Not so much 'copied', considering it was the TS team that has gotten them added to JS. They just didn't want to commit to anything in the TS codebase that wasn't at least stage 3 in JS.
Out of curiosity, have there been features that were thought to be part of JS one day, added by TypeScript, and then abandoned for JS again, leading TS to rip them out as well? Or do such vestiges remain in TypeScript and have to be changed into different constructs on compilation?
