Would be a syntactic improvement (if you ask me) if you could pass a configuration object to FusionAuthClient instead of directly passing the constructor values.
For readability it looks a tad nicer if you got
const client= new FusionAuthClient(
  { 
    apiKey: "asda-sd-asd",
    host: "https://localhost",
    tennantId: "bla-bla-bla"
  }
)