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HTTP Headers

Before diving into the concept of HTTP Headers in Angular, let's get a brief idea about what HTTP Headers actually are. HTTP Headers are an important part of the HTTP protocol that is used for transferring data over the web. They define parameters for HTTP requests and responses. HTTP headers are used to carry out additional information between the client and the server, under the HTTP protocol.

In Angular, we use the HTTP Client module to handle HTTP requests. This module has a built-in method to set HTTP headers.

Creating HTTP Headers​

To create an HTTP header, you need to import HttpHeaders from '@angular/common/http' and then create an instance of HttpHeaders. You can set the header key/value pairs during instantiation.

Let's see an example:

import { HttpHeaders } from '@angular/common/http';
 
let httpHeaders = new HttpHeaders()
.set('Content-Type', 'application/json')
.set('Cache-Control', 'no-cache');

In this example, we are setting 'Content-Type' and 'Cache-Control' headers. The 'set' method returns a new HttpHeaders instance with the new header, it does not update the existing instance.

Sending HTTP Headers​

After creating HTTP Headers, we can send them along with HTTP requests. The HttpClient methods like 'get', 'post', 'put', 'delete' etc., accept an options object as the third argument where we can set headers.

Here's an example of sending headers with a GET request:

this.http.get('YOUR_API_URL', { headers: httpHeaders });

In this example, we have passed our HttpHeaders instance 'httpHeaders' in the options object of the get method.

Modifying HTTP Headers​

To modify an existing HTTP header, you can use the 'set' method. This method will replace the existing header value with the new value. 

httpHeaders = httpHeaders.set('Content-Type', 'text/plain');

Deleting HTTP Headers​

You may need to delete an HTTP header from the HttpHeaders instance. You can use the 'delete' method for this. 

httpHeaders = httpHeaders.delete('Content-Type');

Checking HTTP Headers​

The HttpHeaders class provides the 'has' method to check if a header exists.

let hasContentType = httpHeaders.has('Content-Type');

This will return true if 'Content-Type' header exists, otherwise false.

In conclusion, HTTP Headers are an essential part of the HTTP communication handled by the HttpClient in Angular. Understanding how to set, send, modify, delete, and check HTTP headers is crucial for developing web applications with Angular. We've covered most of the basic operations related to HTTP headers in Angular. For more complex operations, you can refer to the official Angular documentation.