When comparing Bugsnag vs TrackJS JavaScript Error Monitoring, the Slant community recommends TrackJS JavaScript Error Monitoring for most people. In the question“What are the best JavaScript client-side error logging services?” TrackJS JavaScript Error Monitoring is ranked 4th while Bugsnag is ranked 7th. The most important reason people chose TrackJS JavaScript Error Monitoring is:
You can see how your site is doing every day with a summary of all the interesting activity and errors that should be checked into.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Great documentation
Clear, concise documentation. Plus docs on cross-domain script errors and enabling CORS.
Pro Email, chat, sms notifications
Bugsnag can be set up to send email notifications as soon as exceptions occur. It can also integrate with a number of third-party services that can be used for notifications including Campfire, HipChat, Twilio SMS and custom solutions via Webhook.
Pro Automatic issues/ticket creation
Bugsnag integrates with a wide variety of 3rd party tools to automatically create issues or tickets. Integrations include GitHub, BitBucket, UserVoice, Trello and many, many others.

Pro Has a free plan
After a two-week full-featured free trial period the user is limited to 2,000 errors/month, 1 user and 1 project.
Pro Exceptions are grouped
Bugsnag groups similar exceptions together displaying the number of occurences so that there's no need to deal with each instance of the error separately.
Pro Exceptional documentation organization
Their documentation was well organized, easy to search through for the platform of choice and quick work of full product integration.
Pro Supports Unity 3D
Drop in place integration with Unity 3D, only a simple API Token configuration on game object and everything was up & running with almost zero code written.
Pro Supports phabricator integrations
Not many products have decided to finally start doing this, phabricator now has a champion.

Pro Source map support

Pro Stack traces
Pro Client libraries are open source

Pro Report uncaught errors
Pro Good web UI
Pro Daily summary email
You can see how your site is doing every day with a summary of all the interesting activity and errors that should be checked into.
Pro Telemetry timeline
Understanding the console, network, and user events that led up to an error makes it possible to recreate it in development.
Pro Analytics dashboard
A snapshot of how you are doing, comparing page view volume to error volume.
Pro Error trends
Reports show what are the important errors to focus on fixing.
Pro Realtime notifications
See when new errors are happening in Slack or Hipchat rooms.
Pro Inline sourcecode snippets
Error messages and stacktraces are shown with snippets of the raw source at the location. This gives a ton of context to quickly find the error.
Pro Wraps console.log for errors
All messages sent into console are included with error messages for additional context and logging
Cons

Con Blocked by ad blockers by default
Bugsnag is blocked by the AdBlock EasyPrivacy filter.
Con No deployments using a REST API call
Con Multiple errors from the same user can bloat your metrics
No "distinct" error count. If a loop ends up inflating a single user's error count, all reports are affected. Dashboard skewed by weird outliers.
Recently (as of February 2018) sorting by "users impacted" was introduced to reduce this problem.