fix: preserve mixed case enum keys with underscores like _123ValCamelCase, Val_Snake_Case, _Val_12_CamelCase

[PSiemieniuk]

Summary

Fixes enum key formatting to preserve mixed case names with underscores while still transforming pure lowercase snake_case to PascalCase. This resolves TypeScript compilation errors caused by stripping leading underscores from numeric-prefixed identifiers.

Problem

When generating TypeScript enums from OpenAPI schemas, the formatter was incorrectly transforming enum keys that contained underscores and mixed case (e.g., _123ValCamelCase, _Val_12_CamelCase, Val_Snake_Case) into camelCase variants, losing the original casing and underscore structure.

Critical Issue: The transformation of _123ValCamelCase to 123ValCamelCase produced invalid TypeScript code that failed to compile, since identifiers cannot start with a digit. The leading underscore is semantically important and must be preserved.

The issue occurred because the regex pattern in type-name-formatter.ts only checked for all-uppercase constants (/^(?!\d)([A-Z0-9_]{1,})$/g), causing mixed case values to fall through to the lodash.startCase transformation, which would strip meaningful casing information and create syntax errors.

Example Issue

Given this enum schema:

{
"type": "string",
"enum": [
"VAL_1", // SCREAMING_SNAKE_CASE (uppercase only)
"_123ValCamelCase", // Mixed case starting with underscore + digit
"_Val_12_CamelCase", // Mixed case with underscores
"Val_Snake_Case", // Mixed case with underscores
"local_only" // Pure lowercase snake_case
]
}Before:

• _123ValCamelCase → 123ValCamelCase ❌ TypeScript compilation error!
• _Val_12_CamelCase → Val12CamelCase1 ❌
• Val_Snake_Case → ValSnakeCase ❌
• local_only → LocalOnly ✅ (correct)

After:

• _123ValCamelCase → _123ValCamelCase ✅ Valid TypeScript
• _Val_12_CamelCase → _Val_12_CamelCase ✅
• Val_Snake_Case → Val_Snake_Case ✅
• local_only → LocalOnly ✅

Solution

Added specific logic in src/type-name-formatter.ts to detect and preserve mixed case enum keys with underscores. The fix distinguishes between:

1. SCREAMING_SNAKE_CASE (all uppercase + underscores) - already preserved
2. Mixed case with underscores (e.g., _123ValCamelCase, _Val_12_CamelCase) - now preserved
3. Pure lowercase snake_case (e.g., local_only) - still transformed to PascalCase

The solution adds a new condition specifically for enum-key types that checks for:

• Valid identifier pattern with alphanumeric + underscores
• Presence of underscores
• Presence of at least one uppercase letter (mixed case detection: /[A-Z]/.test(name))

This prevents unwanted transformation of intentionally mixed case enum keys while maintaining the existing behavior for pure lowercase snake_case, and critically prevents TypeScript syntax errors from removing semantically important leading underscores.

Changes

Core Fix

• src/type-name-formatter.ts: Added mixed case detection logic (lines 39-46) that preserves enum keys containing both uppercase and lowercase letters with underscores

Test Coverage

• tests/spec/enumIncludesNumber/schema.json: Includes test case for _123ValCamelCase (underscore + digit prefix) and added LowercaseSnakeCaseEnum to test pure lowercase transformation
• tests/spec/enumIncludesNumber/basic.test.ts: Added third test case "transform pure lowercase snake_case to PascalCase" to verify both behaviors
• tests/spec/enumIncludesNumber/snapshots/basic.test.ts.snap: Updated snapshots to reflect correct formatting

Meta

• .changeset/preserve-mixed-case-enum-keys.md: Added changeset (patch)

Testing

All 134 tests pass, including:

• Original enum formatting tests continue to work (SCREAMING_SNAKE_CASE preserved)
• New test cases validate mixed case preservation, especially _123ValCamelCase → _123ValCamelCase (preventing TS errors)
• Existing tests confirm pure lowercase snake_case still transforms to PascalCase (local_only → LocalOnly)
• No regression in other test suites (extended.test.ts, simple.test.ts, etc.)

Note

Fixes enum key formatting by detecting mixed case (uppercase + lowercase) in enum keys and preserving their original form, preventing TypeScript compilation errors from invalid identifiers like 123ValCamelCase, while maintaining PascalCase transformation for pure lowercase snake_case values.

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Latest commit: de5d6ec

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
swagger-typescript-api	Patch

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