Object (un)flattening in TypeScript

flatten.ts

// Based on original implementation from Tofandel’s answer: https://stackoverflow.com/a/59787588/247441

/* Aggregates parts (mapped to slash-separated paths) into a nested object.
   E.g.:
     { /some/path: A, /foo: B, /some/other/path: C }
   
   gets turned into:
     { foo: B, some: { path: A, other: { path: C } } }
*/

export function unflattenObject<T extends Record<string, any>>
(parts: Record<string, any>): T {
  const result: Record<string, any> = {};
  // Ideally should be typed as Partial<T>, but that causes problems down the line

  for (const partPath of Object.keys(parts)) {
    if (Object.prototype.hasOwnProperty.call(parts, partPath)) {

      const keys = partPath.match(/^\/+[^\/]*|[^\/]*\/+$|(?:\/{2,}|[^\/])+(?:\/+$)?/g);

      if (keys) {
        keys.reduce((accumulator, val, idx) => {
          return accumulator[val] ?? (
            (accumulator[val] = isNaN(Number(keys[idx + 1]))
              ? (keys.length - 1 === idx
                ? parts[partPath]
                : {})
              : [])
          );
        }, result);
      }
    }
  }

  return result as T;
}


/* Recursively decomposes an arbitrarily nested object into a flat record
   of slash-separated part paths mapped to respective structures.
   E.g.:
     { foo: B, some: { path: A, other: { path: C } } }
   
   gets turned into:
     { /some/path: A, /foo: B, /some/other/path: C }
*/
export function flattenObject(
  obj: Record<string, any>,
  _prefix: false | string = false,
  _result: Record<string, any> | null = null,
): Record<string, any> {
  const result: Record<string, any> = _result ?? {};

  // Preserve empty objects and arrays, they are lost otherwise
  if (_prefix !== false &&
      typeof obj === 'object' &&
      obj !== null &&
      Object.keys(obj).length === 0) {
    result[_prefix] = Array.isArray(obj) ? [] : {};
    return result;
  }

  const prefix = _prefix !== false
    ? (_prefix + path.posix.sep)
    : path.posix.sep;

  for (const i in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, i)) {
      if (typeof obj[i] === 'object' && obj[i] !== null) {
        // Recursion on deeper objects
        flattenObject(obj[i], prefix + i, result);
      } else {
        result[prefix + i] = obj[i];
      }
    }
  }
  return result;
}

Cannot find name 'path' in flattenObject function