Angular — HTTP Interceptors

🅰️ Angular 18+ 🟢 Chapter 29 of 50 📂 Phase 10: HttpClient and APIs 📅 2026 Edition

📌 Covered in this chapter: Interceptor ante enti? · Functional interceptor · Request logging · Auth token · Global error handling · Loading indicator · Retry logic · Cache interceptor · Request headers · Interceptor testing

Welcome to Angular — HTTP Interceptors in our Angular Complete Masterclass! Intercept outbound HTTP requests using functional HttpInterceptorFn to inject Bearer auth tokens and handle global errors.

1Simple Introduction

In Angular web development, understanding HTTP Interceptors is essential for building structured, scalable frontend applications. Angular components and directives participate in Angular's reactive system and dependency injection system seamlessly.

2What You Will Learn

📚 Learning Objectives:

Master TypeScript mechanics and Angular decorators for HTTP Interceptors
Understand templates, data binding, Signals, and dependency injection
Implement clean, production-ready Angular components and services
Avoid common binding mistakes, subscription memory leaks, and change detection pitfalls

3Why HTTP Interceptors is Useful

💡 Practical Utility

Angular components and directives dependency injection system lo automatically participate chestayi; dependencies ni inject chesi use cheyyachu. Mastering HTTP Interceptors gives you full control over dynamic user interfaces in enterprise web apps.

4Required Component / Service Design

Target Class / Artifact: AuthInterceptor. Configured using Angular metadata decorators (e.g. @Component, @Injectable, @Directive, @Pipe).

5Syntax & Mechanism

Mechanism: HttpInterceptor. Target File: src/app/interceptors/auth.interceptor.ts.

6Basic Example Code

Angular Component / Service Code

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

export const authInterceptor: HttpInterceptorFn = (req, next) => {
  const token = localStorage.getItem('token');
  if (token) {
    req = req.clone({ setHeaders: { Authorization: `Bearer ${token}` } });
  }
  return next(req);
};

7Browser Output

UI Render / Execution Output

Authorization: Bearer  attached to outbound requests

8Line-by-Line Explanation

Angular Bootstrapping -> Component Instantiation -> Dependency Injection -> Data Binding / Signal Update -> Template Render -> Zone.js / Signal Change Detection -> DOM Tree Sync

9Practical Example Usage

Template / CLI Invocation

export const authInterceptor: HttpInterceptorFn = (req, next) => { ... };

10Concept Comparison / Signal vs Normal Variable

Verification Status: Header Attached

Angular Signals reactive state store cheyyadaniki use avutayi; signal value read cheyyadaniki signal ni function laga call chestaru. Normal variables require manual ChangeDetectorRef triggering.

11Common Mistakes & Anti-Patterns

⚠️ Anti-Patterns to Avoid

Forgetting to unsubscribe from manual RxJS Observables, leading to memory leaks.
Omitting track in @for loops causing full list DOM re-renders on array updates.
Mutating objects directly instead of updating Signals via .set() or .update().
Putting heavy computation directly inside HTML template expression interpolation.
Injecting services in non-injection contexts instead of constructors or inject() functions.

12Coding Challenge

🎯 Hands-On Challenge:

Build an Angular component for HTTP Interceptors using standalone component syntax. Bind properties dynamically to your template and test user interactions in your browser at http://localhost:4200!

13Mini Quiz

❓ Question: What is the primary advantage of HTTP Interceptors in Angular?

Answer: It provides structured TypeScript type safety and Interceptor ante enti?, streamlining single-page app development.

14Quick Recap

Intercept outbound HTTP requests using functional HttpInterceptorFn to inject Bearer auth tokens and handle global errors.
Angular components participate automatically in the Dependency Injection system.
Utilize standalone components, Angular Signals, and modern control flow syntax (@if, @for).

Written by Our Compiler Technical Editorial Team
Reviewed for accuracy & tested on Angular 18+ Standards · Last updated August 2026