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Make API Calls the Right Way in Angular

About Angular 10Angular 10 is the latest version of the Angular family. Angular applications are basically designed for Single Page Applications, where all controls are required in single page architecture. Angular is an application based design framework, which provides the Cross-Platform development environment for creating the most sophisticated SPAs. These SPAs are efficient enough for giving us a component to view to model environment, making it an MVC or MVVM application.Angular provides a Progressive Web App architecture, which means that an application made in Angular gives us an App like experience with high performance. Angular has zero-step installation, making its up- gradation using modern web capabilities possible, even if offline. Angular has the capability of making cross platform Desktop Apps which can be created using MAC, Windows, or Linux OS using the same Angular methods. Angular can be rendered in Node.js, Microsoft .Net, PHP and many other servers by giving the output in HTML-CSS format. This even optimizes the app for SEO. Angular framework turns our templates into JavaScript Virtual machine codes which are highly optimized. With Angular CLI, the Command Line tools, we can build and deploy Apps faster than ever before. Angular uses Karma for unit tests and Protractor for scenario tests making the applications made in Angular more stable.IntroductionFront-end applications in today’s programming world use HTTP protocol modules to talk to the back-end services. Almost all current browsers make this communication via two ways, one by HTTPrequests through XMLHttpRequest interface and the other by the API fetch() method call.We usually make API calls to remote HTTP servers via HttpClient module of Angular 10 by sending HTTP requests. HttpClient has methods which perform HTTP requests. HttpClient is an injectable class for Angular 10. The methods of this class have request methods with many types of signatures, and their return types are different based on the signature of the request, thus giving different response types.We will study about HttpClient in detail in the subsequent passage.The Common MistakeOne thing for sure is that we need to call APIs in our programs. If we do so correctly, it can help us in upgrading to further versions, in making test modules, and even in handling errors. One common mistake that programmers usually make is in the call they use API URLs directly in the component. This means that if the API URL is upgraded for any reason, we will need to upgrade the URLs in every single component. For Eg: this.httpClient.get('https://www.userdomain.com/api_name/data/' + this.id);  Again, since we are not validating the path by using the Path variable of Angular, this call might fail if the URL is not valid or not available for some reason. Also, changing the modules with direct call to httpClient will require a lot of changes in the component. Let us now talk about the right approach.The Right WayThe correct approach towards calling APIs in Angular includes the following:Create ConstantsWe need to set global variables in Angular 10 to access our variables synchronously throughout our application. We declare Global variables in a Global Constant file. Create a folder under src/app and name it config. To create the global constant file, name it constants.ts and place it under src/app/config folder.Eg: create the following file: src/app/config/constants.ts Open this file and put global constants like API_ENDPOINT, API_MOCK_ENDPOINT etc. These global variables will be used later in Component files like AppComponent so that they can be used like Site constants. Declaring Global Constants // Angular Modules import { Injectable } from '@angular/core';  @Injectable()  export class Constants { public readonly API_ENDPOINT: string = ' https://www.userdomain.com/';  public readonly API_MOCK_ENDPOINT: string = 'https://www.userdomainmock.com/';  public static TitleOfSite: string = " Making API calls the Right Way by Monica";  }  An example of AppComponent using the above global constant is:  Edit the app.component.ts file from the src/app folder:  src/app/app.component.ts:  import { Component, OnInit } from '@angular/core'; import{ Constants } from './config/constants';  @Component({    selector: 'app-root',    templateUrl: './app.component.html',    styleUrls: ['./app.component.css']  })  export class AppComponent implements OnInit{      title = Constants.TitleOfSite;      constructor() {          console.log(GlobalConstants.API_ENDPOINT);      }      ngOnInit() {          console.log(this.title);      }  } Run the project by navigating to the project folder and calling ng serve : $ cd hello-world  $ ng serve  The output will be seen in the browser: https://www.userdomain.com/ Making API calls the Right Way by MonicaCreate a ServiceTo create a service in Angular 10, create a file api-http.service.ts in the src/app/core/services folder. Add the following code in the file as below: // Angular Modules import { Injectable } from '@angular/core'; import { HttpClient } from '@angular/common/http'; @Injectable() export class ApiHttpService { constructor( // Angular Modules private http: HttpClient ) { } public get(url: string, options?: any) { return this.http.get(url, options); } public post(url: string, data: any, options?: any) { return this.http.post(url, data, options); } public put(url: string, data: any, options?: any) { return this.http.put(url, data, options); } public delete(url: string, options?: any) { return this.http.delete(url, options); } }Use HTTPClient LibraryHttpClient Library is present in @angular/common/http folder. The older version of HttpClient was in @angular/http folder, this is replaced by the upgraded version of the same in @angular/common/http folder.Almost all current browsers communicate with APIs via two ways, one by HTTPrequests through XMLHttpRequest interface and the other by the API fetch() method call.The HttpClient module is built over XMLHttpRequest interface. It wraps all the complexities of this interface and provides extra features like: RxJS Obervables, Interceptors for requests and responses, Typed requests and responses, Error handling techniques, ease in Testing Modules, etc. Benefits of using HTTPClient LibraryHttpClient is a module found in angular/common/http module. HttpClient is based on XMLHttpRequest interface, the Interface which is commonly given by most modern browsers. HttpClient gives us a lot more than XMLHttpRequest, and some of its benefits are:It gives us ease in using testing modules.It has request and response objects in strongly typed manner.It has APIs Support with Observable class object.It provides error handling modules.Importing the libraryIn order to use HttpClient library we need to import HttpClientModule from the @angular/common/http package and include the library in the imports array of the App module : Edit the AppModule file in the src/app folder and add the line on top of the file as follows: src/app/app.module.ts: import { HttpClientModule } from '@angular/common/http'; Configure HTTPClientIncluding HttpClient: Once done, include HttpClientModule in the imports array of the app.module and further use it:import { NgModule }         from '@angular/core'; import { BrowserModule }    from '@angular/platform-browser'; import { HttpClientModule } from '@angular/common/http'; @NgModule({   imports: [     BrowserModule,     // import HttpClientModule after BrowserModule.     HttpClientModule,   ],   declarations: [     AppComponent,   ],   bootstrap: [ AppComponent ] }) export class AppModule {} …Now we are ready to use the HttpClient library to send HTTP requests or receive API response. Injecting HttpClient into our Application: Once we have  imported HttpClientModule into the AppModule, we can inject the HttpClient into our application as:import { Injectable } from '@angular/core'; import { HttpClient } from '@angular/common/http'; @Injectable() export class ConfigService {   constructor(private http: HttpClient) { } }Create a query parameterGetting JSON dataIn this example we will request JSON data from the API server. Here our app needs a configuration file on the server, config.json, that specifies resource URLs.