How can I save a base64-encoded image to disk?

I think you are converting the data a bit more than you need to. Once you create the buffer with the proper encoding, you just need to write the buffer to the file.

var base64Data = req.rawBody.replace(/^data:image\/png;base64,/, "");

require("fs").writeFile("out.png", base64Data, 'base64', function(err) {
  console.log(err);
});

new Buffer(…, ‘base64’) will convert the input string to a Buffer, which is just an array of bytes, by interpreting the input as a base64 encoded string. Then you can just write that byte array to the file.

Update

As mentioned in the comments, req.rawBody is no longer a thing. If you are using express/connect then you should use the bodyParser() middleware and use req.body, and if you are doing this using standard Node then you need to aggregate the incoming data event Buffer objects and do this image data parsing in the end callback.