We use TrackJS to track and monitor JavaScript errors across more than a dozen websites, and recently we noticed an error that had fired just under 100 times in a short period of time for a single user.

A quick look at the captured information and there appeared to be problem with an older version of Android, in this case 4.0.3. Unfortunately the error was a rather unhelpful undefined is not a function triggered on minified vendor code. The user wasn’t performing any action such as a click (which TrackJS will report), so all signs pointed towards a timeout or scroll-based event. The browser was reported as the Android Browser, a default browser that has slowly been phased out since Android 4.

I didn’t have any physical device to test on so my next option was an emulator. Android Studio allows you to create virtual devices all the way from Android 6 down to Android 1. I created a virtual device matching the device resolution and Android version, and spun it up.

When your virtual device is running, you can use adb to tail logcat logs for the browser like so:

adb logcat browser:* *:S

(Note: If adb is not in $PATH then you should be able to find it under the location where your android SDK is installed - for example, on OSX: ~/Library/Android/sdk/platform-tools/)

Once you’re tailing the log you can simply navigate to a webpage in the Android Browser and see the console messages coming through. Within the virtual device 10.0.2.2 will map to localhost on your machine, which can be used to debug a local web server.

After debugging the unminified code I found that the error was coming from a 3rd-party package that assumed the existence of some form of requestAnimationFrame, which the Android Browser doesn’t support until 4.4. One polyfill later and the issue has been resolved.