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Mastering Sequential API Calls in Angular with RxJS for Smoother User Experiences

Introduction

Angular is a popular front-end framework that allows developers to build dynamic and responsive web applications. One common requirement in web development is making API calls to fetch and display data from a server. When dealing with complex applications, it’s often necessary to call multiple APIs in sequence, where the result of one API call depends on the response of the previous one. This can be achieved efficiently using RxJS (Reactive Extensions for JavaScript), which is a library for handling asynchronous operations in a more organized and manageable way. In this article, we will explore how to call APIs in sequence in Angular using RxJS.

Prerequisites

Before we dive into the example, make sure you have Angular and RxJS installed in your project. You can create a new Angular project using Angular CLI with the following command:

ng new angular-api-sequence

Install RxJS by running:

npm install rxjs

Calling APIs in Sequence — Step by Step

Let’s break down the process of calling APIs in sequence using Angular and RxJS into a few key steps.

Step 1: Import HttpClientModule and RxJS

In your Angular project, you need to import the HttpClientModule from @angular/common/http for making HTTP requests and RxJS operators to handle the asynchronous calls. Open your app.module.ts file and make the following changes:

import { HttpClientModule } from '@angular/common/http';
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';

@NgModule({
  declarations: [
    // ...
  ],
  imports: [
    BrowserModule,
    HttpClientModule, // Add this line
  ],
  bootstrap: [AppComponent],
})
export class AppModule {}

Step 2: Create an Angular Service

To keep your code organized, it’s a good practice to create a service that handles the API calls. Generate a service using the Angular CLI:

ng generate service api

This command will create an api.service.ts file in the src/app folder. In this service, we'll define methods to call APIs in sequence.

Step 3: Define API Endpoints

In your api.service.ts file, define the API endpoints that you want to call. For this example, let's assume you have two API endpoints that fetch user data and post data. You should replace these URLs with the actual endpoints you need in your project:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root',
})
export class ApiService {
  private baseUrl = 'https://api.example.com';
  constructor(private http: HttpClient) {}
  getUserData(): Observable<any> {
    const url = `${this.baseUrl}/users`;
    return this.http.get(url);
  }
  postData(data: any): Observable<any> {
    const url = `${this.baseUrl}/post`;
    return this.http.post(url, data);
  }
}

Step 4: Call APIs in Sequence using RxJS

Now, let’s create a method in the service to call the APIs in sequence. In this example, we’ll call getUserData first, and then based on the response, we'll call postData. We'll use RxJS operators like switchMap to achieve this:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';
import { switchMap } from 'rxjs/operators';

@Injectable({
  providedIn: 'root',
})
export class ApiService {
  private baseUrl = 'https://api.example.com';
  constructor(private http: HttpClient) {}
  getUserData(): Observable<any> {
    const url = `${this.baseUrl}/users`;
    return this.http.get(url);
  }
  postData(data: any): Observable<any> {
    const url = `${this.baseUrl}/post`;
    return this.http.post(url, data);
  }
  callApisInSequence(): Observable<any> {
    return this.getUserData().pipe(
      switchMap((userData) => {
        // Do something with userData if needed
        const postData = { /* your data to post */ };
        return this.postData(postData);
      })
    );
  }
}

Step 5: Consume the Service in Your Component

Now that the service is ready to call APIs in sequence, you can consume it in your Angular component. Here’s a simple example of how to do that:

import { Component } from '@angular/core';
import { ApiService } from './api.service';

@Component({
  selector: 'app-root',
  template: `
    <button (click)="callApis()">Call APIs</button>
    <div *ngIf="result">{{ result | json }}</div>
  `,
})
export class AppComponent {
  result: any;

  constructor(private apiService: ApiService) {}

  callApis() {
    this.apiService.callApisInSequence().subscribe((data) => {
      this.result = data;
    });
  }
}

In this example, when the “Call APIs” button is clicked, the callApisInSequence method from the service is called, and the result is displayed in the template.

Conclusion

In this article, we’ve learned how to call APIs in sequence in Angular using RxJS. By using RxJS operators like switchMap, you can efficiently manage the flow of asynchronous API calls, making your code more readable and maintainable. This approach is particularly useful in scenarios where one API call depends on the result of a previous call. Remember to replace the example URLs and data with your actual API endpoints and requirements. Angular, combined with RxJS, provides a powerful toolset for handling complex asynchronous operations in your web applications.

Angular
Typescript
JavaScript
Tutorial
Programming
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