Abort previous ajax request on new request

 var currentRequest = null;    

currentRequest = jQuery.ajax({
    type: 'POST',
    data: 'value=' + text,
    url: 'AJAX_URL',
    beforeSend : function()    {           
        if(currentRequest != null) {
            currentRequest.abort();
        }
    },
    success: function(data) {
        // Success
    },
    error:function(e){
      // Error
    }
});

var filterCandidates = function(form){
    //Previous request needs to be aborted.
    var request = $.ajax({
        type: 'POST',
        url: '/echo/json/',
        data: {
            json: JSON.stringify({
                count: 1
            })
        },
        success: function(data){
            if(typeof data !== 'undefined'){
                jQuery('.count').text(data.count)
                console.log(data.count);
            }
        }
    });
    return request;
};

var ajax = filterCandidates(form);

save in a variable, and then, before sending second time, check its readyState and call abort() if needed

a variation on the accepted answer and adopted from a comment on the question - this worked great for my application....

using jQuery's $.post()....

var request = null;

function myAjaxFunction(){
     $.ajaxSetup({cache: false}); // assures the cache is empty
     if (request != null) {
        request.abort();
        request = null;
     }
     request = $.post('myAjaxURL', myForm.serialize(), function (data) {
         // do stuff here with the returned data....
         console.log("returned data is ", data);
     });
}

Call myAjaxFunction() as many times as you like and it kills all except the last one (my application has a 'date selector' on it and changes price depending on the days selected - when someone clicks them fast without the above code, it is a coin toss as to if they will get the right price or not. With it, 100% correct!)