TypeScript catches a lot, but "this" bugs are still one of the most common things it misses. For example, when factoring a method into a free function, I'll often miss a this:
Check out the option "noImplicitThis" in TS2, for this exact case. It will ban using `this` when it's implicitly `any`. I agree that it was much needed!
A very happy way to live as a Javascript coder is to avoid the `this` keyword, in my experience. You really don't need it. Instead write this function as
(x, y) => Math.sqrt(x*x + y*y)
That can never have any state bugs, because it's a pure function (just like you can never have type errors with a sufficiently good type system).
And you might be interested in the native JS function (since ES6) called Math.hypot. Happy hacking :-)
