How to do `tail -f logfile.txt`-like processing in node.js?

The canonical way to do this is with fs.watchFile.

Alternatively, you could just use the node-tail module, which uses fs.watchFile internally and has already done the work for you. Here is an example of using it straight from the documentation:

Tail = require('tail').Tail;

tail = new Tail("fileToTail");

tail.on("line", function(data) {
  console.log(data);
});