Intercept XHR requests so that we can do something useful with it.

intercept-xhr.js

// Taken from https://stackoverflow.com/a/27363569
// Allows for the object to be interrogated

((() => {
    const origOpen = XMLHttpRequest.prototype.open;
    XMLHttpRequest.prototype.open = function() {
        console.log('request started!');
        this.addEventListener('load', function() {
            console.log('request completed!');
            console.log(this.readyState); //will always be 4 (ajax is completed successfully)
            console.log(this.responseText); //whatever the response was
        });
        origOpen.apply(this, arguments);
    };
}))();

log-xhr.js

// Taken from https://stackoverflow.com/a/5202999
// This is only good for logging.
function addXMLRequestCallback(callback){
    let oldSend;
    let i;
    if( XMLHttpRequest.callbacks ) {
        // we've already overridden send() so just add the callback
        XMLHttpRequest.callbacks.push( callback );
    } else {
        // create a callback queue
        XMLHttpRequest.callbacks = [callback];
        // store the native send()
        oldSend = XMLHttpRequest.prototype.send;
        // override the native send()
        XMLHttpRequest.prototype.send = function(){
            // process the callback queue
            // the xhr instance is passed into each callback but seems pretty useless
            // you can't tell what its destination is or call abort() without an error
            // so only really good for logging that a request has happened
            // I could be wrong, I hope so...
            // EDIT: I suppose you could override the onreadystatechange handler though
            for( i = 0; i < XMLHttpRequest.callbacks.length; i++ ) {
                XMLHttpRequest.callbacks[i]( this );
            }
            // call the native send()
            oldSend.apply(this, arguments);
        }
    }
}

// e.g.
addXMLRequestCallback( xhr => {
    console.log( xhr.responseText ); // (an empty string)
});
addXMLRequestCallback( xhr => {
    console.dir( xhr ); // have a look if there is anything useful here
});