Six months ago I decided to make the switch from plain JavaScript to TypeScript. The learning curve was steeper than I expected, but the benefits are enormous.
The biggest advantage is IDE support. With TypeScript you get autocompletion that actually works, and errors are caught while you type instead of at runtime. This has significantly boosted my productivity.
Type annotations also act as living documentation. When I see a function with clear parameter and return types, I immediately understand what it does without having to read the implementation.