oh-my.ts

interface ErrorResponse {
  errorMessage: string;
  redirectUrl?: string;
}

interface SessionResponse {
  username: string;
  email: string;
}

function fetchSession(): Promise<SessionResponse | ErrorResponse> {
  return fetch('/session').json()
}
  
async function bootUpDaApp() {
  // .ts treats response as SessionResponse | ErrorResponse here.
  // Would need to cast w/ `as` normally.
  const response = await fetchSession()
  
  if ('errorMessage' in response) {  
    // Since this property is only in one of the types,
    // .ts treats response as ErrorResponse in this block.
    alert(response.errorMessage)
    return
  }
  
  // Based on the handling / return above, .ts treats response as SuccessResponse
  // from here on out!
  document.body = `Welcome home, ${ response.username }`
}