Smart option assigining code for TypeScript

roroOptions.ts

/**
 * Inject all key values written in userOptions
 * If a key value doesn't exist, use the value from defaultOptions
 * In this case, even if userOptions[key] is undefined, it overwrites if given
 * If you want behavior identical to Object.assign, set `isUndefinedIgnored` to `true`
 *
 * @param userOptions 사용자가 입력할 부분적인 값
 * @param defaultOptions 기본적으로 제공할 값
 * @param isUndefinedIgnored undefined 값을 무시할 지 여부
 * @returns
 */
export function fillOptions<T extends object>(
  userOptions: Partial<T>,
  defaultOptions: T,
  isUndefinedIgnored = false,
) {
  const out = { ...defaultOptions };
  for (const key in userOptions) {
    if (isUndefinedIgnored && userOptions[key] === undefined) {
      continue;
    }
    out[key] = userOptions[key]!;
  }
  return out;
}