What is HTTP?
The Hypertext Transfer Protocol (HTTP) is designed to enable communications between clients(frontend) and servers(backend).

HTTP works as a request-response protocol between a client and server.

Example: A client (browser) sends an HTTP request to the server; then the server returns a response to the client. The response contains status information about the request and may also contain the requested content.

HTTP Methods

GET, POST, PUT, HEAD, DELETE, PATCH, OPTIONS, CONNECT and TRACE

The GET Method
GET is used to request data from a specified resource. and know that it is only used to request data (not modify).

The POST Method
POST is used to send data to a server to create/update a resource.

The PUT Method
PUT is used to send data to a server to create/update a resource.

The difference between POST and PUT is that PUT requests are idempotent. That is, calling the same PUT request multiple times will always produce the same result. In contrast, calling a POST request repeatedly have side effects of creating the same resource multiple times.

HTTP response status codes

HTTP response status codes indicate whether a specific HTTP request has been successfully completed. Responses are grouped in five classes:

    • Informational responses (100 – 199)
    • Successful responses (200 – 299)
1- 200 ok. The result meaning of "success" depends on the HTTP method
2- 201 created. The request succeeded, and a new resource was created as a result
3- 202 accepted. The request has been received but not yet acted upon.
    • Redirection messages (300 – 399)
1- 302 Found This response code means that the URI of requested resource has been changed temporarily.
    • Client error responses (400 – 499)
1- 403 Forbidden The client does not have access rights to the content; that is, it is unauthorized, so the server is refusing to give the requested resource. 
2- 404 Not Found The server cannot find the requested resource. In the browser, this means the URL is not recognized. In an API, this can also mean that the endpoint is valid but the resource itself does not exist. Servers may also send this response instead of 403 Forbidden to hide the existence of a resource from an unauthorized client. This response code is probably the most well known due to its frequent occurrence on the web.
    • Server error responses (500 – 599)

For more click this link:- https://developer.mozilla.org/en-US/docs/Web/HTTP/Status

URL - Uniform Resource Locator

Web browsers request pages from web servers by using a URL.

URL Encoding (Percent Encoding)
URL encoding converts characters into a format that can be transmitted over the Internet.

URLs can only be sent over the Internet using the ASCII character-set.

Since URLs often contain characters outside the ASCII set, the URL has to be converted into a valid ASCII format

URL encoding replaces unsafe ASCII characters with a "%" followed by two hex digits.

URLs cannot contain spaces. URL encoding normally replaces a space with a plus (+) sign or with %20.

AJAX - XMLHttpRequest (XHR)

XMLHttpRequest (XHR) is a JavaScript API to create HTTP requests. Its methods provide the ability to send network requests between the browser and a server.

API - API stands for Application Programming Interface, which is a software intermediary that allows different applications to communicate with each other. APIs are a set of rules and protocols that define how applications request and respond to each other

The Fetch API is the modern replacement for XMLHttpRequest.