It’s the nullish coalescing operator that has been proposed for ecmascript and has been implemented in Typescript. You can read more here or here
The gist of it is that
const dealType = currentDealType ?? originalDealType;
is equivalent to:
const dealType = currentDealType !== null && currentDealType !== void 0 ? currentDealType : originalDealType;
Or in words: if currentDealType is null or undefined use originalDealType otherwise use currentDealType