add a new method, deleteExpenseEntry() in our ExpenseEntryService to delete existing expense entry as mentioned below −
deleteExpenseEntry(expenseEntry: ExpenseEntry | number) : Observable<ExpenseEntry> {
   const id = typeof expenseEntry == 'number' ? expenseEntry : expenseEntry.id
   const url = `${this.expenseRestUrl}/${id}`;

   return this.httpClient.delete<ExpenseEntry>(url, this.httpOptions)
   .pipe(
      retry(3),
      catchError(this.httpErrorHandler)
   );
}

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