How do determine the type of data in S3.getObject()

I don't know if you can control in advance the type of the data.Body field provided in the getObject() callback. If all you want to do is determine if you've received a buffer, you can try Node's Buffer.isBuffer(data.Body) class method.

Alternately, you might want to avoid the issue altogether and use this approach from Amazon's S3 documentation:

var s3 = new AWS.S3();
var params = {Bucket: 'myBucket', Key: 'myImageFile.jpg'};
var file = require('fs').createWriteStream('/path/to/file.jpg');
s3.getObject(params).createReadStream().pipe(file);

Presuming you'll be using this code in a typical node.js async callback environment, it might make more sense to see the code like so:

var fs = require('fs');

function downloadFile(key, localPath, callback) {
  var s3 = new AWS.S3();
  var params = {Bucket: 'myBucket', Key: key};
  var file = fs.createWriteStream(localPath);

  file.on('close') {
    callback();
  }

  file.on('error', function(err) {
    callback(err);
  });

  s3.getObject(params).createReadStream().pipe(file);
}

I couldn't find any way to change the Body type either, however after noticing the Body was a buffer, I transformed the buffer into a ReadableStream with this handy & pretty straightforward function: AWS.util.buffer.toStream (or perhaps you might want to use another lib like streamifier).

I was looking for something where I could validate errors before doing anything else, in Amazon's example it translates to "create the Write Stream only if there were no errors".

s3.getObject(params, function(err, data) {
    if (err) {
        console.log(err);
        return;
    }

    var file = require('fs').createWriteStream(name);
    var read = AWS.util.buffer.toStream(data.Body);

    read.pipe(file);
    read.on('data', function(chunk) {
        console.log('got %d bytes of data', chunk.length);
    });
});