Interceptors for request modification in Angular

Introduction

Interceptors let you change or add information to every request before it is sent. This helps keep your code clean and consistent.

Add an authorization token to every request automatically.

Log or track outgoing requests for debugging.

Modify headers or parameters for all requests in one place.

Handle errors globally before they reach components.

Syntax

Angular

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class MyInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    const modifiedReq = req.clone({
      // modify request here, e.g. add headers
    });
    return next.handle(modifiedReq);
  }
}

The intercept method runs for every HTTP request.

Use req.clone() to create a changed copy without altering the original request.

Examples

Adds an Authorization header with a token to every request.

Angular

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  const token = 'my-token';
  const modifiedReq = req.clone({
    setHeaders: { Authorization: `Bearer ${token}` }
  });
  return next.handle(modifiedReq);
}

Logs the URL of each outgoing request without modifying it.

Angular

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  console.log('Request URL:', req.url);
  return next.handle(req);
}

Sample Program

This interceptor adds a bearer token to every HTTP request's headers automatically. You register it in your app module so Angular uses it for all requests.

Angular

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    const token = '12345';
    const authReq = req.clone({
      setHeaders: { Authorization: `Bearer ${token}` }
    });
    return next.handle(authReq);
  }
}

// Usage in app.module.ts providers array:
// { provide: HTTP_INTERCEPTORS, useClass: AuthInterceptor, multi: true }

Important Notes

Always return next.handle() with the (possibly modified) request to continue the request chain.

Register interceptors in the providers array with multi: true to allow multiple interceptors.

Summary

Interceptors let you change HTTP requests before sending.

Use req.clone() to safely modify requests.

Register interceptors globally to apply changes everywhere.

Practice

1. What is the main purpose of an HTTP interceptor in Angular?

easy

A. To modify HTTP requests before they are sent

B. To create new HTTP requests from scratch

C. To handle user input events

D. To style components dynamically

Solution

1. Step 1: Understand the role of interceptors

   Interceptors are designed to catch HTTP requests and responses to modify or handle them.

2. Step 2: Identify the correct purpose

   Among the options, only modifying HTTP requests before sending matches the interceptor's role.

3. Final Answer:

   To modify HTTP requests before they are sent -> Option A

4. Quick Check:

   Interceptor purpose = modify requests [OK]

Hint: Interceptors change requests before sending [OK]

Common Mistakes:

• Thinking interceptors create requests
• Confusing interceptors with event handlers
• Assuming interceptors style components

2. Which method is used to safely modify an HTTP request inside an Angular interceptor?

easy

A. req.clone()

B. req.change()

C. req.update()

D. req.modify()

Solution

1. Step 1: Recall Angular HTTP request immutability

   HTTP requests are immutable, so you cannot change them directly.

2. Step 2: Use the clone method to modify

   The correct way is to create a copy with changes using req.clone().

3. Final Answer:

   req.clone() -> Option A

4. Quick Check:

   Modify request = req.clone() [OK]

Hint: Use req.clone() to change requests safely [OK]

Common Mistakes:

• Trying to modify request directly
• Using non-existent methods like req.modify()
• Confusing clone with update or change

3. Given this interceptor code snippet, what will be the value of the 'Authorization' header in the outgoing request?

intercept(req, next) {
  const cloned = req.clone({
    setHeaders: { Authorization: 'Bearer token123' }
  });
  return next.handle(cloned);
}

medium

A. An error will occur due to incorrect syntax

B. No Authorization header will be set

C. 'token123'

D. 'Bearer token123'

Solution

1. Step 1: Analyze the clone method usage

   The code clones the request and sets the 'Authorization' header to 'Bearer token123'.

2. Step 2: Understand header setting effect

   The cloned request will have the header exactly as set, so the header value is 'Bearer token123'.

3. Final Answer:

   'Bearer token123' -> Option D

4. Quick Check:

   Header set to 'Bearer token123' [OK]

Hint: Headers set in clone appear in outgoing request [OK]

Common Mistakes:

• Assuming header is missing
• Confusing header value with token only
• Thinking syntax causes error

4. Identify the error in this Angular interceptor code:

intercept(req, next) {
  req.setHeaders({ 'Content-Type': 'application/json' });
  return next.handle(req);
}

medium

A. Missing return statement

B. req.setHeaders is not a function

C. next.handle should be called with a cloned request

D. Headers cannot be set in interceptors

Solution

1. Step 1: Check method existence on req

   The HttpRequest object is immutable and does not have a setHeaders method.

2. Step 2: Understand correct way to set headers

   Headers must be set by cloning the request with new headers using req.clone().

3. Final Answer:

   req.setHeaders is not a function -> Option B

4. Quick Check:

   setHeaders method does not exist [OK]

Hint: HttpRequest is immutable; no setHeaders method [OK]

Common Mistakes:

• Trying to call setHeaders directly
• Ignoring immutability of HttpRequest
• Thinking headers can't be changed at all

5. You want to add a custom header 'X-App-Version' with value '1.0' to all outgoing HTTP requests in your Angular app. Which is the best way to do this?

hard

A. Add the header manually in every HTTP call

B. Modify the HttpClient service to include the header

C. Create an interceptor that clones requests and sets the header, then register it globally

D. Use a component to add the header before each request

Solution

1. Step 1: Understand global header addition

   Manually adding headers everywhere is error-prone and repetitive.

2. Step 2: Use interceptor for centralized modification

   Interceptors allow modifying all requests in one place by cloning and setting headers, then registering globally.

3. Final Answer:

   Create an interceptor that clones requests and sets the header, then register it globally -> Option C

4. Quick Check:

   Global header addition = interceptor + clone + register [OK]

Hint: Use interceptor to add headers globally [OK]

Common Mistakes:

• Adding headers manually in every call
• Trying to modify HttpClient directly
• Using components for HTTP header changes