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What is jquery Deferred and Promise?

jQuery the java-script framework help us to write concise java-script code, in this blog I am going to introduce about powerful feature of jQuery some people found it difficult to understand or ignore/forgot it once they done with the task. So let's begin: The definition of deferred and promise can be found easily by googling it :-), so we are directly begin with the implementation or coding. Since most of the developer are familiar with ajax so first I would like to start with ajax and then we will look how to use deferred. A html div :- <div id="mydiv"> </div> <div id="mydiv2"> </div> js code :- $.ajax({ url:'/echo/json/',   context:document.body,   success:function() {   $('#mydiv').text('ajax success');   },   error:function() {   $('#mydiv').text('ajax error');   } }); Here success and error handlers are the callback functions and will be invoked when ajax succeeded