Joined a new company as front end developer after working for a legal tech for 3 years. The codebase is shit here. There is this ReactJS developer who has done everything except for following standard code practices. No modularity, no reusable components. Some files go beyond 2000 lines of codes. It is a literal nightmare.

The worst part is, he does not understand that he has written the worst code. I dunno how to handle this now.

Well best thing IMO is to 'show' developers.

"Hey check this out, I did this thing, I think it could save us some time."

That's pretty much every react codebase I've ever seen in the wild not part of an OS project from Facebook

So nobody likes being told that you are beeing in the wrong.
What you need is to do is to carefully observe his behavior when he codes and collect his pain points with the codebase. That will take time.
To convince him to change, craft a list with personal benefits for him with each pain point solution. That may be less time spent on tedious task X or more praise from Boss for x.
Present him that in a whiteboard session. But put your ego away and always think what he desires. That way you get it your way.

What @heyheni said.

Or just shaming, yelling, etc...

@heyheni haha.. thats longer!! But will try that

@SortOfTested trust me.. this is a very small dashboard application. And he tool 2 years to come up with the shit. Now that they are expanding need to revamp

@kirodian The book "This is Service Design Doing" may help you with some systematic methods on how to observe and introduce change.
The methods are free of charge on the website. https://thisisservicedesigndoing.com/...

https://amazon.com/This-Service-Des...
This Is Service Design Doing: Applying Service ... - Amazon.com

@heyheni Thanks for this.. will look this up