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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"require.extensions is undefined on synthetic require for CJS loaded through ESM loader (regression in v24.15.0)","articleBody":"### Version\n\nv24.15.0 (fails). Works on v24.14.1 and earlier. Platform: Linux x64 (also reproduced inside `node:24.15.0-slim` Docker image).\n\n### Platform\n\n```text\nLinux x64\n```\n\n### Subsystem\n\n`loaders`, `module`, specifically the CJS-from-ESM translator (`node:internal/modules/esm/translators`).\n\n### What steps will reproduce the bug?\n\n**Reproducer repo:** https://github.com/kabo/node-24.15-require-extensions-repro\n\n```sh\ngit clone https://github.com/kabo/node-24.15-require-extensions-repro\ncd node-24.15-require-extensions-repro/variant-b\ntouch yarn.lock\ncorepack yarn install\ncorepack yarn node run.mjs   # crashes on v24.15.0; succeeds on v24.14.1\n```\n\nThe reproducer is ~10 lines of code split across 4 files: a Yarn 4.14.1 PnP project (`package.json`), an ESM entry (`run.mjs`) that does `await import('dummy-cjs')`, and a trivial local CJS dependency (`dummy-cjs/index.cjs`) whose only relevant line is:\n\n```js\nconst oldJSHook = require.extensions['.js']  // line that throws on v24.15.0\n```\n\nSee the repo's `README.md` for full context, and `output-variant-b-24.14.0.txt` / `output-variant-b-24.15.0.txt` for the side-by-side outputs that establish the regression.\n\n\n### How often does it reproduce? Is there a required condition?\n\nEvery time on v24.15.0 when a CJS file is loaded via the ESM loader through a custom resolver/loader that produces a non-file URL (Yarn PnP loads from `file://.../cache/....zip/node_modules/...`).\n\n\n### What is the expected behavior? Why is that the expected behavior?\n\n`require.extensions` should be defined on the `require` function passed into the CJS module wrapper. It is deprecated (DEP0007) but has been part of the public `require` surface since 0.12. Multiple libraries still in active use (Next.js `next-config-ts/require-hook.js`, ts-node, esbuild-register, pirates, etc.) read it at CJS module top level.\n\nConfirmed behavior on v24.14.1:\n\n```\nnode: v24.14.1\ntypeof require.extensions: object\n.js hook: function\n```\n\n\n### What do you see instead?\n\nOn v24.15.0:\n\n```\n[dummy-cjs] node: v24.15.0\n[dummy-cjs] typeof require: function\n[dummy-cjs] typeof require.extensions: undefined\nfile:///.../.yarn/berry/cache/dummy-cjs-file-6872e80d06-10.zip/node_modules/dummy-cjs/index.cjs:6\nconst oldJSHook = require.extensions['.js']\n                                    ^\n\nTypeError: Cannot read properties of undefined (reading '.js')\n    at Object.\u003canonymous\u003e (file:///.../.yarn/berry/cache/dummy-cjs-file-6872e80d06-10.zip/node_modules/dummy-cjs/index.cjs:6:37)\n    at loadCJSModule (node:internal/modules/esm/translators:185:3)\n    at ModuleWrap.\u003canonymous\u003e (node:internal/modules/esm/translators:231:7)\n    at ModuleJob.run (node:internal/modules/esm/module_job:437:25)\n    at process.processTicksAndRejections (node:internal/process/task_queues:104:5)\n    at async node:internal/modules/esm/loader:639:26\n    at async file:///.../run.mjs:1:1\n\nNode.js v24.15.0\n```\n\nNote `typeof require` is `function` — a `require` is provided — but `require.extensions` is `undefined` on it, which differs from what CJS modules (and the `Module.prototype.require` used in filesystem-loaded CJS) see.\n\n\n### Additional information\n\n- **Suspected cause**: [nodejs/node#61769](https://github.com/nodejs/node/pull/61769) (\"lib: reduce cycles in esm loader and load it in snapshot\"), which rewrote the ESM loader initialization and the synthesised `require` construction for CJS-from-ESM. That PR landed in v24.x for the first time in v24.15.0.\n- **Related bug in same subsystem**: [nodejs/node#62012](https://github.com/nodejs/node/issues/62012) — EBADF fstat on zip fds in the same `createCJSModuleWrap` path. Same Node version, same Yarn PnP setup, different symptom in the same synthetic-CJS-wrap code.\n- **Real-world impact**: `next build` on Next.js 15+ fails for all users on Yarn PnP + Node 24.15 because Next's [`next/dist/build/next-config-ts/require-hook.js`](https://github.com/vercel/next.js/blob/canary/packages/next/src/build/next-config-ts/require-hook.ts#L35) reads `require.extensions['.js']` at module top level.\n- **Not reproducible** when the CJS file is loaded from a filesystem path (without a custom PnP-like loader in front), which suggests the bug is specific to how the ESM→CJS wrap constructs `require` for modules whose source URL is not a plain `file:///.../something.cjs`.\n- **Likely also affects v25.7+** since PR #61769 landed on v25 first.\n\n## Workaround\n\n- Downgrade to Node v24.14.1.\n- Or, for library authors reading `require.extensions`: use optional chaining (`require.extensions?.['.js']`) so the call at module load doesn't throw.\n","author":{"url":"https://github.com/kabo","@type":"Person","name":"kabo"},"datePublished":"2026-04-17T10:43:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/62786/node/issues/62786"}