type Expense = {
  // ...
};

async function postExpense(projectId: string, expense: Omit<Expense, 'id'>): Promise<string> {
  const res = await axios.post(`/projects/${projectId}/expenses`, expense);

  // `res.headers.location` is considered as a string, even though it could be `undefined`
  // e.g. when the response from the server is an error one
  const location = res.headers.location;

  // this causes this variable to be `undefined`, and leads to errors when the function is called
  return location;
}