res.set('Content-Type','application/force-download');
res.set('Content-Disposition','attachment;filename=aa.txt');
/*
//直接读取文件
var fReadStream = fs.createReadStream(filePath, {
    encoding : 'binary',
    bufferSize : 1024 * 1024
});
*/
//复用你之前得到的buffer 需要引用[stream-buffer](https://www.npmjs.com/package/stream-buffers)
var myReadableStreamBuffer = new streamBuffers.ReadableStreamBuffer({
  frequency: 10       // in milliseconds.
  chunkSize: 2048     // in bytes.
}); 

myReadableStreamBuffer.put(aBuffer);
myReadableStreamBuffer.on('data', function(chunk) {
    res.write(chunk, 'binary');
});
myReadableStreamBuffer.on('end', function() {
    res.end();
});