I didn't expect TypeScript to be a problem. I was more worried about not understanding other code and libraries that are written in JavaScript style. It seems a lot of design patterns used in the JavaScript world make sense once you understand the strengths and weaknesses of JavaScript.
You'll need to understand JavaScript. One of the things I like about TypeScript is that it tries to 'fit' JavaScript, but the consequence is that you need to understand JavaScript to be proficient in TypeScript.
That said, I suspect it won't be too difficult, and quite worth your while, to learn 'idiomatic JavaScript' if you have experience with at least one other language. And especially with ES6 (which is 'included' in TS), if you have any experience with Ruby or Python you'll be fine with JS and it might even be a relatively pleasant experience.
You can learn one before the other and a lot of the skills are transferable.