{   "demoUrl": "api/demo",   "filename": "assets/demotextfile.txt" } The ConfigService fetches this file with a get() method from HttpClient library as follows: configUrl = 'assets/config.json'; getConfig() {   return this.http.get(this.configUrl); }A component, such as ConfigComponent, injects the ConfigService and this getConfig service method is called from the component as for example:showConfig() {   this.configService.getConfig()     .subscribe((data: Config) => this.config = {         demoUrl: data['demoUrl'],         filename:  data['filename']     }); }The service method returns an Observable object of configuration data. Thus the component subscribes to the method's return value. The subscription callback copies the data fields into the component's config object, which binds the data in the component template for display.We can get the Observable object of typed HTtpResponse from the HttpClient as follows:getConfigResponse(): Observable<HttpResponse<Config>> {   return this.http.get<Config>(     this.configUrl, { observe: 'response' }); }Here, HttpClient.get() returns an Observable object of typed HttpResponse rather than just the JSON data.The showConfigResponse() method of the component displays the response headers as well as the configuration.Create Error HandlerHttpClient returns an error object instead of a successful response if a request fails on the server. This usually happens due to a request fail, poor network connection or other network related issues.We can create an error handler component by adding a second callback to the.subscribe():showConfig() {   this.configService.getConfig()     .subscribe(       (data: Config) => this.config = { ...data }, // success path       error => this.error = error // error path     ); }We can give different reasons and feedback for a failed request, but displaying the error object can also help at times. @angular/common/http@angular/common/http implements an HTTP client API for Angular apps that relies on the XMLHttpRequest interface exposed by browsers.This includes testability features, typed request and response objects, request and response interception, observable APIs, and streamlined error handling.Entry point exportsNgModulesHttpClientJsonpModuleConfigures the dependency injector for HttpClient with supporting services for JSONP. Without this module, Jsonp requests reach the backend with method JSONP, where they are rejected.HttpClientModuleConfigures the dependency injector for HttpClient with supporting services for XSRF. Automatically imported by HttpClientModule.HttpClientXsrfModuleConfigures XSRF protection support for outgoing requests.ClassesHttpBackendA final HttpHandler which will dispatch the request via browser HTTP APIs to a backend.HttpClientPerforms HTTP requests. This service is available as an injectable class, with methods to perform HTTP requests. Each request method has multiple signatures, and the return type varies based on the signature that is called (mainly the values of observe and responseType).HttpErrorResponseA response that represents an error or failure, either from a non-successful HTTP status, an error while executing the request, or some other failure which occurred during the parsing of the response.HttpHandlerTransforms an HttpRequest into a stream of HttpEvents, one of which will likely be a HttpResponse.HttpHeaderResponseA partial HTTP response which only includes the status and header data, but no response body.HttpHeadersRepresents the header configuration options for an HTTP request. Instances are immutable. Modifying methods return a cloned instance with the change. The original object is never changed.HttpParamsAn HTTP request/response body that represents serialized parameters, per the MIME type application/x-www-form-urlencoded.HttpRequestAn outgoing HTTP request with an optional typed body.HttpResponseA full HTTP response, including a typed response body (which may be null if one was not returned).HttpResponseBaseBase class for both HttpResponse and HttpHeaderResponse.HttpUrlEncodingCodecProvides encoding and decoding of URL parameter and query-string values.HttpXhrBackendUses XMLHttpRequest to send requests to a backend server.HttpXsrfTokenExtractorRetrieves the current XSRF token to use with the next outgoing request.JsonpClientBackendProcesses an HttpRequest with the JSONP method, by performing JSONP style requests.JsonpInterceptorIdentifies requests with the method JSONP and shifts them to the JsonpClientBackend.XhrFactoryA wrapper around the XMLHttpRequest constructor.StructuresHttpDownloadProgressEventA download progress event.HttpEventTypeType enumeration for the different kinds of HttpEvent.HttpInterceptorIntercepts and handles an HttpRequest or HttpResponse.HttpParameterCodecA codec for encoding and decoding parameters in URLs.HttpProgressEventBase interface for progress events.HttpSentEventAn event indicating that the request was sent to the server. Useful when a request may be retried multiple times, to distinguish between retries on the final event stream.HttpUploadProgressEventAn upload progress event.HttpUserEventA user-defined event.TypesHTTP_INTERCEPTORSA multi-provider token that represents the array of registered HttpInterceptor objects.HttpEventUnion type for all possible events on the response stream.HttpClient vs HttpBackendHttpBackendA final HttpHandler which will dispatch the request via browser HTTP APIs to a backend.Interceptors sit between the HttpClient interface and the HttpBackend.When injected, HttpBackend dispatches requests directly to the backend, without going through the interceptor chain.abstract class HttpBackend implements HttpHandler {   abstract handle(req: HttpRequest<any>): Observable<HttpEvent<any>> }HttpClientPerforms HTTP requests. This service is available as an injectable class, with methods to perform HTTP requests. Each request method has multiple signatures, and the return type varies based on the signature that is called (mainly the values of observe and responseType).Note that the response Type options value is a String that identifies the single data type of the response. A single overload version of the method handles each response type. The value of responseType cannot be a union, as the combined signature could imply.class HttpClient { request(first: string | HttpRequest<any>, url?: string, options: { body?: any; headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   delete(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   get(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   head(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   jsonp<T>(url: string, callbackParam: string): Observable<T>   options(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   patch(url: string, body: any, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   post(url: string, body: any, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>   put(url: string, body: any, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any> }The request that we send by HttpClient always go through interceptor of Angular application and the request we send by HttpBackend will dispatch requests directly to the backend, without going through the interceptor chain. So we should use both HttpClient and HttpBackend to send a request.Citation.ConclusionThe above article included all the information that we need for working with APIs for Angular Applications. We worked with the Http module and HttpClient Module. We worked with services and efficient API Calls. We understood how to improve on calling API from an Angular Application.

