javascript - Return value from a asyncronous request using a callback function -

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• how return response asynchronous call? 25 answers

i have httprequest returns value. i'm capturing such value callback function.

the code executes , alert if username duplicate in db. "return false" not working , form submitted (savenewuser) duplicated username anyway. examples i've seen far stop @ callback alert have in code. how accomplish return false stop execution in other cases: first, last name , password checks?

thank much.

function checkusername(callbackusername){     var username = document.getelementbyid('username_id').value;     var ajaxrequest = getxmlhttp();            ajaxrequest.onreadystatechange = function()           {             if(ajaxrequest.readystate == 4 && ajaxrequest.status==200){                 var response = trim(ajaxrequest.responsetext);                callbackusername(response);             }            };         var url = "../admin/check_unique_username.php";             var parameters = "username="+username;         ajaxrequest.open("post", url, true);          ajaxrequest.setrequestheader("content-type", "application/x-www-form-urlencoded");                 ajaxrequest.send(parameters);        }  function checknewuserform(){     if(document.getelementbyid('first_name_id').value === ""){         alert("first name cannot blank!");         document.getelementbyid('first_name_id').focus();         return false;     }     if(document.getelementbyid('last_name_id').value === ""){         alert("last name cannot blank!");         document.getelementbyid('last_name_id').focus();         return false;     }     checkusername(function(result) {         if(result > 0){         alert("error: username exist. please select different username!!");         document.getelementbyid('username_id').focus();         return false;         }        });     re = /[0-9]/;      if(!re.test(document.getelementbyid('password_id').value)) {         alert("error: password must contain @ least 1 number (0-9)!");         document.getelementbyid('password_id').focus();          return false;     }     savenewuser(first_name,last_name,username,password); }  

by return false return callback function. have return checknewuserform() function stop saving new user. happen, can use boolean variable hasduplicateusername check whether duplicate username entry there, can make true, inside callback. , before savenewuser() should check whether false.

function checknewuserform(){     var hasduplicateusername = false;     if(document.getelementbyid('first_name_id').value === ""){         alert("first name cannot blank!");         document.getelementbyid('first_name_id').focus();         return false;     }     if(document.getelementbyid('last_name_id').value === ""){         alert("last name cannot blank!");         document.getelementbyid('last_name_id').focus();         return false;     }     checkusername(function(result) {         if(result > 0){         alert("error: username exist. please select different username!!");         document.getelementbyid('username_id').focus();         hasduplicateusername = true;         }     });     re = /[0-9]/;      if(!re.test(document.getelementbyid('password_id').value)) {         alert("error: password must contain @ least 1 number (0-9)!");         document.getelementbyid('password_id').focus();          return false;     }     if(!hasduplicateusername) {         savenewuser(first_name,last_name,username,password);     } }