utils.merge does not merge headers correctly if config.headers is an instance of AxiosHeaders

[magom001]

Describe the bug

axios/lib/core/Axios.js

Line 73 in ab77a40

contextHeaders = headers && utils.merge(

if headers.common is not an instance of AxiosHeaders then headers[config.method] (config.method === 'get') will return not the headers object but a reference to the getter method of the AxiosHeaders class instance.

In that case the utils.merge will fall into the default else case:

axios/lib/utils.js

Line 305 in ab77a40

const assignValue = (val, key) => {

And assign a function to the headers object. The request is malformed at this point.

To Reproduce

https://github.com/magom001/axios-jwt-auth/tree/1682dd6bdd20b1d4a619aff71554f92f14ae1b1e, update axios to 1.1.3 and run unit tests.

Code snippet

const responseErrorInterceptor =
  (
    axios: AxiosInstance,
    {
      tokensStorage = defaultTokensStorage,
      shouldRefresh = defaultShouldRefresh,
      refreshTokens,
      onFailedToRefresh,
    }: Config,
  ) =>
  async (error: AxiosError) => {
    if (await shouldRefresh(error)) {
      if (refreshPromise) {
        await refreshPromise;

        return axios.request(error.config as any);
      }

      refreshPromise = new Promise(async (resolve, reject) => {
        try {
          const refreshToken = await tokensStorage.getRefreshToken();
          if (!refreshToken) {
            throw error;
          }

          const tokens = await refreshTokens(refreshToken);

          await tokensStorage.saveTokens(tokens);

          resolve(tokens.accessToken);
        } catch (e) {
          await tokensStorage.clearTokens();

          reject(e);

          onFailedToRefresh?.(e as AxiosError<any>);
        } finally {
          refreshPromise = null;
        }
      });

      await refreshPromise;

      return axios.request(error.config as any);
    }

    throw error;
  };

### Expected behavior

Headers are merged correctly. Headers object is well formed.

### Axios Version

1.1.3

### Adapter Version

n/a

### Browser

n/a

### Browser Version

n/a

### Node.js Version

n/a

### OS

n/a

### Additional Library Versions

_No response_

### Additional context/Screenshots

_No response_

[DigitalBrainJS]

1.1.3

I believe this has been fixed in the latest version.

[timothymctim]

1.1.3

I believe this has been fixed in the latest version.

Indeed, fixed in #5224, I guess.

[philBrown]

@DigitalBrainJS Shouldn't bug fixes appear in 1.1.4? This approach to "versioning" is just painful.