Interceptors for request modification in Angular - Mini Project: Build & Apply

Interceptors for request modification

📖 Scenario: You are building an Angular app that talks to a server. Sometimes you need to add extra information to every request, like a secret token.Interceptors help you change requests before they go out.

🎯 Goal: Create an Angular HTTP interceptor that adds a custom header X-Custom-Token with value abc123 to every outgoing HTTP request.

📋 What You'll Learn

Create an Angular interceptor class named AuthInterceptor

Add the header X-Custom-Token with value abc123 to all HTTP requests

Register the interceptor in the providers array using HTTP_INTERCEPTORS

Use the intercept method with parameters req and next

💡 Why This Matters

🌍 Real World

Interceptors are used in real Angular apps to add tokens, handle errors, or log requests automatically.

💼 Career

Understanding interceptors is important for Angular developers working on secure and maintainable web applications.

1

Create the interceptor class

Create an Angular class called AuthInterceptor that implements HttpInterceptor. Import HttpInterceptor from @angular/common/http. Add the method intercept(req, next) that returns next.handle(req).

Angular

import { HttpInterceptor } from '@angular/common/http';

// Create the AuthInterceptor class implementing HttpInterceptor
// Your code here

Hint

Remember to import HttpInterceptor and create the intercept method that returns next.handle(req).

2

Add the custom header

Inside the intercept method, clone the req object and add a header X-Custom-Token with value abc123. Use req.clone({ setHeaders: { 'X-Custom-Token': 'abc123' } }) and assign it to a variable modifiedReq. Then pass modifiedReq to next.handle().

Angular

import { HttpInterceptor } from '@angular/common/http';

export class AuthInterceptor implements HttpInterceptor {
  intercept(req, next) {
    // Clone req and add header X-Custom-Token with value abc123
    // Your code here
  }
}

Hint

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

3

Register the interceptor provider

In your Angular module or component providers array, add a provider object with provide: HTTP_INTERCEPTORS, useClass: AuthInterceptor, and multi: true. Import HTTP_INTERCEPTORS from @angular/common/http.

Angular

import { HTTP_INTERCEPTORS } from '@angular/common/http';

// Add the interceptor provider to the providers array
// Your code here

Hint

Use multi: true to allow multiple interceptors.

4

Add the provider to the module

Add the interceptorProvider to the providers array of your Angular module or component decorator. For example, in @NgModule({ providers: [interceptorProvider] }).

Angular

import { NgModule } from '@angular/core';

@NgModule({
  providers: [
    // Add interceptorProvider here
    // Your code here
  ]
})
export class AppModule {}

Hint

Put interceptorProvider inside the providers array in your module.

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