Failing promise gets catched by the Application.onerror function

Question

There's this weird behavior which I don't know if it is supposed to be happening or it's a winjs bug.

I'm using the WinJS.UI.Pages.render to render the content of a page under a specific element.

The problem I'm facing is that no matter if I handle the failing of the render function, the 'error' keeps propagating and it gets "catched" by the onerror function.

Here's a quick view of my code:

function goFunction() {
    WinJS.UI.Pages.render('otherpage.html', root).then(null, failedNavigating);
}

function noGoFunction() {
    WinJS.UI.Pages.render('missingpage.html', root).then(null, failedNavigating);
}

function failedNavigating() {
    console.log('failed to navigate');
}

WinJS.Application.onerror = function (args) {
    debugger;
};

The otherpage.html page exists so the rendering gets executed and everything is fine. But the missingpage.html does not exists. In that case the failedNavigating function executes, so far so good, but then the WinJS.Application.onerror handler gets 'called'.

Is that how that is supposed to work? What am I missing?

Here's a full project with a repro in case anybody wants to take a look at it.

Something else is going on here. Even if you didn't have `.then(null, failedNavigating)` there it _still_ shouldn't have triggered `WinJS.Application.onerror` since it's `.then` and not `.done` and WinJS promises aren't smart enough to figure out it's done on their own. I suspect that WinJS.UI.Pages.Render calls `WinJS.Application.onerror` directly and regardless of whether or not you handle the error. — Benjamin Gruenbaum, Oct 07 '14 at 06:10
I can't find anything in the WinJS source code to back that up though. — Benjamin Gruenbaum, Oct 07 '14 at 06:14

score 2 · Accepted Answer · answered Nov 07 '14 at 00:23

2

I finally got around to debugging this in your GitHub issue. It's because WinJS.UI.Pages.render implicitly creates a Page, which by default has an error handler that creates an Error promise, which Application was detecting as an "unhandled error promise" since nothing was continuing off of it.

You can work around it for now with something like this:

function noGoFunction() {

    WinJS.UI.Pages.define('missingpage.html', {
        error: function (e) {
            console.log("Page failed to load");
        }

    });

    WinJS.UI.Pages.render('missingpage.html', root).then(null, failedNavigating);
}

function failedNavigating() {
    console.log('failed to navigate');
}

answered Nov 07 '14 at 00:23

Xirzec

151
5

Cool! Good to know. The workaround is no good to me cause I need something more generic. missingpage.html is what I used for the sake of the sample, but it could actually be anything I'm not aware of. Will it be threated as a bug? – sebagomez Nov 07 '14 at 14:26
This would be a breaking change to Pages.ready(), so it would have to be opened as a feature request. – Xirzec Dec 03 '14 at 00:05

score 0 · Answer 2 · answered Oct 08 '14 at 14:05

Only the .done has the ability to catch and throw an error.

see here: http://msdn.microsoft.com/en-us/library/windows/apps/br211867.aspx for the differencies.

As you can see the two are almost identical apart from this: "After the handlers have finished executing, this function throws any error that would have been returned from then as a promise in the error state." that is only present in the .done.

Try to replace you .then with a .done and see if it works.

see my other answer, I would still use a done though. :) – Maurizio In denmark Oct 08 '14 at 14:30 — Maurizio In denmark, Oct 08 '14 at 14:30

score 0 · Answer 3 · answered Oct 08 '14 at 14:30

Look at this: http://msdn.microsoft.com/en-us/library/windows/apps/br229728.aspx

promise.then(onComplete, onError, onProgress).done( /* Your success and error handlers */ );

onComplete

Type: function

The function to be called if the promise is fulfilled successfully with a value. The value is passed as the single argument. If the value is null, the value is returned. The value returned from the function becomes the fulfilled value of the promise returned by then. If an exception is thrown while this function is being executed, the promise returned by then moves into the error state.

So basically this happens because you have null in the onCompleted parameter of the .done

Try replacing it with a function instead

no, still same thing. I agree in my sample the done is enough. But I simplified it for the sake of the sample. You can download the project and try it yourself — sebagomez, Oct 08 '14 at 14:40

Failing promise gets catched by the Application.onerror function

3 Answers3