Microsoft vs Javascript standards


Microsoft’s flagship browser has been causing web developers much anguish for over a decade now, and they appear to have unleashed an “improved” version of that mediocrity upon us via WinJS in their offering for HTML apps for Windows.

In Javascript norms, when a function throws an error, the error and its stack trace get output to the console, all functions on this stack get aborted, and execution of other parts of the page resume. Even IE gets this right. Microsoft has, however, decided to “improve” upon this behaviour in their WinJS run time, where they decide that in the above scenario, what should happen instead is to simply crash the program, in a manner similar to that of a SEGFAULT in a C program.

This is most likely the root cause of the plentiful random crashes in an app that we have been working on.

Even if we wrote our own code in defensively to work around this limitation by wrapping everything in a try-catch block, we simply cannot enforce that the 3rd-party libraries we include do so as well.

… But there is light at the end of the tunnel. After much sleuthing, Chris and I have worked out that there is indeed a way to catch these errors: by setting a window.onerror handler function, and doing evt.preventDefault() inside it.

But that would be too easy, wouldn’t it? Why yes, of course, we should have had the foresight to realise that the spec would be ignored yet again, and be required to use a parallel API instead. Here’s the workaround that we ended up with that appears to work:

window.WinJS.Application.addEventListener('error', globalOnError);
function globalOnError(evt) {
  console.log('globalOnError', evt); // Replace with proper logging functions
  return true;

In a 3rd instance of a flagrant disregard for Javascript norms, the evt parameter here has neither preventDefault() nor stopPropagation() methods available. On the other hand, evt.details.error does, but that does not matter, because calling those will crash the program SEGFAULT style as well - but of course! Instead, we have to do a return true to do so, which takes us all the way back to Javascript event handling patterns that fell out of favour in the 90’s.


We may have found a workaround for (at least some of) the random crashes, and in doing so, found this caution to be vital: When doing anything Microsoft Javascript platforms, tread carefully, and be prepared to write a parallel set of code from specification compliant Javascript. Do not expect anything that “should work” because “it’s standard Javascript” to actually work, because that does not appear to be a priority.

Javascript is a language that is supposed to be both community-driven (ECMAScript and W3C) and open; and vendor should be creating platforms that go forward with the standards as they progress forward rather than break them with works-for-me-only solutions.

Please reconsider your Javascript strategy.

Copyright © 2008-present Brendan Graetz