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+?