I want to return response data when XMLHttpRequest transaction completes successfully. upon calling my function trashAnswer() to a variable I expect to assign the value returned to the variable and do something with it like below:
I can see resData is always undefined when i try to access data from it. Please how can I solve this?
var resData = trashAnswer({answerid:58,answererid:65,questionid:458});
This is the code:
//This function will trash answer object
function trashAnswer(param){
if (typeof param !== 'object' || param == null ) {
throw "trashAnswer(): Strictly expect valid object.";
}
var param = $.extend({answerid:0,answererid:0},param),
fd = new FormData();
fd.append('answerData',JSON.stringify(param));
fd.append('trash-answer',true);
var req = AJAX_REQEUST_OB();
req.open(bigPipe.formMethod.a,ajax.ac,true);
req.onload = function(){
if (req.readyState === req.DONE && req.status === 200) {
//ParseJSON is a custom function to check if response is a valid JSON...if its valid then then function will return response Data else return false
var Data = ParseJSON(req.responseText);
//process data...
var EvalData = !Data ? function(){
throw "Invalid JSON";
}:function(Data){
//do something...
return Data;
};
//call EvalData ****method
EvalData(Data);
}
}
//send request.
req.send(fd);
}
This is an asynchronous call. When you do trashAnswer({...});
you are triggering the request and your code continues to execute. At some point in the future your request may work. Here the "may" part is important: your request may fail for several reasons, but you are only checking the HTTP status 200, that is an "OK" response).
An easy solution:
var resData;
trashAnswer({answerid:58,answererid:65,questionid:458});
//This function will trash answer object
function trashAnswer(param){
// ...
req.onload = function(){
if (req.readyState === req.DONE && req.status === 200) {
// ...
// in your "//do something...":
resData = // your result here. If you do not have a local variable,
// it will change the global one (not recommended, see comment below)
}
}
// ...
}
Your solution is even better if you avoid using a global variable ( See here why ). But I believe this is a good starting point for you to understand how async calls work.
You can also use promises (including the async/await syntax). A good start reading about how this works is https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_promises
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