Make API Calls the Right Way in Angular

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Make API Calls the Right Way in Angular

About Angular 10

Angular 10 is the latest version of the Angular family. Angular applications are basically designed for Single Page Applications, where all controls are required in single page architecture. Angular is an application based design framework, which provides the Cross-Platform development environment for creating the most sophisticated SPAs. These SPAs are efficient enough for giving us a component to view to model environment, making it an MVC or MVVM application.

Angular provides a Progressive Web App architecture, which means that an application made in Angular gives us an App like experience with high performance. Angular has zero-step installation, making its up- gradation using modern web capabilities possible, even if offline. Angular has the capability of making cross platform Desktop Apps which can be created using MAC, Windows, or Linux OS using the same Angular methods. Angular can be rendered in Node.js, Microsoft .Net, PHP and many other servers by giving the output in HTML-CSS format. This even optimizes the app for SEO. Angular framework turns our templates into JavaScript Virtual machine codes which are highly optimized. With Angular CLI, the Command Line tools, we can build and deploy Apps faster than ever before. Angular uses Karma for unit tests and Protractor for scenario tests making the applications made in Angular more stable.

Introduction

Front-end applications in today’s programming world use HTTP protocol modules to talk to the back-end services. Almost all current browsers make this communication via two ways, one by HTTPrequests through XMLHttpRequest interface and the other by the API fetch() method call.

We usually make API calls to remote HTTP servers via HttpClient module of Angular 10 by sending HTTP requests. HttpClient has methods which perform HTTP requests. HttpClient is an injectable class for Angular 10. The methods of this class have request methods with many types of signatures, and their return types are different based on the signature of the request, thus giving different response types.

We will study about HttpClient in detail in the subsequent passage.

The Common Mistake

One thing for sure is that we need to call APIs in our programs. If we do so correctly, it can help us in upgrading to further versions, in making test modules, and even in handling errors. One common mistake that programmers usually make is in the call they use API URLs directly in the component. This means that if the API URL is upgraded for any reason, we will need to upgrade the URLs in every single component.
For Eg: 

this.httpClient.get('https://www.userdomain.com/api_name/data/' + this.id);  

Again, since we are not validating the path by using the Path variable of Angular, this call might fail if the URL is not valid or not available for some reason. Also, changing the modules with direct call to httpClient will require a lot of changes in the component. 

Let us now talk about the right approach.

The Right Way

The correct approach towards calling APIs in Angular includes the following:

Create Constants

We need to set global variables in Angular 10 to access our variables synchronously throughout our application. 

We declare Global variables in a Global Constant file. Create a folder under src/app and name it config. To create the global constant file, name it constants.ts and place it under src/app/config folder.

Eg: create the following file: 
src/app/config/constants.ts 

Open this file and put global constants like API_ENDPOINT, API_MOCK_ENDPOINT etc. These global variables will be used later in Component files like AppComponent so that they can be used like Site constants.

 Declaring Global Constants

// Angular Modules
import { Injectable } from '@angular/core'; 
@Injectable() 
export class Constants {
public readonly API_ENDPOINT: string = ' https://www.userdomain.com/'; 
public readonly API_MOCK_ENDPOINT: string = 'https://www.userdomainmock.com/'; 
public static TitleOfSite: string = " Making API calls the Right Way by Monica"; 

An example of AppComponent using the above global constant is: 
Edit the app.component.ts file from the src/app folder: 
src/app/app.component.ts: 
import { Component, OnInit } from '@angular/core';
import{ Constants } from './config/constants'; 
@Component({ 
  selector: 'app-root', 
  templateUrl: './app.component.html', 
  styleUrls: ['./app.component.css'] 
}) 
export class AppComponent implements OnInit{ 
    title = Constants.TitleOfSite; 
    constructor() { 
        console.log(GlobalConstants.API_ENDPOINT); 
    } 
    ngOnInit() { 
        console.log(this.title); 
    } 

Run the project by navigating to the project folder and calling ng serve : 
$ cd hello-world  

$ ng serve  

The output will be seen in the browser: https://www.userdomain.com/ 

Making API calls the Right Way by Monica

Create a Service

To create a service in Angular 10, create a file api-http.service.ts in the src/app/core/services folder. 

Add the following code in the file as below: 

// Angular Modules 
import { Injectable } from '@angular/core'; 
import { HttpClient } from '@angular/common/http'; 
@Injectable() 
export class ApiHttpService { 
constructor( 
// Angular Modules 
private http: HttpClient 
) { } 
public get(url: string, options?: any) { 
return this.http.get(url, options); 
} 
public post(url: string, data: any, options?: any) { 
return this.http.post(url, data, options); 
} 
public put(url: string, data: any, options?: any) { 
return this.http.put(url, data, options); 
} 
public delete(url: string, options?: any) { 
return this.http.delete(url, options); 
} 
}

Use HTTPClient Library

HttpClient Library is present in @angular/common/http folder. The older version of HttpClient was in @angular/http folder, this is replaced by the upgraded version of the same in @angular/common/http folder.

Almost all current browsers communicate with APIs via two ways, one by HTTPrequests through XMLHttpRequest interface and the other by the API fetch() method call.

The HttpClient module is built over XMLHttpRequest interface. It wraps all the complexities of this interface and provides extra features like: RxJS Obervables, Interceptors for requests and responses, Typed requests and responses, Error handling techniques, ease in Testing Modules, etc. 

Benefits of using HTTPClient Library

HttpClient is a module found in angular/common/http module. HttpClient is based on XMLHttpRequest interface, the Interface which is commonly given by most modern browsers. HttpClient gives us a lot more than XMLHttpRequest, and some of its benefits are:

  1. It gives us ease in using testing modules.
  2. It has request and response objects in strongly typed manner.
  3. It has APIs Support with Observable class object.
  4. It provides error handling modules.

