Javascript provides a good integration for logging and debugging. You can use Chrome Dev tools as a great tool to help you debug frontend applications.
On your locale machine you can set breakpoints add error logs. But then the user has an error! You cannot access his console or debugger tools. It maybe always happen at a certain hour. No log is available for you personally. The user cannot explain the flow he has followed to this error, even explain you which browser version he has is hard!
During this talks we do a deeper dive into the console and chrome dev tools, but also how can you log your errors and warnings from a remote computer.