Why? Because this is the only way to find all errors your users see but you don't. Only thing left that could fail without you knowing about it is the service or framework itself so let's hope that's well tested or... errorception!
I guess it should possible to log something like "okay, I'm loaded and ready to catch them errors" to make sure everything works as expected but I degress.
You can also read something like http://blog.meldium.com/home/2013/9/30/so-youre-thinking-of-tracking-your-js-errors about JS error tracking.
Below you will find some resources I collected during a very not extensive Google search, I might add more later.
Services do all the logging for you and give you a nice (hopefully) GUI to use. Included some pricing information but for the full picture visit their websites.
- http://trackjs.com/, free to 10k hits/month and $29.99 to 500k hits/month
- https://errorception.com/, free for 30 days and $5 to 500 errors/day
- https://raygun.io/, I find it hard to compare their pricing with the others
- http://jslogger.com/, free to 50 errors/day and $7 to 5000 errors/day
Frameworks are only the client side code, you need to do all the logging yourself. At least these are free!