Importing the library

In order to use HttpClient library we need to import HttpClientModule from the @angular/common/http package and include the library in the imports array of the App module : 

Edit the AppModule file in the src/app folder and add the line on top of the file as follows: 

src/app/app.module.ts: 

import { HttpClientModule } from '@angular/common/http'; 

Configure HTTPClient

Including HttpClient: 

Once done, include HttpClientModule in the imports array of the app.module and further use it:

import { NgModule }         from '@angular/core';
import { BrowserModule }    from '@angular/platform-browser';
import { HttpClientModule } from '@angular/common/http';
@NgModule({
  imports: [
    BrowserModule,
    // import HttpClientModule after BrowserModule.
    HttpClientModule,
  ],
  declarations: [
    AppComponent,
  ],
  bootstrap: [ AppComponent ]
})
export class AppModule {}
…

Now we are ready to use the HttpClient library to send HTTP requests or receive API response. 

Injecting HttpClient into our Application: 

Once we have  imported HttpClientModule into the AppModule, we can inject the HttpClient into our application as:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
@Injectable()
export class ConfigService {
  constructor(private http: HttpClient) { }
}

Create a query parameter

Getting JSON data

In this example we will request JSON data from the API server. Here our app needs a configuration file on the server, config.json, that specifies resource URLs.

{
  "demoUrl": "api/demo",
  "filename": "assets/demotextfile.txt"
}
The ConfigService fetches this file with a get() method from HttpClient library as follows:

configUrl = 'assets/config.json';
getConfig() {
  return this.http.get(this.configUrl);
}

A component, such as ConfigComponent, injects the ConfigService and this getConfig service method is called from the component as for example:

showConfig() {
  this.configService.getConfig()
    .subscribe((data: Config) => this.config = {
        demoUrl: data['demoUrl'],
        filename:  data['filename']
    });
}

The service method returns an Observable object of configuration data. Thus the component subscribes to the method's return value. The subscription callback copies the data fields into the component's config object, which binds the data in the component template for display.

We can get the Observable object of typed HTtpResponse from the HttpClient as follows:

getConfigResponse(): Observable<HttpResponse<Config>> {
  return this.http.get<Config>(
    this.configUrl, { observe: 'response' });
}

Here, HttpClient.get() returns an Observable object of typed HttpResponse rather than just the JSON data.

The showConfigResponse() method of the component displays the response headers as well as the configuration.

Create Error Handler

HttpClient returns an error object instead of a successful response if a request fails on the server. This usually happens due to a request fail, poor network connection or other network related issues.

We can create an error handler component by adding a second callback to the.subscribe():

showConfig() {
  this.configService.getConfig()
    .subscribe(
      (data: Config) => this.config = { ...data }, // success path
      error => this.error = error // error path
    );
}

We can give different reasons and feedback for a failed request, but displaying the error object can also help at times. 

@angular/common/http

@angular/common/http implements an HTTP client API for Angular apps that relies on the XMLHttpRequest interface exposed by browsers.

This includes testability features, typed request and response objects, request and response interception, observable APIs, and streamlined error handling.

Entry point exports

NgModules
HttpClientJsonpModuleConfigures the dependency injector for HttpClient with supporting services for JSONP. Without this module, Jsonp requests reach the backend with method JSONP, where they are rejected.
HttpClientModuleConfigures the dependency injector for HttpClient with supporting services for XSRF. Automatically imported by HttpClientModule.
HttpClientXsrfModuleConfigures XSRF protection support for outgoing requests.
Classes
HttpBackendA final HttpHandler which will dispatch the request via browser HTTP APIs to a backend.
HttpClientPerforms HTTP requests. This service is available as an injectable class, with methods to perform HTTP requests. Each request method has multiple signatures, and the return type varies based on the signature that is called (mainly the values of observe and responseType).
HttpErrorResponseA response that represents an error or failure, either from a non-successful HTTP status, an error while executing the request, or some other failure which occurred during the parsing of the response.
HttpHandlerTransforms an HttpRequest into a stream of HttpEvents, one of which will likely be a HttpResponse.
HttpHeaderResponseA partial HTTP response which only includes the status and header data, but no response body.
HttpHeadersRepresents the header configuration options for an HTTP request. Instances are immutable. Modifying methods return a cloned instance with the change. The original object is never changed.
HttpParamsAn HTTP request/response body that represents serialized parameters, per the MIME type application/x-www-form-urlencoded.
HttpRequestAn outgoing HTTP request with an optional typed body.
HttpResponseA full HTTP response, including a typed response body (which may be null if one was not returned).
HttpResponseBaseBase class for both HttpResponse and HttpHeaderResponse.
HttpUrlEncodingCodecProvides encoding and decoding of URL parameter and query-string values.
HttpXhrBackendUses XMLHttpRequest to send requests to a backend server.
HttpXsrfTokenExtractorRetrieves the current XSRF token to use with the next outgoing request.
JsonpClientBackendProcesses an HttpRequest with the JSONP method, by performing JSONP style requests.
JsonpInterceptorIdentifies requests with the method JSONP and shifts them to the JsonpClientBackend.
XhrFactoryA wrapper around the XMLHttpRequest constructor.
Structures
HttpDownloadProgressEventA download progress event.
HttpEventTypeType enumeration for the different kinds of HttpEvent.
HttpInterceptorIntercepts and handles an HttpRequest or HttpResponse.
HttpParameterCodecA codec for encoding and decoding parameters in URLs.
HttpProgressEventBase interface for progress events.
HttpSentEventAn event indicating that the request was sent to the server. Useful when a request may be retried multiple times, to distinguish between retries on the final event stream.
HttpUploadProgressEventAn upload progress event.
HttpUserEventA user-defined event.
Types
HTTP_INTERCEPTORSA multi-provider token that represents the array of registered HttpInterceptor objects.
HttpEventUnion type for all possible events on the response stream.

HttpClient vs HttpBackend

HttpBackend

A final HttpHandler which will dispatch the request via browser HTTP APIs to a backend.

Interceptors sit between the HttpClient interface and the HttpBackend.

