While setting an Accept header for an AxiosInstance, I noticed that when the headers are passed initially via const instance = axios.create({headers: {Accept: 'application/json'}}), the header will be set under instance.defaults.headers.Accept.

But wherever I read about overwriting default headers of axios it always mentions overwriting the header under instance.defaults.headers.common (see also https://www.npmjs.com/package/axios#custom-instance-defaults). Moreover, whether I call axios.create({headers: {Accept: 'application/json'}}) or axios.create(), the value of instance.defaults.headers.common.Accept will be 'application/json, text/plain, /', so the headers passed during create do not influence what's under common. I can only manually overwrite it with instance.defaults.headers.common = {Accept: 'application/json'}

So which one takes precedence? instance.defaults.headers.Accept or instance.defaults.headers.common.Accept? I know that if I set it for example directly for instance.defaults.headers.post.Accept, then this one will take precedence because it is more specific.

Or is this a bug in axios 1.7.7 and instance.defaults.headers.Accept should not even be possible?