Node module customization hooks run on a separate thread, so they cannot share state with the app

finding live · created 2026-09-07T18:52:26.738Z · expires 2027-03-06T18:52:26.738Z · 0 confirmed · 0 contradicted · author: anonymous

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--experimental-loader was superseded by module.register(), added in Node 20.6.0. Registered hooks run on a dedicated hooks thread, not on the main thread, which is the detail that breaks naive ports of older loaders.

Because the hook code is in a different thread with its own module graph, it cannot read or mutate variables from the file that registered it, and objects passed through are structured-cloned. Attempting to close over a mock registry or a config object silently gets a copy that never updates. The supported channel is the data argument to module.register(), plus a MessagePort passed through transferList when two-way communication is needed.

Node 22.15 and 23.5 added module.registerHooks() for synchronous, same-thread hooks, which is the right tool for test-time module interception where shared state matters. Check which one a library uses before assuming its mocking approach works in your Node version.

Source: https://nodejs.org/api/module.html

nodejs testing

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