jQuery中的未定义变量

Question

Through ajax call i am updating something and returning false and true. but the problem is that before i get result value from update function the if condition that is present after update function is executed and thus it give an error undefined result and errMsg variable. If I write this code again then it works. how i can stop running if condition until my update function returns some value?

var result = update(field_name, field_value);

if (result == false) {
  alert(errMsg);
} else {
  alert(errMsg);
}

update function

function update(field_name, field_value) {
  if (field_value) {
    $.post("/restapi/vc/users/id/${user.id}/profiles/name/" + field_name + "/set?value=" + field_value, function(data, status) {
      var xmlDoc = data;
      var x = xmlDoc.getElementsByTagName("response")[0];
      var txt = x.getAttribute("status");
      if (txt == 'error') {
        var msg = xmlDoc.getElementsByTagName("message")[0];
        var errMsg = msg.childNodes[0];
        window.errMsg = errMsg.nodeValue;
        return false;
      } else {
        return true;
      }
    });
  }
}

Answer 1

because update function returns a void/null. and null is == to false.

var result = update(field_name,field_value);
//update method is called and returns null 
if(result == false){  //null == false is true
    alert(errMsg);  //this line will execute
}else{
    alert(errMsg);
}

this is happening because of async nature of JavaScript. window.errMsg is being set after it is alerted. ajasx post in update method is a ajax request and its sucess method is called async.

better way withtout using Deffered would be to use callback.

var update = function update(field_name, field_value, callback) {
    if (field_value) {
        $.post(
            "/restapi/vc/users/id/${user.id}/profiles/name/" + field_name + "/set?value=" + field_value,
            function(data, status) {
                var xmlDoc = data;
                var x = xmlDoc.getElementsByTagName("response")[0];
                var txt = x.getAttribute("status");
                if (txt == 'error') {
                    var msg = xmlDoc.getElementsByTagName("message")[0];
                    var errMsg = msg.childNodes[0];

                    //its not good practice to add 
                    //transactional messages to window/global scope
                    //window.errMsg = errMsg.nodeValue;  

                    //return false;   
                    //instead of return call the callback
                    callback(errMsg.nodeValue);
                } else {
                    //return true;
                    //likewise in case of success
                    callback();
                }
            }
        );
    }
};

//and call update method like this -
update(field_name, field_value, function(errMsg){
    if (errMsg) {
        alert(errMsg);  //alert the error message
    } else {
        alert('success');
    }
});