ES6 to TS is more refactoring than a re-write. Just start with the core library and some TS linting and fix things as you go. The time it takes to 80-20 a microservice project into TS is usually less than it takes to hunt down a confused type bug (ie. someone thought this was an array but it's a string). This is not a "re-write":

function(param1, param2) {}

function(param1: string, params2: number) {}