CORS is driving me crazy - intermittent issues - ajax

Question

It seems like I've tried everything and I finally just switched to using the CORS npm module:
var cors = require('cors');
And my one route I want to use CORS on:
app.post('/hangouts', cors(), hangoutsController.hangouts); // user CORS
I'm implementing a custom app in Google Hangouts, but need to post to my server, and the Hangout is run from a Google server. I put the AJAX call on a loop so that it will keep trying - this post going through is crucial to my app.
Here's the relevant AJAX call in the Hangout app:
var shouldpostHangoutId = true;
/* Post the Hangout ID to server */
var postHangoutId = function(hangoutId) {
var startData = gapi.hangout.getStartData();
$.ajax({
type: 'POST',
url: rootURL + "/hangouts",
crossDomain: true,
dataType: "json",
data: {
"hangouts_id" : hangoutId,
"start_data" : startData
},
success: function( response ) {
console.log( "postHangoutId -- success" ); // server response
console.log( response ); // server response
shouldpostHangoutId = false;
},
error: function(xhr, textStatus, error){
console.log( "postHangoutId -- error" ); // server response
console.log(xhr.statusText);
console.log(textStatus);
console.log("error = " + error);
// Try again
if (shouldpostHangoutId) {
postHangoutId(hangoutId); // Try again
};
}
});
};
What's driving me crazy is that sometimes it goes through on the first go, sometimes it takes 5 times. And the whole process is super slow. Here's the log I get when it doesn't come through:
XMLHttpRequest cannot load https://www.foo.bar/hangouts. No 'Access-Control-Allow-Origin' header
is present on the requested resource. Origin 'https://ts6d5n5om59gt6cin9c39faccjf890k5-a-hangout-
opensocial.googleusercontent.com' is therefore not allowed access.
I'm using Node + Express ~4 on Heroku.

Answer 1

I think the problem had something to do with pre-flight requests. I changed the AJAX call to the following:
$.ajax({
type: 'POST',
url: rootURL + "/hangouts",
dataType: "json",
data: {
"hangouts_id" : hangoutId,
"start_data" : startData
},
error: function( error ){
// Log any error.
console.log( "ERROR:", error );
// Try again
if (shouldpostHangoutId) {
postHangoutId(hangoutId); // Try again
};
},
complete: function(){
console.log( "postHangoutId -- success" ); // server response
shouldpostHangoutId = false;
}
});
And it goes right through, first time without delay.