how to make @ user reply or user selection method

[JAI PRAKASH SINGH]

hello all, 

    i am very new in coding line, i am trying to build a small code so that on 

    1) type of "@" character it will make a ajax call and get user list  ( i have login for it )

    2) on remove a "@" character username (after @) act as just a string ( but unable to do it  )

    for example in facebook

    

    1) in text box if we use @ and usernem both at same time user get notification 

    2) but if we remove @ character the  it cat as normal string 

   please reply i am stucking here  

   my small code 

   $('#btn-input-comment-text').keypress(function(e){

    console.log('keypress', String.fromCharCode( e.which )); // i will write a ajax call on type @ here 

                                                        // but on remove @ how to process further i dont know 
});