code: function copyFile(source, target, callback) {
      var reader = fs.createReadStream(source)
        , writer = fs.createWriteStream(target)
        , done = false;
      reader.on('error', oncomplete);
      writer.on('error', oncomplete);
      writer.on('close', oncomplete);
      reader.pipe(writer);
      function oncomplete(err) {
          if (!done) {
              callback(err);
              done = true;
          }
      }
  }
docs: Copy a file.