When injected, HttpBackend dispatches requests directly to the backend, without going through the interceptor chain.

abstract class HttpBackend implements HttpHandler {
  abstract handle(req: HttpRequest<any>): Observable<HttpEvent<any>>
}

HttpClient

Performs HTTP requests. This service is available as an injectable class, with methods to perform HTTP requests. Each request method has multiple signatures, and the return type varies based on the signature that is called (mainly the values of observe and responseType).

Note that the response Type options value is a String that identifies the single data type of the response. A single overload version of the method handles each response type. The value of responseType cannot be a union, as the combined signature could imply.

class HttpClient {
request(first: string | HttpRequest<any>, url?: string, options: { body?: any; headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  delete(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  get(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  head(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  jsonp<T>(url: string, callbackParam: string): Observable<T>
  options(url: string, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  patch(url: string, body: any, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  post(url: string, body: any, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
  put(url: string, body: any, options: { headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: HttpObserve; params?: HttpParams | { [param: string]: string | string[]; }; reportProgress?: boolean; responseType?: "arraybuffer" | ... 2 more ... | "json"; withCredentials?: boolean; } = {}): Observable<any>
}

The request that we send by HttpClient always go through interceptor of Angular application and the request we send by HttpBackend will dispatch requests directly to the backend, without going through the interceptor chain. So we should use both HttpClient and HttpBackend to send a request.

Citation.

Conclusion

The above article included all the information that we need for working with APIs for Angular Applications. We worked with the Http module and HttpClient Module. We worked with services and efficient API Calls. We understood how to improve on calling API from an Angular Application.

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Handling React Events - A Detailed Guide

Event handling essentially allows the user to interact with a webpage and do something specific when a certain event like a click or a hover happens. When the user interacts with the application, events are fired, for example, mouseover, key press, change event, and so on. The application must handle events and execute the code. In short, events are the actions to which javascript can respond.   The actions to which javascript can respond are called events. Handling events with react is  very similar to handling events in DOM elements. Below are some general events that you would see in and out when dealing with react based websites:  Clicking an element  Submitting a form Scrolling page Hovering an element  Loading a webpage Input field change User stroking a key Image loading Naming Events in React Handling events with react is very similar to handling events in DOM elements, although there are some syntactic differences.   React events are written in camelCase.   A function is passed as the event handler rather than string. The way to write events in html / DOM is below:        click me onclick is written in lower case in html as shown above and what action to take when this onclick event triggers is taken care of by handleClick.In React, events are named using camel case and you pass a function as event handler as shown below:  Like in a functional component, event is written like below:       click me   In class based component ,event is written like below        click me Defining Events:Events are normally used in combination with functions, and the function is not executed until the event occurs, and the combination of event, HTML element, and javascript function is called binding which means to map all three. Generic syntax is:      Example:  Create a button element and what happens when onClick event triggered is driven by the function which is func() shown below     click me Let’s see some of the event attributes:   onmouseover : The mouse is moved over an element onmouseup : The mouse button is released onmouseout : The mouse  is moved off an element onmousemove: The mouse is moved Onmousedown: mouse button is pressed  onload : A image is done loading onunload: Existing the page  onblur : Losing Focus  on element  onchange : Content of a field changes onclick: Clicking an object  ondblclick: double clicking an object  onfocus element getting a focus  Onkeydown: pushing a keyboard key Onkeyup: keyboard key is released Onkeypress: keyboard key is pressed  Onselect: text is selected These are some examples of events:                                         Events                               function testApp (){                        alert((“Hello Event”);                                                   test Clicked                  test double Clicked                     Synthetic Events When you specify an event in JSX, you are not directly dealing with regular DOM events, you are dealing with a react event type called a synthetic event.It's a simple wrapper for native event instances and every synthetic event created needs to be garbage-collected which can be resource intensive in terms of CPU. The synthetic event object has properties mentioned below:  Boolean isTrusted  DOMEvent nativeEvent number timeStamp   void preventDefault() number eventPhase Synthetic events provide an interface and reduce browser inconsistencies and the event contains required information for its propagation to work. Synthetic event is reused for performance reasons in the browser, A synthetic event is a cross-browser wrapper around the browser’s native event it has the same interface as the native event. Synthetic events are delegated to the document node. Therefore native events are triggered first and the events bubble up to document, after which the synthetic events are triggered. The synthetic event object will be reused and all the properties will be nullified after the event callback has been invoked and this is for performance reasons.The workflow of synthetic event in react is:    Element ---- > Event ---- > synthetic event  ---- > handler(e)                                |                                                      |                                |  _______  Component ________|  umber timeStamp The Basics of React Event Handling Let’s explore how to handle events in react and we will showcase the click event and how it holds good for other types of events. Let’s start with functional components by creating a  file as clickAppHandler.js.In this file let’s create a  functional component  as shown below                        Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler  When onClick event triggers clickHandler function is called as shown below and when you click the button console will print the string “clicked” After this you need the add a component in the app component. In our code above you can see on click we pass the function as event handler and you will notice that we haven't added parentheses as it becomes a function, and we do not want that and we want handler to be a function not a function call. When a new component is rendered its event handler functions are added to the mapping maintained by the react.When the event is triggered and it hits and DOM object ,react maps the event to the handler, if it matches it calls the handler. The event handling in react is declarative and the advantage of declarative way to handlers is that they are part of the User interface structure.  