How to force a program to wait until an HTTP request is finished in JavaScript?

EDIT: Synchronous requests are now deprecated; you should always handle HTTP requests in an async way.

There is a 3rd parameter to XmlHttpRequest‘s open(), which aims to indicate that you want the request to by asynchronous (and so handle the response through an onreadystatechange handler).

So if you want it to be synchronous (i.e. wait for the answer), just specify false for this 3rd argument.
You may also want to set a limited timeout property for your request in this case, as it would block the page until reception.

Here is an all-in-one sample function for both sync and async:

function httpRequest(address, reqType, asyncProc) {
  var req = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
  if (asyncProc) { 
    req.onreadystatechange = function() { 
      if (this.readyState == 4) {
        asyncProc(this);
      } 
    };
  }
  req.open(reqType, address, !(!asyncProc));
  req.send();
  return req;
}

which you could call this way:

var req = httpRequest("http://example.com/aPageToTestForExistence.html", "HEAD");  // In this example you don't want to GET the full page contents
alert(req.status == 200 ? "found!" : "failed");  // We didn't provided an async proc so this will be executed after request completion only