Describe the issue

Headers are typed as Record<string, string>. This includes axios.defaults.headers, which is configured as:

This seems incompatible with headers being of type Record<string, string>.

Example Code

The following line produces a TypeScript error beacuse headers.commons is of type string: "Element implicitly has an 'any' type because index expression is not of type 'number'.ts(7015)".

axios.defaults.headers.common['Authorization'] = AUTH_TOKEN

Expected behavior, if applicable

I expect to be able to configure default headers without TypeScript errors. Using the code examples from the Axios documentation, I would exspect axios.defaults.headers.common to be of type Record<string, string> not of type string.

Environment

• Axios Version 0.22.0
• Adapter N/A
• Browser N/A
• Browser Version N/A
• Node.js Version 15.14.0
• OS: Ubuntu 21.04
• Additional Library Versions TypeScript 4.4.3

Additional context/Screenshots

Add any other context about the problem here. If applicable, add screenshots to help explain.