Let’s take a look at event handling in class components                       Import React, { Component } from ‘...react’                         class TestApp extends Component {                              clickHandler() {                                  console.log(“clicked”)                                }                                render(){                                      return(                                                                                     Click me                                                                                 )                                 }                            }                       export default TestApp You cannot return false to prevent default behaviour in React. You must call preventDefault explicitly.  In HTML it looks like below:    Click Output: It will print “Clicked”  And in React, like this:  function clickHandle(e) {       e.preventDefault();       console.log(“Handled”);   }  Click  Output : console will print “Handled”  There are some  event handlers triggered by an event in the bubbling phase which is the same as with the normal DOM API; simply attach a handler to an eventual parent of an element and any events triggered on that element will bubble to the parent as long as it's not stopped via stopPropagation along the way   Click me  Below are some of the event handlers triggered in the bubbling phase:  MouseEvents           onClick           onDrag          onDoubleClick Keyboard Events                    onKeyDown                    onKeyPress                    onKeyUp Focus Events                  onFocus   onBlur To capture an event handler for the capture phase, append capture to the event name. For example, instead of using onClick, use onClickCapture to handle the click event.  Capture event example:                  Click me    Additional ExamplesExample1                       Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler   Example2       This example is along with HTML in a single file                                                            Events                               function testApp (){                        alert((“Hello Event”);                                                   test Clicked                  test double Clicked                     Adding Events: Below example is how you add an event. Highlighted in bold                      Import React from ‘...react’                         function clickAppHandler() {                                function clickHandler() {                                        console.log(‘clicked’)                                         }                                  return (                                                                                  Click                                                                          )                         }                       export default clickAppHandler  Passing Arguments to Event HandlerThere are two ways arguments are passed to event handler  Arrow function                    this.handleClick(id,e)}>Click                onClick is the event                e is the event object                 id can be state or props or some data Bind method      Click  In this case event object is automatically passed In both methods e represents the react event and its passed after the ID as second argument,With an arrow function this event e is passed explicitly but with bind method its automatically passed.                                     Import React,{ Component } from “react”;                                         class TestApp extends Component {                                           state = {                                                       id: 2,                                                      Name: “TestApp Dummy”                                                };                                                             //arrow function                                                 handleClick = (id,e) => {                                                       console.log(id);                                                       console.log(e);                                                  };                                               handleArg = (e) => { this.handleClick(this.state.id,e);}                                                          render() {     return (                    TestApp,{this.state.name}            onClick={this.handleArg}>Display            );   }  }  The react event is an object and obtained from react. Instead of creating a separate function for passing argument, you can directly pass the anonymous arrow function as shown in the render function below:     render() {        return (                                                                                                       TestApp,{this.state.name}                                                {                           this.handleClick(this.state.id,e);                                                               }}>Display                                                                                                         );                                                 }                                            }    Output:   click on button  “TestApp Dummy “                   Let’s see only how bind method looks like in the render function    render() {                                         return (                                                                                                 TestApp,{this.state.name}                                                   Display                                                                                                       );                                                  }                                              } Output: this will display the h1 tag and when you click the button handleClick function gets invoked and the console will display id of the state object as shown above. Building a Practice to Thoroughly Understand Events This blog focuses on event handling, which in turn teaches about event handlers declared in JSX markup.This approach helps in tracking down the element mapped with events in an easy way.  We also learned how to handle multiple event handlers in a single element by using JSX attributes.we also learned about ways to bind event handler and  parameter values. Then we learned about synthetic events which are abstractions around native events. The best way you can retain this learning is by practicing more and tackling the complexities that may arise as you practice. You can find several tutorials on the internet or share your questions with us here. Happy learning! 
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Handling React Events - A Detailed Guide

