Axios response headers can be undefined (TS typing isn't strict enough)

[gprst]

Describe the bug

Axios response headers can actually be undefined. For instance, a response can sometimes contain the location header, and sometimes not.

To Reproduce

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;
}

Expected behavior

AxiosResponseHeaders should reflect the fact that headers can be undefined.

Environment

• Axios Version: 0.27.2
• Adapter: N/A
• Browser: N/A
• Browser Version: N/A
• Node.js Version: 16.15.1
• OS: N/A
• Additional Library Versions: N/A

Additional context/Screenshots

N/A

[wei-beamery]

The response headers were always available in previous versions. Is the expectation that response headers should be checked for undefined, or is this a recent breaking change?

[gprst]

I don't know about the history of the codebase. I just know that currently, you can get a value that'd be typed as string, although it could actually be undefined — so yes, the expectation is that response headers should be typed as string | undefined, to be more accurate.

I've created a PR for this.

[jasonsaayman]

Merged the PR will release soon, closing.