While everything in Node.js is generally done asynchronously, require() is not one of those things. Since modules in practice only need to be loaded once, it is a blocking operation and should be used properly.
Modules are cached after the first time they are loaded. Should you be editing a module in development, you will need to delete its entry in the module cache in order to use new changes. That being said, even if a module is cleared out of the module cache, the module itself is not garbage collected, so care should be taken for its use in production environments.