According to the Serverless Overview doc page (https://docs.runpod.io/serverless/workers/overview), we can write functions in the language you're most comfortable with.
There's a Runpod SDK on NPM (https://www.npmjs.com/package/runpod-sdk), but that looks like it's meant to call existing endpoints, not to create handler functions.
Is this possible? If so, are there any templates available to create the handler function in Node.js?