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Ajax Advantages and Dis-Advantages
Ajax Advantages :
Asynchronous calls
AJAX make asynchronous calls to a web server. This means client browsers are avoid waiting for all data arrive before start the rendering.

AJAX is much responsive, whole page(small amount of) data transfer at a time.

Form Validation
This is the biggest advantage. Form are common element in web page. Validation should be instant and properly, AJAX gives you all of that, and more.

Speed and Bandwidth
AJAX is improve the speed and performance. Reduce the server traffic time consuming on both side response.
Ajax based application use less server bandwidth, because no need to reload complete page.. 
Ajax Dis-Advantages :

Less Security in AJAX application. Anyone can view the source code written for Ajax.

AJAX application would be a mistake because search engines would not be able to index an AJAX application.

No support of XMLHttpRequest in old browsers, Only supports Future versions (IE9, Google Chrome V40+, Firefox V36+ )