why this JavaScript method return undefined?

Question

in this code this method return undefined despites alert statement print a value ?

function getNearestPoint(idd) 
        {
            var xmlhttp;
                     


        

Answer 1

result isn't defined at that point, it only gets defined once your callback executes. The order of execution:

• getNearestPoint starts
• XHR is fired off
• getNearestPoint returns undefiend
• XHR comes back and runs xmlhttp.onreadystatechange
• result gets set

If you need result from OUTSIDE of this, you should use a callback:

getNearestPoint(idd, cb){
   ...
   xmlhttp.onreadystatechange = function(){
      ...
      cb(result);
   }
}

and your calling code changes from:

var result = getNearestPoint(id);

to:

getNearestPoint(id, function(result){
   // do something with result;
});