Understanding HTTP Interceptors in Angular
Angular is a powerful framework that allows developers to build large-scale and complex web applications with ease. A fundamental part of any web application is the ability to communicate with the server. In Angular, this communication is achieved using HTTP requests. HTTP requests are used to send and receive data from the server. However, sometimes we need to modify, intercept or control HTTP requests and responses. That’s where HTTP interceptors come in.
HTTP interceptors are a feature in Angular that allow you to intercept HTTP requests and responses. An interceptor sits between your Angular application and the server and can modify or replace the request and response objects before they reach their final destination. This gives you the opportunity to modify the request headers, add authentication to the request, or transform the response data.
There are many use cases for HTTP interceptors, ranging from adding global headers to every request to handling token-based authentication. Understanding how to use HTTP interceptors in Angular is a critical skill for any developer looking to build complex web applications.
Defining an HTTP interceptor
To define an HTTP interceptor, we need to create a class that implements the HttpInterceptor interface and override its methods. The HttpInterceptor interface includes two methods, `intercept()` and `handle()`.
The `handle()` method sends the request on to the server and returns an `Observable` of the response. The `intercept()` method takes a request and a next `HttpHandler` object that will eventually send the request to the server. The `intercept()` method can manipulate the request before sending it and then return the `Observable` of the response.
Here’s an example of a simple interceptor that adds a header to every request:
“`
import { Injectable } from ‘@angular/core’;
import { HttpEvent, HttpInterceptor, HttpHandler, HttpRequest } from ‘@angular/common/http’;
import { Observable } from ‘rxjs’;
@Injectable()
export class AuthInterceptor implements HttpInterceptor {
intercept(req: HttpRequest
const authReq = req.clone({
setHeaders: {
Authorization: ‘Bearer your-auth-token’
}
});
return next.handle(authReq);
}
}
“`
In this example, we’ve created a new class `AuthInterceptor` that implements the `HttpInterceptor` interface. The `intercept()` method takes the request object and a `next` handler object and creates a new request object that includes an `Authorization` header with a bearer token. Finally, the handler sends the modified request to the server and returns an `Observable` of the response.
Using an HTTP interceptor in Angular
Once we’ve defined our interceptor, we need to add it to our Angular module’s `providers` array so that Angular knows to use it.
In our application module, we can add the `AuthInterceptor` to the `providers` array like this:
“`
import { NgModule } from ‘@angular/core’;
import { HTTP_INTERCEPTORS } from ‘@angular/common/http’;
import { AuthInterceptor } from ‘./auth-interceptor’;
@NgModule({
providers: [
{ provide: HTTP_INTERCEPTORS, useClass: AuthInterceptor, multi: true }
]
})
export class AppModule { }
“`
In this example, we’ve imported `HTTP_INTERCEPTORS` and `AuthInterceptor` and added `AuthInterceptor` to the `providers` array using the `HTTP_INTERCEPTORS` token. The `multi: true` option tells Angular to merge our interceptor with any other interceptors that we might have defined.
Once we’ve defined and registered our interceptor in our application module, it will be applied to every HTTP request that our application makes.
Frequently Asked Questions (FAQs)
Q: Why do I need an HTTP interceptor?
A: HTTP interceptors are used to modify or control HTTP requests and responses. They can be used to add headers to every request, handle errors, or add authentication.
Q: How do I create an HTTP interceptor in Angular?
A: To create an HTTP interceptor, you need to create a class that implements the `HttpInterceptor` interface and override its methods.
Q: How do I register an HTTP interceptor in Angular?
A: To register an HTTP interceptor in Angular, you need to add it to the `providers` array in your Angular module using the `HTTP_INTERCEPTORS` token.
Q: Can I use more than one HTTP interceptor in my application?
A: Yes, you can use more than one HTTP interceptor in your application. You can add multiple interceptors to the `providers` array and Angular will apply them in the order that they are specified.
Q: Are there any limitations to using HTTP interceptors in Angular?
A: HTTP interceptors can add additional overhead to your application and can impact performance. Additionally, any modifications made to requests or responses can have unintended consequences if not handled carefully.
Conclusion
HTTP interceptors are a powerful feature in Angular that allow developers to modify and control HTTP requests and responses. They can be used for a wide range of use cases, from adding authentication to handling errors. By understanding how to create and use HTTP interceptors in Angular, you can build more robust and flexible web applications.