How to keep JSContext from crashing your iOS 7 app

I found a pretty sneaky JavaScript Core bug recently which was triggered by getting an exception when evaluating JavaScript with a JSContext. When the JSContext that caught the exception is deallocated, it would crash my app. Luckily I ran into this not long before WWDC so I was able to get some help with it in the WebKit lab. As it turns out there was a bug in JSContext’s dealloc method that was only just recently fixed, but unfortunately we won’t see that fix until iOS8 is out. If you need to support iOS 7 and want to use JSContext to evaluate JS, the fix is very simple; before your context gets released, simply nil out its exception property. Here’s a simple code example:

@autoreleasepool {
    JSContext *context = [[JSContext alloc] initWithVirtualMachine:[[JSVirtualMachine alloc] init];
    [context evaluateScript:@"bad"];

    // This needs to get nil'd before deallocation
    context.exception = nil;
}

So save yourself the trouble and make sure to always clear the exception of any contexts you have used to evaluate code before deallocating it if you’re targeting iOS 7.

Top 10 moments from my first day at BuzzFeed

• Eating a delicious catered lunch.
• Getting a BuzzFeed hoodie. You can never have too many hoodies.
• Playing with a shiny, brand-new Retina Macbook Pro.
• Walking into the dedicated snack room for the first time. Holy shit.
• Getting an invite to try out Github’s new Atom editor.
• Getting a text from a friend that he picked up an early copy of Dark Souls 2 for me.
• Running into my old co-worker Shaun at the front desk who was also coming in for his first day.
• Remembering that I now work directly across the street from ShakeShack.
• Commuting with Jen. It’s nice taking the same train again and working similar schedules.
• Getting to make this list joke.