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fix(types): constrain AxiosHeaders custom fields#10941
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Narrow custom header field assignments to values axios accepts while keeping declared header methods callable. Constraint: Preserve declared AxiosHeaders methods while narrowing arbitrary string fields. Rejected: Keep the any index signature | It allows non-header values to type-check. Confidence: high Scope-risk: narrow Directive: Keep ESM and CJS declarations in sync for AxiosHeaders type changes. Tested: .\\node_modules\\.bin\\mocha --timeout 10000 tests\\typings.module.test.cjs (from tests/module/cjs) Tested: npx vitest run --config vitest.config.js --project module tests/typings.module.test.js (from tests/module/esm) Tested: npx prettier --check changed typing files Tested: git diff --check Not-tested: full test suite
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1 issue found across 6 files
Confidence score: 4/5
- This PR looks safe to merge with minimal risk: the reported issue is moderate (
5/10) and focused on type constraints rather than a clear runtime break. - In
index.d.cts, theAxiosHeadersstring index still allows arbitrary function values, so custom header fields are not fully restricted to valid header value types. - The impact appears to be mostly TypeScript typing looseness (potentially weaker compile-time safety) rather than a concrete user-facing regression.
- Pay close attention to
index.d.cts- tighten theAxiosHeadersindex signature so custom headers cannot accept arbitrary functions.
Prompt for AI agents (unresolved issues)
Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.
<file name="index.d.cts">
<violation number="1" location="index.d.cts:13">
P2: The new `AxiosHeaders` string index type still permits arbitrary function values, so custom header fields are not fully constrained to header values.</violation>
</file>
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Brand declared AxiosHeaders methods so the string index can accept the class API while custom header fields remain limited to AxiosHeaderValue. Constraint: The AxiosHeaders class needs a string index for custom headers and string-named methods on the same instance. Rejected: Keep a broad function index signature | It lets arbitrary custom header functions type-check. Confidence: high Scope-risk: narrow Directive: Keep ESM and CJS AxiosHeaders declarations aligned. Tested: .\node_modules\.bin\mocha --timeout 10000 tests\typings.module.test.cjs (from tests/module/cjs) Tested: npx vitest run --config vitest.config.js --project module tests/typings.module.test.js (from tests/module/esm) Tested: npx prettier --check index.d.ts index.d.cts tests/module/cjs/tests/helpers/headers-types.ts tests/module/esm/tests/helpers/headers-types.ts tests/module/cjs/tests/typings.module.test.cjs tests/module/esm/tests/typings.module.test.js Tested: git diff --check Not-tested: full test suite
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jasonsaayman
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May 27, 2026
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We may need to hold off on this merge, but let's make these changes in the meantime. This would be a breaking change if it lands.
| export class AxiosHeaders { | ||
| constructor(headers?: RawAxiosHeaders | AxiosHeaders | string); | ||
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| [key: string]: AxiosHeaderValueOrMethod; |
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Generated custom accessors from AxiosHeaders.accessor('X-Foo') are no longer callable by type because unknown keys resolve to AxiosHeaderValueOrMethod, a union with non-callable members. Add ESM and CJS type tests for setXFoo / getXFoo / hasXFoo, then preserve callable typings, likely with template-literal signatures for set${string}, get${string}, and has${string}.
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Add positive assertions that declared methods still call correctly, not only negative custom-field assignments.
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Summary
Fixes #7487
Tests
Summary by cubic
Constrain
AxiosHeadersstring index to valid header values while preserving declared method calls across ESM and CJS. Branded class methods so custom function fields are rejected; type-only change, no runtime impact.Description
AxiosHeadersmethods and typed them as properties to distinguish them from arbitrary functions.[key: string]toAxiosHeaderValueOrMethodinindex.d.tsandindex.d.ctsso custom fields accept only header values; methods remain callable.Promiseand custom function assignments./docs/to list allowed custom header value types and note that promises/functions are not allowed.Semantic version impact
Written for commit d31a7a0. Summary will update on new commits. Review in cubic