Interceptors for authentication tokens in Angular - Mini Project: Build & Apply

Interceptors for authentication tokens

📖 Scenario: You are building a simple Angular app that needs to send an authentication token with every HTTP request to a server. This token proves the user is logged in.To avoid adding the token manually in every request, you will create an HTTP interceptor. This interceptor will automatically add the token to the request headers.

🎯 Goal: Create an Angular HTTP interceptor that adds a fixed authentication token to the Authorization header of all outgoing HTTP requests.

📋 What You'll Learn

Create a constant token string called authToken with value 'Bearer abc123token'.

Create an Angular HTTP interceptor class called AuthInterceptor.

In the interceptor, add the Authorization header with the authToken to every outgoing HTTP request.

Register the interceptor as a provider in the Angular dependency injection system.

💡 Why This Matters

🌍 Real World

Most web apps need to send authentication tokens with API requests to prove the user is logged in. Interceptors automate this process.

💼 Career

Understanding Angular interceptors is essential for frontend developers working with secure APIs and authentication.

1

DATA SETUP: Create the authentication token constant

Create a constant string called authToken and set it to 'Bearer abc123token'.

Angular

# Create a constant authToken with value 'Bearer abc123token'
# Your code here

Hint

This token will be added to HTTP request headers later.

2

CONFIGURATION: Create the AuthInterceptor class skeleton

Create an Angular HTTP interceptor class called AuthInterceptor that implements HttpInterceptor. Import HttpInterceptor and HttpRequest from @angular/common/http. Add the method intercept(req: HttpRequest, next: HttpHandler) returning next.handle(req).

Angular

const authToken = 'Bearer abc123token';
// Create AuthInterceptor class implementing HttpInterceptor
// Your code here

Hint

Start with a basic interceptor that just passes requests through.

3

CORE LOGIC: Add the Authorization header with the token

Inside the intercept method, clone the incoming req and add a new header Authorization with the value of authToken. Then pass the cloned request to next.handle().

Angular

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

const authToken = 'Bearer abc123token';

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest, next: HttpHandler) {
    // Clone req and add Authorization header
    // Your code here
  }
}

Hint

Use req.clone() to add headers without changing the original request.

4

COMPLETION: Register the interceptor provider

Add a provider object to register AuthInterceptor as an HTTP interceptor. Use provide: HTTP_INTERCEPTORS, useClass: AuthInterceptor, and multi: true. Import HTTP_INTERCEPTORS from @angular/common/http.

Angular

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

const authToken = 'Bearer abc123token';

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest, next: HttpHandler) {
    const authReq = req.clone({
      setHeaders: { Authorization: authToken }
    });
    return next.handle(authReq);
  }
}

// Add provider registration below
// Your code here

Hint

This provider will be added to your Angular module's providers array.

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