Help With 'return' Statement In Functions

I have the following script:

function myFunc() {
// some code to do with AJAX
http_request.onreadystatechange = function() {
// do some things
return "a string";
}

}

alert(myFunc());

However, when I return "a string"; I want that string to be returned by myFunc() (and therefore alert'ed). What seems to be happening is that the string is being returned by the http_request.onreadystatechange = function() { } function, and myFunc() just returns false. How can I make the string be returned by myFunc()?