It's a contradiction I see in this discussion quite often.
If you want "quick feedback loops", a type checker is probably one of the quickest. Almost always quicker to give feedback than tests even. And certainly quicker than ftping code to a webserver and clicking through a webapp manually.
Modern JS (actually TS) tooling has adopted a middle ground.
There is an instant feedback - un-type-checked version that runs (typically used in dev mode) with instant-feedback/hot-reload, and a slower compile and build flow that builds for production.
I was initially not very convinced of the benefits - why have a type-checker if you won't use it in the small? But IDEs provide much of the benefit inline while writing code and this is seldom a real problem from the DX perspective.
My skepticism was unwarranted.
> ...after I deployed bad code and it blew up in someone's face, I got an email that not only shows a perfectly readable stack trace, but also a line of source code for the frames.
This is an odd contradiction. On one hand, he values the fast feedback loop you get from edit-and-reload style development, but on the other, he doesn't appreciate the instant feedback from the type checker saying it's not gonna work.