When first learning it's more enjoyable to see an idea translate quickly to reality.
JS is great for that purpose. It's very easy to make animations and other fun effects in the DOM or build an extension on top of their favorite site. Once invested they might start research and discover CS fundamentals. Immediately throwing them at typescript without understanding JS could lead to frustration/confusion.
They also might learn an appreciation for things like type safety by not having them. For that reason dynamic languages like Ruby /Python are great. They can execute an idea faster, and if they get frustrated by those types of issues then introducing them to those other languages. Kotlin is cool, but it makes a lot more sense if you have had to deal with null pointer errors before.