I can't vouch for either of these, personally, but if you're hell-bent on doing the work in-process, there have been a couple of independent implementations of the WebWorkers API for node, as listed on the node modules page:
At first glance, the second looks more mature, and these would both allow you to essentially do threaded programming, but it's basically actor-model, so it's all done with message passing, and you can't have shared data structures or anything.
Also, for what it's worth, the node.js team intends to implement precisely this API natively, eventually, so these tools, even if they're not perfect, may be a decent stopgap.
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