js: Add a trivial error handler that alert()s

I'm not quite sure how useful it will be, but it does at least catch
an assertion failure in main() and present an opaque message in a box,
which is better than stopping and putting a message in the console
where no-one will see it.

1 files changed, 6 insertions, 0 deletions

diff --git a/emccpre.js b/emccpre.js
index bca2aef..606401a 100644
--- a/emccpre.js
+++ b/emccpre.js
@@ -19,6 +19,12 @@
 // immediately.  On the other hand, the Emscripten runtime hasn't
 // started yet, so Module.cwrap isn't safe.
 
+// Error handler to make any failures from here on visible to the
+// user, maybe.
+window.addEventListener("error", function (e) {
+    alert(e.message);
+});
+
 // To avoid flicker while doing complicated drawing, we use two
 // canvases, the same size. One is actually on the web page, and the
 // other is off-screen. We do all our drawing on the off-screen one