Interceptors for authentication tokens in Angular

Introduction

Interceptors help add authentication tokens to your app's requests automatically. This keeps your app secure without repeating code.

When you want to send a login token with every request to a server.

When you need to refresh tokens before sending requests.

When you want to handle errors like expired tokens globally.

When you want to add headers or modify requests easily.

When you want to keep your components clean from authentication details.

Syntax

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 = 'your-auth-token';
    const clonedReq = req.clone({
      setHeaders: {
        Authorization: `Bearer ${token}`
      }
    });
    return next.handle(clonedReq);
  }
}

The intercept method runs on every HTTP request.

Use req.clone() to avoid changing the original request.

Examples

This example adds the token only if it exists in local storage.

Angular

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  const token = localStorage.getItem('authToken');
  if (token) {
    const clonedReq = req.clone({
      setHeaders: { Authorization: `Bearer ${token}` }
    });
    return next.handle(clonedReq);
  }
  return next.handle(req);
}

This example adds a custom header instead of an auth token.

Angular

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
  const clonedReq = req.clone({
    setHeaders: { 'Custom-Header': 'MyValue' }
  });
  return next.handle(clonedReq);
}

Sample Program

This interceptor checks if a token exists in local storage. If yes, it adds it as a Bearer token in the Authorization header for every HTTP request. This way, your app sends the token automatically without extra code in components.

Angular

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent, HTTP_INTERCEPTORS } 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 = localStorage.getItem('authToken');
    if (token) {
      const clonedReq = req.clone({
        setHeaders: { Authorization: `Bearer ${token}` }
      });
      return next.handle(clonedReq);
    }
    return next.handle(req);
  }
}

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

Important Notes

Remember to register your interceptor in the app module providers.

Interceptors run in the order they are provided.

Do not modify the original request directly; always clone it.

Summary

Interceptors add tokens to HTTP requests automatically.

They keep your app secure and code clean.

Always clone requests before modifying headers.

Practice

1. What is the main purpose of an HTTP interceptor in Angular when dealing with authentication tokens?

easy

A. To create new HTTP requests manually

B. To automatically add authentication tokens to outgoing HTTP requests

C. To block all HTTP requests from the app

D. To change the URL of HTTP requests

Solution

1. Step 1: Understand the role of interceptors

   Interceptors are used to modify HTTP requests or responses globally in Angular.

2. Step 2: Identify the use of tokens in interceptors

   They commonly add authentication tokens to requests to keep the app secure without repeating code.

3. Final Answer:

   To automatically add authentication tokens to outgoing HTTP requests -> Option B

4. Quick Check:

   Interceptors add tokens = B [OK]

Hint: Interceptors modify requests automatically, especially for tokens [OK]

Common Mistakes:

• Thinking interceptors create requests
• Believing interceptors block requests
• Assuming interceptors change URLs

2. Which of the following is the correct way to clone an HTTP request before adding an authentication token in an Angular interceptor?

easy

A. request.updateHeaders({ Authorization: 'Bearer ' + token })

B. request.headers.set('Authorization', 'Bearer ' + token)

C. request.modifyHeaders('Authorization', 'Bearer ' + token)

D. request.clone({ headers: request.headers.set('Authorization', 'Bearer ' + token) })

Solution

1. Step 1: Recall Angular HttpRequest immutability

   HttpRequest objects are immutable, so you must clone them to modify headers.

2. Step 2: Use the clone method with headers.set()

   The correct syntax is to call clone() and set the new header inside it.

3. Final Answer:

   request.clone({ headers: request.headers.set('Authorization', 'Bearer ' + token) }) -> Option D

4. Quick Check:

   Clone request with headers.set() = A [OK]

Hint: Always clone requests before changing headers [OK]

Common Mistakes:

• Trying to modify headers directly without cloning
• Using non-existent methods like modifyHeaders or updateHeaders
• Not using the clone method

3. Given this Angular interceptor code snippet, what will be the Authorization header value in the outgoing HTTP request if the token is 'abc123'?

intercept(request: HttpRequest<any>, next: HttpHandler) {
  const token = 'abc123';
  const cloned = request.clone({
    headers: request.headers.set('Authorization', `Bearer ${token}`)
  });
  return next.handle(cloned);
}

medium

A. Authorization: Bearer abc123

B. Authorization: abc123

C. Authorization: Bearer token

D. No Authorization header will be added

Solution

1. Step 1: Check how the token is added

   The code sets the Authorization header to 'Bearer ' plus the token string 'abc123'.

2. Step 2: Understand string interpolation

   The template string `Bearer ${token}` becomes 'Bearer abc123'.

3. Final Answer:

   Authorization: Bearer abc123 -> Option A

4. Quick Check:

   Header value = 'Bearer abc123' [OK]

Hint: Template strings insert token value directly [OK]

Common Mistakes:

• Forgetting 'Bearer ' prefix
• Using the literal word 'token' instead of variable
• Assuming header is not added

4. Identify the error in this Angular interceptor code that tries to add an auth token:

intercept(req: HttpRequest<any>, next: HttpHandler) {
  const token = 'xyz';
  req.headers.set('Authorization', 'Bearer ' + token);
  return next.handle(req);
}

medium

A. Headers are immutable; must clone request before setting headers

B. Token variable is not defined

C. next.handle() is missing a return statement

D. Authorization header should be lowercase

Solution

1. Step 1: Understand HttpRequest immutability

   HttpRequest headers cannot be changed directly; they are immutable.

2. Step 2: Correct way is to clone the request with new headers

   You must clone the request and set headers on the clone before passing it on.

3. Final Answer:

   Headers are immutable; must clone request before setting headers -> Option A

4. Quick Check:

   Immutable headers require cloning = A [OK]

Hint: Headers are immutable; clone request before modifying [OK]

Common Mistakes:

• Trying to set headers directly on request
• Ignoring immutability of HttpRequest
• Not returning next.handle()

5. You want to create an Angular interceptor that adds an authentication token only to requests targeting your API domain 'api.example.com'. Which approach correctly implements this conditional logic?

hard

A. Modify the token inside next.handle() without cloning

B. Add the token to all requests unconditionally

C. Check if request.url.includes('api.example.com') before cloning and adding the token

D. Use a global variable to store the token and add it after the request completes

Solution

1. Step 1: Identify the need for conditional token addition

   Only requests to 'api.example.com' should have the token added to avoid leaking tokens to other domains.

2. Step 2: Use URL check before cloning and modifying request

   Check request.url.includes('api.example.com') and then clone the request with the token header.

3. Step 3: Avoid modifying requests without cloning or adding tokens globally

   Modifying without cloning or adding tokens to all requests is insecure or incorrect.

4. Final Answer:

   Check if request.url.includes('api.example.com') before cloning and adding the token -> Option C

5. Quick Check:

   Conditional token addition by URL check = C [OK]

Hint: Add tokens only after checking request URL [OK]

Common Mistakes:

• Adding tokens to all requests blindly
• Modifying requests without cloning
• Trying to add tokens after request completes