jquery prevent form from submiting if ajax error exists -

i couldn't find answer i'm looking here. i'm using ajax check if username exists in database, if user exists error displayed in form. i'm trying prevent form being submitted if error exists. i've tried event.preventdefault() i'm not sure if i'm using right, or if i'm using wrong.

my js code is:

$(".check").blur(function () {     var username = $(".check").val();     $.ajax({         url: "validateuser.php",         type: "post",         data: "username=" + username     }).done(function (result) {         if (result == "true") {             // code run if error doesn't exist         } else {             // code run if error exist             // create error message , displays             event.preventdefault();         }     }); }); 

do validation on form submit blur.

function checkusername(username) {     var username = $(".check").val();     return $.ajax({         url: "validateuser.php",         type: "post",         data: "username=" + username     }).then(function (result) {         var def = $.deferred();         if (result == "true") {             // code run if error doesn't exist              def.resolve();         } else {             // code run if error exist             // create error message , displays              def.reject();         }         return def.promise();     }); }  $(".check").blur(checkusername); $("#myform").submit(function(e){     var self = this;     e.preventdefault();     checkusername().done(function(){         // username ok         self.submit();     }); });