JavaScript example:

https://www.typescriptlang.org/play/?filetype=js#code/PTAEAE...

Almost equivalent typescript code:

https://www.typescriptlang.org/play/?#code/C4TwDgpgBA6glsAFg...

(I had to make it a little bit different from the JS code to make it compile)

(Well, this is not exactly about arrow function I guess. I remembered that part wrong.)

Note that I cannot make the type check in JS code to pass. Whatever I do, there is always a error. Meanwhile, it does not take much to TS code to work.