Everything required to add that to regular Javascript: transpilation, extra markup and understanding and mastering a new tool. Another layer to manage and debug if you will.

Assuming you use ES6+ in production, so you need a build step anyways, would you still say TS adds costs that aren't worth it to you? And if so, which costs? I mean that in full honesty, as I generally find myself conflicted whenever I start a new project: TS or ES2015+?

Now browsers support most of ES6, so you only need build step for deployments, and can have fast reloads without a build step when developing.

Oh. But these days most JS projects are in some task runner anyway.