https://www.slideshare.net/BrendanEich/jslol-9539395/110-Alw...
In terms of whether JS will introduce things that conflict with TS, it's hard to say. TypeScript team themselves are active participants in TC39, championing recent features such as Optional Chaining and Nullish Coalescing. So if there were any conflicts, or even opportunities for confusion, it would all be managed way ahead of time. Several TC39 delegates are users of TypeScript so there is no risk of accidents here (in my opinion).