Event handling essentially allows the user to inte... Read More

MongoDB Query Document Using Find() With Example

MongoDB's find() method selects documents from a collection or view and returns a cursor to those documents. There are two parameters in this formula: query and projection.Query – This is an optional parameter that specifies the criteria for selection. In simple terms, a query is what you want to search for within a collection.Projection – This is an optional parameter that specifies what should be returned if the query criteria are satisfied. In simple terms, it is a type of decision-making that is based on a set of criteria.MongoDB's Flexible SchemaA NoSQL database, which stands for "not only SQL," is a way of storing and retrieving data that is different from relational databases' traditional table structures (RDBMS).When storing large amounts of unstructured data with changing schemas, NoSQL databases are indeed a better option than RDBMS. Horizontal scaling properties of NoSQL databases allow them to store and process large amounts of data.These are intended for storing, retrieving, and managing document-oriented data, which is frequently stored in JSON format (JavaScript Object Notation). Document databases, unlike RDBMSs, have a flexible schema that is defined by the contents of the documents.MongoDB is one of the most widely used open-source NoSQL document databases. MongoDB is known as a 'schemaless' database because it does not impose a specific structure on documents in a collection.MongoDB is compatible with a number of popular programming languages. It also offers a high level of operational flexibility because it scales well horizontally, allowing data to be spread or 'sharded' across multiple commodity servers with the ability to add more servers as needed. MongoDB can be run on a variety of platforms, including developer laptops, private clouds, and public clouds.Querying documents using find()MongoDB queries are used to retrieve or fetch data from a MongoDB database. When running a query, you can use criteria or conditions to retrieve specific data from the database.The function db.collection is provided by MongoDB. find() is a function that retrieves documents from a MongoDB database.In MongoDB, the find method is used to retrieve a specific document from the MongoDB collection. In Mongo DB, there are a total of six methods for retrieving specific records.find()findAndModify()findOne()findOneAndDelete()findOneAndReplace()findOneAndUpdate()Syntax:find(query, projection)We can fetch a specific record using the Find method, which has two parameters. If these two parameters are omitted, the find method will return all of the documents in the MongoDB collection.Example:Consider an example of employees with the database of employee_id and employee_name and we will fetch the documents using find() method.First, create a database with the name “employees” with the following code:use employeesNow, create a collection “employee” with:db.createCollection("employee")In the next step we will insert the documents in the database:db.employee.insert([{employee_id: 101, employee_name: "Ishan"}, {employee_id: 102, employee_name: "Bhavesh"}, {employee_id: 103, employee_name: "Madan"}])Find all Documents:To get all the records in a collection, we need to use the find method with an empty parameter. In other words, when we need all the records, we will not use any parameters.db.employee.find()Output in Mongo ShellThe pretty() method can be used to display the results in a formatted manner.Syntax:db.COLLECTION_NAME.find().pretty()Let’s check our documents with pretty() method:Query FiltersWe will see examples of query operations using the db.collection.find() method in mongosh.We will use the employee collection in the employees database.db.employee.insert([{employee_id: 101, employee_name: "Ishan", age: 21, email_id: "ishanjain@gmail.com"}, {employee_id: 102, employee_name: "Bhavesh", age: 22, email_id: "bhaveshg@gmail.com"}, {employee_id: 103, employee_name: "Madan", age: 23, email_id: "madan@gmail.com"}])As we have seen earlier that to select all the documents in the database we pass an empty document as the query filter parameter to the find method.db.employee.find().pretty()Find the first document in a collection:db.employee.findOne()Find a document by ID:db.employee.findOne({_id : ObjectId("61d1ae0b56b92c20b423a5a7")})Find Documents that Match Query Criteriadb.employee.find({“age”: “22”})db.employee.find({"employee_name": "Madan"}).pretty()Sort Results by a Field:db.employee.find().sort({age: 1}).pretty()order by age, in ascending orderdb.employee.find().sort({age: -1}).pretty()order by age, in descending orderAND Conditions:A compound query can specify conditions for multiple fields in the documents in a collection. A logical AND conjunction connects the clauses of a compound query indirectly, allowing the query to select all documents in the collection that meet the specified conditions.In the following example, we will consider all the documents in the employee collection where employee_id equals 101 and age equals 21.db.employee.find({"employee_id": 101, "age": "21" }).pretty()Querying nested fieldsThe embedded or nested document feature in MongoDB is a useful feature. Embedded documents, also known as nested documents, are documents that contain other documents.You can simply embed a document inside another document in MongoDB. Documents are defined in the mongo shell using curly braces (), and field-value pairs are contained within these curly braces.Using curly braces, we can now embed or set another document inside these fields, which can include field-value pairs or another sub-document.Syntax:{ field: { field1: value1, field2: value2 } }Example:We have a database “nested” and in this database we have collection “nesteddoc”.The following documents will insert into the nesteddoc collection.db.nesteddoc.insertMany([ { "_id" : 1, "dept" : "A", "item" : { "sku" : "101", "color" : "red" }, "sizes" : [ "S", "M" ] }, { "_id" : 2, "dept" : "A", "item" : { "sku" : "102", "color" : "blue" }, "sizes" : [ "M", "L" ] }, { "_id" : 3, "dept" : "B", "item" : { "sku" : "103", "color" : "blue" }, "sizes" : "S" }, { "_id" : 4, "dept" : "A", "item" : { "sku" : "104", "color" : "black" }, "sizes" : [ "S" ] } ])Place the documents in the collection now. Also, take a look at the results:As a result, the nesteddoc collection contains four documents, each of which contains nested documents. The find() method can be used to access the collection's documents.db.nesteddoc.find()Specify Equality Condition:In this example, we will select the document from the nesteddoc query where dept equals “A”.db.nesteddoc.find({dept: "A"})Querying ArraysUse the query document {: } to specify an equality condition on an array, where is the exact array to match, including the order of the elements.The following query looks for all documents where the field tags value is an array with exactly two elements, "S" and "M," in the order specified:db.nesteddoc.find( { sizes: ["S", "M"] } )Use the $all operator to find an array that contains both the elements "S" and "M," regardless of order or other elements in the array:db.nested.find( { sizes: { $all: ["S", "M"] } } )Query an Array for an Element:The following example queries for all documents where size is an array that contains the string “S” as one of its elements:db.nesteddoc.find( { sizes: "S" } )Filter conditionsTo discuss the filter conditions, we will consider a situation that elaborates this. We will start by creating a collection with the name “products” and then add the documents to it.db.products.insertMany([ { _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }, { _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] }, { _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] }, { _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] }, { _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }])To check the documents, use db.products.find():$gt$gt selects documents with a field value greater than (or equal to) the specified value.db.products.find( { qty: { $gt: “20” } } )$gte:$gte finds documents in which a field's value is greater than or equal to (i.e. >=) a specified value (e.g. value.)db.products.find( { qty: { $gte: 20 } } )$lt:$lt selects documents whose field value is less than (or equal to) the specified value.db.products.find( { qty: { $lt: 25 } } )$lte:$lte selects documents in which the field's value is less than or equal to (i.e. =) the specified value.db.products.find( { qty: { $lte: 20 } } )Query an Array by Array Length:To find arrays with a specific number of elements, use the $size operator. For example, the following selects documents with two elements in the array.db.products.find( { "tags": {$size: 2} } )ProjectionIn MongoDB, projection refers to selecting only the data that is required rather than the entire document's data. If a document has five fields and you only want to show three of them, select only three of them.The find() method in MongoDB accepts a second optional parameter, which is a list of fields to retrieve, as explained in MongoDB Query Document. When you use the find() method in MongoDB, it displays all of a document's fields. To prevent this, create a list of fields with the values 1 or 0. The value 1 indicates that the field should be visible, while 0 indicates that it should be hidden.Syntax:db.COLLECTION_NAME.find({},{KEY:1})Example:We will consider the previous example of products collection. Run the below command on mongoshell to learn how projection works:db.products.find({},{"tags":1, _id:0})Keep in mind that the _id field is always displayed while executing the find() method; if you do not want this field to be displayed, set it to 0.Optimized FindingsTo retrieve a document from a MongoDB collection, use the Find method.Using the Find method, we can retrieve specific documents as well as the fields that we require. Other find methods can also be used to retrieve specific documents based on our needs.By inserting array elements into the query, we can retrieve specific elements or documents. To retrieve data for array elements from the collection in MongoDB, we can use multiple query operators.
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MongoDB Query Document Using Find() With Example

