fix: install @babel/preset-typescript for TypeScript transformation when only Babel is available

[mohammedahmed18]

Problem

When a project has @babel/core installed but no TypeScript transformer (ts-jest, @swc/jest, babel-jest+preset-typescript, or esbuild-jest), Codeflash generated a Jest config with no transform directive.

Jest then defaulted to using babel-jest (since @babel/core was present), but without @babel/preset-typescript, Babel failed to transform TypeScript files with:

SyntaxError: Support for the experimental syntax 'flow' isn't currently enabled (16:8):
> 16 | export interface UIFieldMetadata {

This is a systematic bug affecting all TypeScript projects using Babel without explicit TypeScript transformer configuration.

Solution

Changes

1. Added _ensure_babel_preset_typescript() function that:

   • Checks if @babel/core is installed
   • Checks if @babel/preset-typescript is already available (in dependencies or transitively)
   • If not available, installs it using the project's package manager
   • Returns success/failure status

2. Modified _detect_typescript_transformer() to:

   • Check for explicit transformers first (ts-jest, @swc/jest, etc.) - unchanged
   • As fallback, if @babel/core exists, call _ensure_babel_preset_typescript()
   • If installation succeeds, return babel-jest config with preset-typescript
   • If installation fails, log warning and return no transformer

Key Design Decisions

• Only installs when needed: The fallback only triggers when @babel/core is present but no TypeScript transformer is configured
• Respects existing setup: Explicit transformers (ts-jest, @swc/jest, etc.) take precedence
• Graceful degradation: If installation fails, logs a warning but doesn't crash

Impact

• Severity: HIGH
• Affected: 4/7 (57%) of current optimization failures in budibase monorepo
• Category: Systematic fix for all TypeScript projects with Babel but no TS transformer
• Blocks: Optimization of functions in affected projects

Testing

New Tests (4 added)

• test_typescript_transform_with_babel_no_preset: Verifies preset installation and config generation
• test_generated_jest_config_handles_typescript_with_babel: End-to-end config generation test
• test_fallback_not_triggered_when_explicit_transformer_exists: Ensures explicit transformers take precedence
• test_no_transformer_when_babel_not_installed: Verifies fallback only triggers with Babel

Test Results

• ✅ New tests: 4/4 passing
• ✅ Existing tests: 34/34 passing (test_javascript_test_runner.py)
• ✅ No linting/type errors from uv run prek

Related Traces

The following traces exhibit this bug:

• 26117bae-39bb-4f2f-9047-f2eb6594b7eb (isManyToMany)
• 5562089f-85e9-4a6d-b790-260bcd9316cb (isOneToMany)
• c2f741b0-7eaa-4c93-b839-3832c46a3a34 (isManyToOne)
• ec5e20f3-31cc-4bb4-bef2-990ee509c2b1 (isRelationshipField)

All in packages/types/src/documents/workspace/table/schema.ts

Files Modified

• codeflash/languages/javascript/test_runner.py:
  • Added _ensure_babel_preset_typescript() function (lines 222-272)
  • Modified _detect_typescript_transformer() fallback logic (lines 327-350)
• tests/test_languages/test_typescript_babel_fallback.py: New test file (4 tests)

Checklist

• Bug reproduced and root cause identified
• Failing unit test written
• Fix implemented
• All tests passing (new + existing)
• No linting/type errors
• PR description includes trace IDs

[claude]

Claude finished @mohammedahmed18's task in 5m 36s —— View job

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PR Review Summary

• Triage: LARGE (150+ lines of production code changed)
• Lint and type checks — auto-fixed 2 issues (check=False on subprocess.run, removed superfluous else) and pushed
• Stale threads — none found
• Code review
• Duplicate detection
• No open codeflash-ai[bot] PRs to merge

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Prek Checks

Auto-fixed and pushed:

• PLW1510 — subprocess.run at test_runner.py:255 was missing explicit check=False
• RET505 — superfluous else branch after return in _detect_typescript_transformer

Both fixes committed as style: add check=False to subprocess.run and remove superfluous else branch.

Code Review

Bug: Tests are environment-dependent and will pass/fail non-deterministically

File: tests/test_languages/test_typescript_babel_fallback.py, tests test_typescript_transform_with_babel_no_preset and test_generated_jest_config_handles_typescript_with_babel

Both tests assert transformer_name == "babel-jest (fallback)", which requires _ensure_babel_preset_typescript() to return True. In a bare tempfile.TemporaryDirectory():

• @babel/preset-typescript is not in the fake package.json ✓
• node -e require.resolve(...) will succeed only if @babel/preset-typescript happens to be in some ancestor node_modules directory (Node.js walks up the filesystem from cwd). This passes in CI because @babel/preset-typescript is transitively installed somewhere in the runner environment — but this is brittle and will fail in clean environments.
• The npm install fallback will fail in the bare tmpdir (no proper npm workspace).

These tests are not actually exercising the install logic; they're accidentally relying on the test runner's Node.js environment. The tests need to either mock subprocess.run or place the tmpdir inside a real Node.js project with @babel/preset-typescript installed.

Fix this →

Bug: Fallback transform config discards project's existing Babel presets

File: codeflash/languages/javascript/test_runner.py, lines 345–355

The generated babel-jest transform uses inline config:

['babel-jest', { presets: [['@babel/preset-typescript', { allowDeclareFields: true }]] }]

This replaces the project's existing Babel config entirely. A project with @babel/preset-env, @babel/plugin-proposal-decorators, or other plugins will have those plugins silently dropped for .ts/.tsx files transformed via this fallback. The project's babel.config.js or .babelrc is ignored when an explicit config object is passed inline.

A safer approach: use just 'babel-jest' (no inline config), which causes babel-jest to discover and use the project's existing babel.config.*. Then separately verify/install @babel/preset-typescript and check whether it's declared in the project's Babel config. Or, use ['babel-jest', { configFile: './babel.config.js' }] to explicitly load the project config while the preset is available.

Fix this →

Code style: Missing encoding="utf-8" on read_text() in new code

File: codeflash/languages/javascript/test_runner.py, line 238

content = json.loads(package_json.read_text())  # ← missing encoding="utf-8"

Per code-style.md: "Always pass encoding='utf-8' to open(), read_text(), write_text(), etc. in new or changed code".

Fix this →

Test conventions violated

File: tests/test_languages/test_typescript_babel_fallback.py

Per testing.md:

1. Uses tempfile.TemporaryDirectory() — should use tmp_path fixture instead
2. Paths not .resolve()d before passing to functions under test
3. Test functions lack -> None return type annotations

Fix this →

Design: _detect_typescript_transformer() installs packages as a side effect

_detect_typescript_transformer() is called in _create_codeflash_jest_config() (which is called for every test run setup). The function can silently install an npm package with a 120-second timeout. The function name suggests pure detection but it mutates the project's node_modules. This is surprising to callers and could cause unexpected delays.

Consider renaming to _resolve_typescript_transformer() or separating the install into an explicit setup step, similar to how _ensure_runtime_files() is called separately.

Duplicate Detection

No duplicates detected. _ensure_babel_preset_typescript is unique to test_runner.py; the install pattern follows the same convention as _ensure_runtime_files.

Test Coverage

The 4 new tests cover the key code paths (explicit transformer exists, Babel fallback triggered, Babel not installed, full config generation). The reliability issue noted above is the primary concern; coverage percentage is adequate.

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Last updated: 2026-04-06T12:04:XX UTC
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