Jest transformIgnorePatterns Not Excluding an ESM Dependency

By Kaveh Alemi·Latchkey

You added a package to transformIgnorePatterns but Jest still throws on its ESM syntax. The negative-lookahead regex is almost always slightly wrong - a scoped name, a nested hoist, or a missing alternation makes the pattern fail to match.

What this error means

Despite a transformIgnorePatterns: ["/node_modules/(?!pkg)"] entry, Jest still reports an unexpected token inside that package. The regex looks right but does not actually exclude the file Jest is failing on.

SyntaxError: Unexpected token 'export'

  at node_modules/.pnpm/@scope+ui@2.0.0/node_modules/@scope/ui/index.js
  // transformIgnorePatterns: ['/node_modules/(?!@scope/ui)']  -> still ignored

Diagnose it: flake, environment, or genuine failure?

Before debugging the assertion, establish whether the test is deterministic. A test that fails only in CI is usually order-dependent, time-dependent, or racing something, and fixing the assertion will not help.

# does it fail in isolation?
npx vitest run path/to/file.test.ts

# is it order dependent? run the suite in a random order twice
npx vitest run --sequence.shuffle

# is it a race? run the same file repeatedly
for i in $(seq 1 20); do npx vitest run path/to/file.test.ts || break; done

Common causes

Scoped package name not escaped/grouped

A scoped package like @scope/ui needs the / matched literally and the group anchored correctly. (?!@scope/ui) can miss the path when the slash is consumed elsewhere.

Nested or hoisted copy under a store path

pnpm/Yarn store the package under .pnpm/... or a nested node_modules, so a pattern anchored only at the top-level node_modules/ does not match the real file path.

How to fix it

Write a pattern that matches every install layout

Allow-list multiple packages and account for nested node_modules with a non-greedy prefix.

// jest.config.js
module.exports = {
  transformIgnorePatterns: [
    'node_modules/(?!(?:\\.pnpm/)?(@scope/ui|other-esm)/)',
  ],
};

Confirm the exact failing path

1. Read the path in the stack trace - match your regex against that literal string.
2. For pnpm, include the .pnpm/ store segment in the alternation.
3. Re-run with --no-cache so an old transform result is not reused.

CI-only causes worth ruling out

• Runners have fewer cores than a laptop, so timing-sensitive tests that pass locally fail under contention.
• No TTY and a different locale or timezone. Snapshot tests containing formatted dates or numbers are the usual casualty; pin TZ and LANG in the job.
• Parallel workers sharing a database, a port, or a temp directory. Give each worker its own namespace.
• Default timeouts calibrated on a fast machine. A cold runner is slower on first execution, especially before any cache warms.

How to prevent it

• Test the regex against the real node_modules path, not the package name alone.
• Account for pnpm/Yarn store layouts in the pattern.
• Clear the Jest cache when changing transform config.

Frequently asked questions

Related guides

References

• GitHub Actions documentation