Replace XHR requests with `fetch

[smellyshovel]

Found this code in src/util/ajax.ts.

export const makeRequest = function(requestParameters: RequestParameters, callback: ResponseCallback<any>): Cancelable {
    // We're trying to use the Fetch API if possible. However, in some situations we can't use it:
    // - IE11 doesn't support it at all. In this case, we dispatch the request to the main thread so
    //   that we can get an accruate referrer header.
    // - Safari exposes window.AbortController, but it doesn't work actually abort any requests in
    //   some versions (see https://bugs.webkit.org/show_bug.cgi?id=174980#c2)
    // - Requests for resources with the file:// URI scheme don't work with the Fetch API either. In
    //   this case we unconditionally use XHR on the current thread since referrers don't matter.
    if (/:\/\//.test(requestParameters.url) && !(/^https?:|^file:/.test(requestParameters.url))) {
        if (isWorker() && (self as any).worker && (self as any).worker.actor) {
            return (self as any).worker.actor.send('getResource', requestParameters, callback);
        }
        if (!isWorker()) {
            const protocol = requestParameters.url.substring(0, requestParameters.url.indexOf('://'));
            const action = config.REGISTERED_PROTOCOLS[protocol] || makeFetchRequest;
            return action(requestParameters, callback);
        }
    }
    if (!isFileURL(requestParameters.url)) {
        if (fetch && Request && AbortController && Object.prototype.hasOwnProperty.call(Request.prototype, 'signal')) {
            return makeFetchRequest(requestParameters, callback);
        }
        if (isWorker() && (self as any).worker && (self as any).worker.actor) {
            const queueOnMainThread = true;
            return (self as any).worker.actor.send('getResource', requestParameters, callback, undefined, queueOnMainThread);
        }
    }
    return makeXMLHttpRequest(requestParameters, callback);
};

The comments are fairly interesting in that that I feel like they are not quite relevant anymore.

• IE11 doesn't support it at all. In this case, we dispatch the request to the main thread so that we can get an accruate referrer header.
  Is it really still relevant?
• Safari exposes window.AbortController, but it doesn't work actually abort any requests in some versions (see https://bugs.webkit.org/show_bug.cgi?id=174980#c2)
  The bug was fixed back in 2019
• Requests for resources with the file:// URI scheme don't work with the Fetch API either. In this case we unconditionally use XHR on the current thread since referrers don't matter.
  Where does the map send requests with file:// protocol? Only in dev. mode I believe? Can't they be proxied instead?

The main desire to remove XHR requests alltogether is conditioned by the desire to use fetch API instead, which uses Promises natively under the hood. And there were ideas to rewrite some major callback-related parts of the code to use Promises instead.

So are we OK with moving from XHR to fetch, @HarelM? Not that it's really crucial, but I see it a good point to refactor.

[HarelM]

You have some good points here, IE11 is no longer supported, and the safari might be solved, but in cordova/ionic environment file:// is used so I think xhr is needed (for example loading a style that is on the device).
Having said that, there's no problem in wrapping the xhr code in a promise.
The problem is that I'm not aware of ways to cancel a promise, so I'm not sure xhr would be my first place to try and fix.
In general, there are many places that use callbacks that their code can be simplified a lot using promises.

[smellyshovel]

The problem is that I'm not aware of ways to cancel a promise

I thought more about reurning an object where the request result is wrapped into a Promise, and separately a cancel method.

but in cordova/ionic environment file:// is used so I think xhr is needed (for example loading a style that is on the device).

OK, makes sense. Thanks.