Restricting a buffer to the output of a nodejs child process

I'm trying to execute a command line command(here: cat) from nodejs using the spawn command.

var spawn = require('child_process').spawn;
var cat = spawn('cat');
result = new Buffer(2048);
cat.stdout.on('data', function (data) 
 {
    data.copy(result);
 });
cat.on('exit', function (code) 
{
    console.log(result.toString());
});
cat.stdin.write(InBUFFER);
cat.stdin.end();

The output of the command seems to be somewhere in the Buffer, but the rest is outputted as well. Is there some kind of null character that I could use to determine when the output ends?

Update: To clarify things: There's a lot of random characters in the ouput and they seem to be part of the Buffer. I'm looking for a way to divide the useful output of the command line command from the rest of the buffer.