assertion-example.ts

// Given this type
type ResultSuccess<T> = {
  data: T
  success: true
}

type ResultFailure<E> = {
  error: E
  success: false
}

type Result<T, E> = ResultSuccess<T> | ResultFailure<E>

const Result = {
  success: <T>(data: T): ResultSuccess<T> => ({ data, success: true }),
  failure: <E>(error: E): ResultFailure<E> => ({ error, success: false }),
  isSuccess: <T, E>(result: Result<T, E>): result is ResultSuccess<T> => result.success,
  isFailure: <T, E>(result: Result<T, E>): result is ResultFailure<E> => !result.success,
}

// assertion handler
type HandleResultFailure = <T, E>(result: Result<T, E>) => asserts result is ResultSuccess<T>
const handleResultFailure: HandleResultFailure = (result) => {
  if (Result.isSuccess(result)) return
  throw new Error('FAIL')
}


// CASE 1
// Standard handling

function standardWay() {
  const result: Result<number, Error> = Result.success(1) as any

  // standard way
  if (result.success === false) {
    console.error(result.error)
    return
  }

  console.log(result.data)
}

// CASE 2 
// Assertion way
function assertionWay() {
  const result: Result<number, Error> = Result.success(1) as any
  handleResultFailure(result)

  console.log(result.data)
}