Summary

Constrain the AxiosHeaders string index signature so dynamic header assignments must be Axios header values instead of any.

🏄 I used an AI coding assistant while preparing this PR, and manually reviewed the issue, implementation, tests, and verification output before submitting.

Linked issue

Fixes #7487

Changes

• Change the AxiosHeaders index signature in index.d.ts and index.d.cts from any to AxiosHeaderValue or callable class members.
• Add ESM and CJS typing coverage that rejects Promise and arbitrary object header assignments while preserving valid dynamic header values.

Checklist

• Tests added or updated
• Docs/types updated if public API changed (index.d.ts and index.d.cts)
• No breaking changes

Verification:

• npm run test:module:esm -- tests/typings.module.test.js
• ./node_modules/.bin/mocha --timeout 10000 tests/typings.module.test.cjs --grep "type-checks commonjs axios typings"
• npx prettier --check index.d.ts index.d.cts tests/module/esm/tests/helpers/esm-index.ts tests/module/cjs/tests/helpers/cjs-typing.ts
• git diff --check
• npm run build

Note: Full local module tests require the CI tarball install step. Running them without installing the packed axios tarball into tests/module/* fails at runtime with Cannot find module 'axios', while the targeted typing tests above pass.

Summary by cubic

Tightens AxiosHeaders index signature to only allow AxiosHeaderValue, undefined, or callable members. Blocks Promises and objects to improve type safety in both ESM and CJS.

Description

• Summary of changes
  • Constrain AxiosHeaders indexer to AxiosHeaderValue | undefined | ((...args: any[]) => any) in index.d.ts and index.d.cts.
• Reasoning
  • Prevents invalid dynamic header assignments (Promises, objects) while allowing optional headers via undefined.
• Additional context
  • Fixes typings bug (not type safety): class field type of AxiosHeaders => any #7487.

Docs

Update /docs/ to:

• List allowed dynamic header values: string, number, boolean, null, undefined.
• Show examples for setting/removing headers; note Promises/objects are rejected.

Testing

• Updated ESM and CJS typing tests to:
  • Accept null and undefined values.
  • Reject Promise and object assignments, including inside interceptors.
• No runtime changes; type-only verification.

Semantic version impact

Patch: type definitions only, no runtime API changes.

^{Written for commit 11dfa88. Summary will update on new commits.}