`@hapi/lab` crashes when TypeScript tests depend on ESM packages

[setthase]

Runtime

node.js

Runtime version

24.13.1 (any version >= 22.12.0 with unflagged require(esm))

Module version

26.0.0

Last module version without issue

N/A — the bug has existed since addTsEsmHook was introduced, but only manifests on Node 22.12+ where require(esm) is unflagged.

Used with

standalone (reproducible with minimal setup)

Any other relevant information

Affects projects using lab --typescript whose package.json has "type": "module" and that depend on an ESM package which itself has CJS sub-dependencies. The bug triggers when Node resolves a CJS sub-dependency of an ESM module during loadESMFromCJS, calling Module._resolveFilename with parent set to undefined.

---

What are you trying to achieve or the steps to reproduce?

Run TypeScript tests with lab --typescript in an ESM project ("type": "module") that depends on an ESM package which has its own CJS dependencies.

Trigger conditions

All four must be true:

1. The project has "type": "module" in its package.json (so addTsEsmHook is activated).
2. Lab is invoked with --typescript (or typescript: true in .labrc).
3. Test code (transitively) imports from an ESM package (a dependency with "type": "module").
4. That ESM dependency itself imports a CJS sub-dependency whose specifier ends in .js.

Condition 4 is the non-obvious one: a leaf ESM package with zero dependencies does not trigger the bug. The crash only occurs when Node resolves a .js request during the ESM→CJS boundary transition inside loadESMFromCJS, at which point Module._resolveFilename is called with parent === undefined.

Minimal reproduction

Create a directory with the following structure:

lab-esm-repro/
├── package.json
├── tsconfig.json
├── src/
│   └── math.ts
├── test/
│   └── math.ts
└── node_modules/
    ├── esm-dep/
    │   ├── package.json
    │   └── index.js
    └── cjs-helper/
        ├── package.json
        └── lib/
            └── utils.js

package.json

{
  "name": "lab-esm-repro",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "lab test --typescript"
  },
  "devDependencies": {
    "@hapi/lab": "^26.0.0",
    "@hapi/code": "^9.0.3",
    "typescript": "^5.6.0"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "target": "ES2022",
    "outDir": "dist",
    "rootDir": ".",
    "strict": true
  }
}

src/math.ts

import { add } from 'esm-dep';

export function addOne(n: number): number {
    return add(n, 1);
}

test/math.ts

import * as Lab from '@hapi/lab';
import { expect } from '@hapi/code';
import { addOne } from '../src/math.js';

export const lab = Lab.script();
const { describe, it } = lab;

describe('addOne', () => {
    it('adds 1 to a number', () => {
        expect(addOne(4)).to.equal(5);
    });
});

node_modules/esm-dep/package.json

{
  "name": "esm-dep",
  "version": "1.0.0",
  "type": "module",
  "exports": {
    ".": "./index.js"
  },
  "dependencies": {
    "cjs-helper": "1.0.0"
  }
}

node_modules/esm-dep/index.js — ESM module that imports a CJS sub-dependency

import { double } from 'cjs-helper/lib/utils.js';

export function add(a, b) {
    return double(a) - a + b;
}

node_modules/cjs-helper/package.json — plain CJS package

{
  "name": "cjs-helper",
  "version": "1.0.0"
}

node_modules/cjs-helper/lib/utils.js

'use strict';

exports.double = function (n) {
    return n * 2;
};

Then run:

npm install
npx lab test --typescript

Why the CJS sub-dependency matters

Lab's addTsEsmHook patches Module._resolveFilename globally. When lab loads a .ts test file via require.extensions['.ts'] → Module._compile, the compiled code require()s esm-dep. Because esm-dep is ESM, Node delegates to loadESMFromCJS → ESM translators. Inside the ESM module graph, when esm-dep/index.js imports cjs-helper/lib/utils.js, Node calls Module._resolveFilename to resolve the CJS module — but in this ESM-to-CJS transition context, parent is undefined. The patched hook does not guard against this and crashes.

A leaf ESM package with no imports does not trigger this path because Module._resolveFilename is never called within the ESM graph for its own dependencies.

---

What was the result you got?

TypeError: Cannot read properties of undefined (reading 'filename')
    at Module._resolveFilename (/path/to/node_modules/@hapi/lab/lib/cli.js:92:48)
    at defaultResolveImpl (node:internal/modules/cjs/loader:1059:19)
    at resolveForCJSWithHooks (node:internal/modules/cjs/loader:1064:22)
    at Module._load (node:internal/modules/cjs/loader:1234:25)
    at loadCJSModuleWithModuleLoad (node:internal/modules/esm/translators:339:3)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/translators:242:7)
    at ModuleJobSync.runSync (node:internal/modules/esm/module_job:534:37)
    at ModuleLoader.importSyncForRequire (node:internal/modules/esm/loader:388:47)
    at loadESMFromCJS (node:internal/modules/cjs/loader:1621:24)
    at Module._compile (node:internal/modules/cjs/loader:1786:5)
    at Object.require.extensions.<computed> [as .js] (.../lab/lib/modules/transform.js:41:28)

Lab crashes when Node resolves the CJS sub-dependency of the ESM package.

---

What result did you expect?

The test should run and pass:

  addOne
    ✔ 1) adds 1 to a number (1 ms)

1 tests complete

[setthase]

Root Cause

internals.addTsEsmHook() in lib/cli.js patches Module._resolveFilename to redirect .js imports to .ts equivalents for TypeScript support. The patched function accesses parent.filename without checking if parent exists:

// lib/cli.js — internals.addTsEsmHook
Module._resolveFilename = function (request, parent, ...rest) {

    if (request.endsWith('.js') && (parent.filename.endsWith('.ts') || parent.filename.endsWith('.tsx'))) {
    //                               ^^^^^^^ crashes here when parent is undefined

The crash path is:

1. Lab installs the hook (project has "type": "module" + --typescript).
2. Lab loads a .ts test file via require.extensions['.ts'] → Module._compile.
3. The compiled code require()s an ESM dependency.
4. Node delegates to loadESMFromCJS → ESM module graph.
5. Inside the ESM graph, when a CJS sub-dependency is resolved, Node calls Module._resolveFilename with parent === undefined (there is no CJS parent module in this context — the call originates from the ESM translator layer).
6. The hook accesses parent.filename → TypeError.

---

Suggested Fix

Add a null guard for parent before accessing parent.filename:

- if (request.endsWith('.js') && (parent.filename.endsWith('.ts') || parent.filename.endsWith('.tsx'))) {
+ if (request.endsWith('.js') && parent && parent.filename && (parent.filename.endsWith('.ts') || parent.filename.endsWith('.tsx'))) {

When parent is undefined (ESM-to-CJS transition context), the hook falls through to the original _resolveFilename, which handles the resolution natively. This is the correct behavior — the .js → .ts redirection should only apply when the parent module is a TypeScript file.

---

Related

• Dynamic import ESM based dependency in Typescript CJS based repo #1077 — similar area (TypeScript + ESM), different root cause (lab overrides module in tsconfig)
• Node.js require(esm) docs: https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require