Because vanilla JS won't even detect a typo before you run it. TypeScript gives you amazing tooling and huge productivity gains, making your code more reliable and dependable. You'll spend more time implementing features instead of tracking down typo bugs and reading the source of a function just to remind yourself of it's signature. That's why a vanilla JS should use TypeScript. As a JS dev turned TypeScript fan, I reject the idea that TypeScript is turning JS into Java. I think it turns JS into the beautiful language it was meant to be by taming it where necessary without sacrificing any of the goodness of dynamic typing when you need it.