MongoDB's find() method selects documents from a c... Read More

Implementing MongoDb Map Reduce using Aggregation

Algorithms and applications in today's data-driven market collect data about people, processes, systems, and organisations 24 hours a day, seven days a week, resulting in massive amounts of data. The problem is figuring out how to process this massive amount of data efficiently without sacrificing valuable insights.What is Map Reduce? The MapReduce programming model comes to the rescue here. MapReduce, which was first used by Google to analyse its search results, has grown in popularity due to its ability to split and process terabytes of data in parallel, generating results faster. A (Key,value) pair is the basic unit of information in MapReduce. Before feeding the data to the MapReduce model, all types of structured and unstructured data must be translated to this basic unit. The MapReduce model, as the name implies, consists of two distinct routines: the Map-function and the Reduce-function.  MapReduce is a framework for handling parallelizable problems across huge files using a huge number of devices (nodes), which are collectively referred to as a cluster (if all nodes are on the same local network and use similar hardware) or a grid (if the nodes are shared across geographically and administratively distributed systems, and use more heterogeneous hardware).  When data stored in a filesystem (unstructured) or a database(structured) is processed, MapReduce can take advantage of data's locality, processing it close to where it's stored to reduce communication costs. Typically, a MapReduce framework (or system) consists of three operations: Map: Each worker node applies the map function to local data and saves the result to a temporary storage. Only one copy of the redundant input data is processed by a master node. Shuffle: worker nodes redistribute data based on output keys (produced by the map function), ensuring that all data associated with a single key is stored on the same worker node. Reduce: each group of output data is now processed in parallel by worker nodes, per key. This article will walk you through the Map-Reduce model's functionality step by step. Map Reduce in MongoDB The map-reduce operation has been deprecated since MongoDB 5.0. An aggregation pipeline outperforms a map-reduce operation in terms of performance and usability. Aggregation pipeline operators like $group, $merge, and others can be used to rewrite map-reduce operations. Starting with version 4.4, MongoDB provides the $accumulator and $function aggregation operators for map-reduce operations that require custom functionality. In JavaScript, use these operators to create custom aggregation expressions. The map and reduce functions are the two main functions here. As a result, the data is independently mapped and reduced in different spaces before being combined in the function and saved to the specified new collection. This mapReduce() function was designed to work with large data sets only. You can perform aggregation operations like max and avg on data using Map Reduce, which is similar to groupBy in SQL. It works independently and in parallel on data. Implementing Map Reduce with Mongosh (MongoDB Shell)  The db.collection.mapReduce() method in mongosh is a wrapper for the mapReduce command. The examples that follow make use of the db.collection.mapReduce(). Example: Create a collection ‘orders’ with these documents: db.orders.insertMany([     { _id: 1, cust_id: "Ishan Jain", ord_date: new Date("2021-11-01"), price: 25, items: [ { sku: "oranges", qty: 5, price: 2.5 }, { sku: "apples", qty: 5, price: 2.5 } ], status: "A" },     { _id: 2, cust_id: "Ishan Jain", ord_date: new Date("2021-11-08"), price: 70, items: [ { sku: "oranges", qty: 8, price: 2.5 }, { sku: "chocolates", qty: 5, price: 10 } ], status: "A" },     { _id: 3, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-08"), price: 50, items: [ { sku: "oranges", qty: 10, price: 2.5 }, { sku: "pears", qty: 10, price: 2.5 } ], status: "A" },     { _id: 4, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-18"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 5, cust_id: "Bhavesh Galav", ord_date: new Date("2021-11-19"), price: 50, items: [ { sku: "chocolates", qty: 5, price: 10 } ], status: "A"},     { _id: 6, cust_id: "Madan Parmar", ord_date: new Date("2021-11-19"), price: 35, items: [ { sku: "carrots", qty: 10, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },     { _id: 7, cust_id: "Madan Parmar", ord_date: new Date("2021-11-20"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 8, cust_id: "Abhresh", ord_date: new Date("2021-11-20"), price: 75, items: [ { sku: "chocolates", qty: 5, price: 10 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },     { _id: 9, cust_id: "Abhresh", ord_date: new Date("2021-11-20"), price: 55, items: [ { sku: "carrots", qty: 5, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 }, { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },     { _id: 10, cust_id: "Abhresh", ord_date: new Date("2021-11-23"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" }  ]) Apply a map-reduce operation to the orders collection to group them by cust_id, then add the prices for each cust_id: To process each input document, define the map function: this refers the document that the map-reduce operation is processing in the function. For each document, the function maps the price to the cust_id and outputs the cust_id and price. var mapFunction1 = function() {emit(this.cust_id, this.price);}; With the two arguments keyCustId and valuesPrices, define the corresponding reduce function: The elements of the valuesPrices array are the price values emitted by the map function, grouped by keyCustId. The valuesPrice array is reduced to the sum of its elements by this function. var reduceFunction1 = function(keyCustId, valuesPrices) {return Array.sum(valuesPrices);};Apply the mapFunction1 map function and the reduceFunction1 reduce function to all documents in the orders collection: db.orders.mapReduce(mapFunction1,reduceFunction1,{ out: "map_reduce_example" }) The results of this operation are saved in the map_reduce_example collection. If the map_reduce_example collection already exists, the operation will overwrite its contents with the map-reduce operation's results. Check the map_reduce_example collection to verify: db.map_reduce_example.find().sort( { _id: 1 } ) Aggregation Alternative:You can rewrite the map-reduce operation without defining custom functions by using the available aggregation pipeline operators: db.orders.aggregate([{$group: { _id:"$cust_id",value:{$sum: "$price" } } },{ $out: "agg_alternative_1" }]) Check the agg_alternative_1 collection to verify: db.agg_alternative_1.find().sort( { _id: 1 } )Implementing Map Reduce with Java Consider the collection car and insert the following documents in it. db.car.insert( [ {car_id:"c1",name:"Audi",color:"Black",cno:"H110",mfdcountry:"Germany",speed:72,price:11.25}, {car_id:"c2",name:"Polo",color:"White",cno:"H111",mfdcountry:"Japan",speed:65,price:8.5}, {car_id:"c3",name:"Alto",color:"Silver",cno:"H112",mfdcountry:"India",speed:53,price:4.5}, {car_id:"c4",name:"Santro",color:"Grey",cno:"H113",mfdcountry:"Sweden",speed:89,price:3.5} , {car_id:"c5",name:"Zen",color:"Blue",cno:"H114",mfdcountry:"Denmark",speed:94,price:6.5} ] ) You will get an output like this:  Let's now write the map reduce function on a collection of cars, grouping them by speed and classifying them as overspeed cars.  var speedmap = function (){  var criteria;  if ( this.speed > 70 ) {criteria = 'overspeed';emit(criteria,this.speed);}}; Based on the speed, this function classifies the vehicle as an overspeed vehicle. The term "this" refers to the current document that requires map reduction. var avgspeed_reducemap = function(key, speed) {       var total =0;       for (var i = 0; i 
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Implementing MongoDb Map Reduce using Aggregation

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