async writes

gistfile1.js

var fs = require('fs'),
  http = require('http');

var out = fs.createWriteStream('some-file', {flags:'w',encoding:'utf8'});

http.createServer(function(req, res){
 var buffer = [];
 req.on('data', [].push.bind(buffer)).on('end', function(){
   out.write(data);
 });
});