Ajax- Asynchronous JavaScript and XML: Sometime we have a requirement to update a particuler portion of a page not whole page so we required AJAX. Conventional web application used iframe for a webpage in which we call a different-2 web page in a single pages. A page will send a request to a server for updating a page and wait for its response. At a time only one request is send and its fully depend on Http(Hyper Text Transfer Protocol) but when ajax comes its overcome this problem in which at a time so many request can be send to the server and a webpage (which will send the request) will not wait for response.
Ajax-A hidden process
When you search about the ajax and its process then everybody just write a line, "Ajax is technique which is used to update the page without refreshing the whole page" or "Ajax is a process which exchange small amount of data with the server in the background for updating a webpage without reloading the whole page" but only few peoples know about what is actually running in the backend. we send a request to server actually an iframe is running in a hidden form. Everything is done in asynchronous mode mean requester will not wait for the response and it can send so many request to server and the response is depend upon the return logic.
Ajax Library- XHR (XMLHttpRequest)
All ajax request is depend upon XHR. It is a Ajax liberary which are responsible for the Ajax request and response. In conventional way everything is depend on http request and response. Every request has to wait for its response from the server but XHR library has overcome this problem and user can send so many request at a time and the response result is depend upon the logic.