There's the key phrase. There's plenty of people that can whip up a POC or MVP in an afternoon of hacking or a week of fiddling, but going from that to something that can safely handle millions of users and hundreds of developer contributions needs types and other assurances. If it fits in a developer or a team's heads, then it's just fine - that's how I did my first major JS application, just Knowing what fields my objects have and what types they are expected to have. But that doesn't scale, people move on, gaps exist in tests, etc etc etc.
There's the key phrase. There's plenty of people that can whip up a POC or MVP in an afternoon of hacking or a week of fiddling, but going from that to something that can safely handle millions of users and hundreds of developer contributions needs types and other assurances. If it fits in a developer or a team's heads, then it's just fine - that's how I did my first major JS application, just Knowing what fields my objects have and what types they are expected to have. But that doesn't scale, people move on, gaps exist in tests, etc etc etc.