JavaScript Error Monitoring from TrackJS gives you context about what's happening in your end-users' browsers. The Telemetry timeline recreates all the application, network, and user activity that leads to an error. The stack trace processing shows you the code you need to investigate.
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Pros
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 Error trends
Reports show what are the important errors to focus on fixing.
Pro Wraps console.log for errors
All messages sent into console are included with error messages for additional context and logging.
Pro Realtime notifications
See when new errors are happening in Slack or Hipchat rooms.
Pro Ability to track an error down to a specific customer (if necessary)
TrackJS supports custom user IDs, which can be random strings that we, the customer can assign to a certain user. This helps a lot when handling a support request or a bug report from customers.
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 Engineer Support
You will always get an engineer who can understand and answer your question right away whenever you need help.
Pro Highly customizable with custom JS code on the page
Hooks in the code that let you customize just about anything, you can get the error object before it's sent to the server and do whatever you like with it.
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 Powerful ignore rules and grouping rules
To ignore spurious errors.
Pro Find bug in product easily
TrackJs is very useful to catch console errors.
Pro Analytics dashboard
A snapshot of how you are doing, comparing page view volume to error volume.
Pro Very useful product for use and tracking issues
Pro Personal touch customer support
Pro Predictable Billing
No surprise bills caused by changes in usage.
Pro Client specs
Some general information like browser, operating system and viewport used by the client may help to determine source of trouble more precisely.
Cons
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.