Function tree

packages/router/src/create_url_tree.ts:182–214

(
  oldRoot: UrlSegmentGroup,
  oldSegmentGroup: UrlSegmentGroup,
  newSegmentGroup: UrlSegmentGroup,
  queryParams: Params | null,
  fragment: string | null,
  urlSerializer: UrlSerializer,
)

}

function tree(
  oldRoot: UrlSegmentGroup,
  oldSegmentGroup: UrlSegmentGroup,
  newSegmentGroup: UrlSegmentGroup,
  queryParams: Params | null,
  fragment: string | null,
  urlSerializer: UrlSerializer,
): UrlTree {
  const qp: Params = {};
  for (const [key, value] of Object.entries(queryParams ?? {})) {
    // This retains old behavior where each item in the array was stringified individually This
    // helps remove special-case handling for empty and single-item arrays where the default
    // serializer removes empty arrays when serialized then parsed or converts them to non-arrays
    // for single-item arrays. Changing this could have breaking change implications. Prior code
    // always returned arrays of strings for array inputs so tests, applications, serializers,
    // etc. may only be set up to handle string arrays. We could consider changing this in the
    // future to serialize the entire array as a single value. For now, this feels safer and is
    // at least a step in the right direction.
    qp[key] = Array.isArray(value)
      ? value.map((v) => normalizeQueryParams(key, v, urlSerializer))
      : normalizeQueryParams(key, value, urlSerializer);
  }

  let rootCandidate: UrlSegmentGroup;
  if (oldRoot === oldSegmentGroup) {
    rootCandidate = newSegmentGroup;
  } else {
    rootCandidate = replaceSegment(oldRoot, oldSegmentGroup, newSegmentGroup);
  }

  const newRoot = createRoot(squashSegmentGroup(rootCandidate));
  return new UrlTree(newRoot, qp, fragment);
}

/**
 * Replaces the `oldSegment` which is located in some child of the `current` with the `newSegment`.