nodejs send download file from buffer
var stream = require('stream');
//...
app.get('/download', function(request, response){
//...
var fileContents = Buffer.from(fileData, "base64");
var readStream = new stream.PassThrough();
readStream.end(fileContents);
response.set('Content-disposition', 'attachment; filename=' + fileName);
response.set('Content-Type', 'text/plain');
readStream.pipe(response);
});