For the most part it is JavaScript. JavaScript is valid Typescript, because Typescript is a superset. Typescript just gets you type annotations and "future" JavaScript features, none of which you're forced to use.
And, when you compile it to JavaScript, you can choose the target language level. So, you could write Typescript and have it compile such that in essence, all it does is strip out the type annotations.
Yes I know, but again, it's not JavaScript and there will always be headaches here and there.