HTTP GET Request

The HTTP GET

TheHTTP standard verbGETcan be used in HTTP protocol to get (retrieve) a resource (data) from the server. The purpose of theGET method is to request data from the server. The server will check for the specified resource in the server and send it back if it is available.

In Angular, the HttpClient service class provides a get() method to request data from the server using the HTTP GET verb. Let’s learn more about this method, including it’s signature, parameters, and real-time usage:

Signature of the get() Method

Following is the signature (different from syntax) of the HttpClient get() method −

get<T>(url:string, options?: Object): Observable<T>

Here,

• url − The URL to which the GET request is sent.
• options − represents the options to be send along with the resource URL.
• Observable<T> − The return type, where ‘T’ represents the expected response type.

Working Example

To work out the HTTP client-server communication, we need to create a backend web application and expose a set of web APIs. These web APIs can be requested from the client. Let’s create a sample server application, Expense API App, to provide CRUD REST APIs (mainly GET requests) for managing expenses.

Step 1: Go to your favorite workspace as shown below −

cd /go/to/your/favorite/workspace

Step 2: Create a new folder, expense-rest-api, and move into the folder −

mkdir expense-rest-api && cd expense-rest-api

Step 3: Create a new application using the init sub-command provided by the npm command as shown below −

npm init

Note: Once you run the above command, it will ask a few questions and answer all of them with default answers.

Step 4: Install express and cors packages to create node-based web applications −

npm install express cors --save

Step 5: Install SQLite package to store the expenses in the SQLite-based database −

npm install sqlite3 --save

Step 6: Create a new file with the name sqlitedb.js, and add the below code to initialize the database with expense table and sample expense entries. An expense table will be used to store the expense items −

var sqlite3 =require('sqlite3').verbose()constDBSOURCE="expensedb.sqlite"let db =newsqlite3.Database(DBSOURCE,(err)=>{if(err){
  console.error(err.message)throw err
   }else{
  console.log('Connected to the SQLite database.')
  db.run(`CREATE TABLE IF NOT EXISTS 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

Step 7: Open the index.js file, and place the 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(express.urlencoded({ extended:true}));
app.use(express.json());varHTTP_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.use(function(req, res){
   res.status(404);});

Here, the code will create six below-mentioned REST API endpoints:

• /endpoint returns an OK message to make sure the application is working fine.
• /api/expense endpoint returns all expense items available in the database.
• /api/expense/:id endpoint returns the expense entry based on the expense entry ID.

Step 8: Now, run the application using the below command −

node index.js

Step 9: To test the application, open your friendly browser (chrome) and go to http://localhost:8000/ URL. It should return the below message if the application is working fine −

{ 
   "message": "Ok" 
}

Let us create a working angular example to get all expense items from the above server application by using the HttpClient service class andget()method −

Angular Sample Application

Step 1: Run the below command to create an angular application −

ng new my-http-app

Enable angular routing and CSS as shown below −

? Would you like to add Angular routing? Yes
? Which stylesheet format would you like to use? CSS

Step 2: Enable HTTP communication in the application by importing HttpClientModule in the root component configuration file (app.component.ts) −

import{ Component }from'@angular/core';import{ CommonModule }from'@angular/common';import{ RouterOutlet }from'@angular/router';import{ HttpClientModule }from'@angular/common/http';

@Component({
  selector:'app-root',
  standalone:true,
  imports:[CommonModule, RouterOutlet, HttpClientModule],
  templateUrl:'./app.component.html',
  styleUrl:'./app.component.css'})exportclassAppComponent{
  title ='my-http-app';}

Here,

• Imported the HttpClientModule from @angular/common/http module.
• Added the HttpClientModule into imports section of the @Component configuration.

Step 3: Create a new interface, ListExpenses to show the expense items from the server −

interfaceExpense{
   id: Number,
   item: String,
   amount: Number,
   category: String,
   location: String,
   spendOn: Date
}exportdefault Expense;

Step 4: Create a new component, ListExpenses to show the expense items from the server −

ng generate component ListExpenses

It will create a new component as shown below −

CREATE src/app/list-expenses/list-expenses.component.css (0 bytes)
CREATE src/app/list-expenses/list-expenses.component.html (28 bytes)
CREATE src/app/list-expenses/list-expenses.component.spec.ts (602 bytes)
CREATE src/app/list-expenses/list-expenses.component.ts (229 bytes)

Step 5: Include our new component into the App root component view, app.component.html as shown below −

<app-list-expenses></app-list-expenses><router-outlet></router-outlet>

Step 6: Inject the HttpClient into the ListExpenses component through the constructor as shown below −

import{ Component }from'@angular/core';import{ HttpClient }from'@angular/common/http';@Component({
   selector:'app-list-expenses',
   templateUrl:'./list-expenses.component.html',
   styleUrls:['./list-expenses.component.css']})exportclassListExpensesComponent{constructor(private http: HttpClient){}}

Step 7: Implement the OnInit life cycle hook to request the server for expenses after the initialization of the ListExpenses component −

exportclassListExpensesComponentimplementsOnInit{constructor(private http: HttpClient){}ngOnInit():void{}}

Step 8: Create a local variable, expenses to hold our expenses from the server −

exportclassListExpensesComponentimplementsOnInit{
   expenses: Expense[]=[];constructor(private http: HttpClient){}ngOnInit():void{}}

Step 9: Call the get method of this.http (HttpClient instance) object by passing the URL and options to get the expense object from the server. Then, set the expenses into our local variable, expenses −

exportclassListExpensesComponentimplementsOnInit{
   expenses: Expense[]=[];constructor(private http: HttpClient){}ngOnInit():void{this.http.get<Expense[]>('http://localhost:8000/api/expense',{'observe':'body','responseType':'json'}).subscribe( data =>{this.expenses = data as Expense[]console.log(this.expenses)})}}

Here,

• Sets the Expense[] as the type of the object returned by the server. The server will send the array of expense objects in its body in JSON format.
• Subscribed to the request (this.http.get) object. Then parsed the subscribed data as an array of expense objects and set it to a local expense variable (this.expenses).

Step 10: The complete code of the ListExpensesComponent is as follows −

import{ Component, OnInit }from'@angular/core';import{ HttpClient, HttpRequest, HttpResponse, HttpEvent }from'@angular/common/http';import Expense from'../Expense';

@Component({
   selector:'app-list-expenses',
   templateUrl:'./list-expenses.component.html',
   styleUrls:['./list-expenses.component.css']})exportclassListExpensesComponentimplementsOnInit{
   expenses: Expense[]=[];constructor(private http: HttpClient){}ngOnInit():void{this.http.get<Expense[]>('http://localhost:8000/api/expense',{'observe':'body','responseType':'json'}).subscribe(data=>{this.expenses = data as Expense[]
     console.log(this.expenses)})}}</pre>


Step 11: Next, get the expenses object from the component and render it in our component template page (list-expenses.component.html) −



<div><h3>Expenses</h3></div><ul><li *ngFor="let expense of expenses">
  {{expense.item}} @ {{expense.location}} for {{expense.amount}} USD on {{expense.spendOn | date:'shortDate' }}
   </li></ul>



Step 12: Finally, run the application using the below command −



ng serve




Step 13: Now, open your friendly (chrome) browser navigate to http://localhost:4200/ URL, and check the output −