Broken AxiosHeaders typings

[nfantone]

Describe the bug

AxiosHeaders types makes it so trying to call methods on the instance raises a typescript compilation error.

AxiosRequestConfig.headers can be either RawAxiosRequestHeaders or AxiosHeaders. RawAxiosRequestHeaders, being an alias of Record<string, AxiosHeaderValue> causes methods to not be callable.

To Reproduce

Invoke any method on AxiosRequestConfig.headers.

Code snippet

const api = axios.create({ ... })

api.interceptors.request.use(req => {
  // Trying to access any method on `req.headers` will result in
  // a "This expression is not callable" TS error
  req.headers.set('foo', 'bar')
})

Expected behavior

Should be able to call any method declared in AxiosHeaders.

Environment

• axios: 1.2.2
• node: 18.12.1
• typescript: 4.9.4
• macOS Ventura 13.1

[nfantone]

In addition to the above, since AxiosRequestConfig.headers is an optional property, it's not straightforward to set headers using regular object access notation — i.e.: config.headers['foo'].

Is the expectation to coerce the type of .headers before accessing it?

[karlhorky]

Seems like this is also similar to #5416, which may also be fixed by #5420