A code sample from the article https://www.joshwulf.com/blog/2020/02/refining-method-signature/

createWorker-decode-implementation.ts

export function decodeCreateZBWorkerSig(config) {
  const isShorthandSig = typeof config.taskTypeOrTaskHandler === 'function'
  const taskHandler: ZB.ZBWorkerTaskHandler = isShorthandSig
    ? (config.taskTypeOrTaskHandler as ZB.ZBWorkerTaskHandler)
    : (config.taskHandlerOrOptions as ZB.ZBWorkerTaskHandler)
  const id: string | null = isShorthandSig
    ? (config.idOrTaskType as string)
    : null
  const taskType: string = isShorthandSig
    ? (config.idOrTaskType as string)
    : (config.taskTypeOrTaskHandler as string)
  const options: ZB.ZBWorkerOptions & ZB.ZBClientOptions =
    (isShorthandSig
      ? config.taskHandlerOrOptions
      : config.optionsOrOnConnectionError) || {}
  const onConnectionError = isShorthandSig
    ? config.optionsOrOnConnectionError
    : config.onConnectionError ||
      options.onConnectionError ||
      config.onConnectionError
  const onReady = options.onReady
  return {
    id,
    onConnectionError,
    onReady,
    options,
    taskHandler,
    taskType,
  }
}