Question

I have a code that XMLHttpRequest. It’s a object to send request to server and it’s asynchronous so when I want to receive the response I need to provide the callback function to onreadystatechange property of this object. And this function is called after response being received:

function send()
{
   XMLHttpRequest req = new...
   req.onreadystatechange = answer;
}

function answer()
{
   //handling the answer...
}

So it’s great but I don’t want to use new function to handle answer so I do it anonymous:

function send()
{
   XMLHttpRequest req = new...
   req.onreadystatechange = function ()
   {
         //handling the answer...
   };
}

But now i want to use the result of send function in another function for example to display result:

display(send())

So how to make this work? Something like:

function send()
{
   XMLHttpRequest req = new...
   req.onreadystatechange = function ()
   {
         //handling the answer...
         return result; //where result is returned by send function
   };
}

Is there any way to do this so that other JS code will still work while this code will handle the response?

Answer 1

With asynchronous functions you need to do it the opposite way. Instead of saying display(send()) you need to sue send(display)

function send(callback)
{
   XMLHttpRequest req = new...
   req.onreadystatechange = function ()
   {
         //handling the answer...
         callback(result);
   };
}

Trying to write display(send()) won’t work. The send() function produces a value asynchronously but returns promptly. It can’t return the result of the async operation.