type File = { id: string; name: string };
type List = {
  id: string;
  name: string;
  files: File[];
}[];

function fileAlreadyInDestination(list: List, source: string, destination: string) {
  const targetFolder: { id: string; name: string; files: File[] } | undefined = list.find(
    (folder) => folder.id === destination,
  );
  const targetFile: undefined | File =
    targetFolder && targetFolder.files.find((f) => f.id === source);

  if (targetFile !== undefined) {
    throw new Error('Given source is already in given folder');
  }
}

function sourceIsNotFile(list: List, source: string) {
  let targetSource: File | undefined;
  list.forEach((folder) => {
    folder.files.forEach((file) => {
      if (file.id === source) {
        targetSource = file;
      }
    });
  });

  if (targetSource === undefined) {
    throw new Error('You cannot move a folder');
  }
}

function destinationIsNotFolder(list: List, destination: string) {
  const targetFolder = list.find((folder) => folder.id === destination);

  if (targetFolder === undefined) {
    throw new Error('Destination is not a folder');
  }
}