[Edit] After reviewing the Mongoose documentation, it looks like you can send each query result as a separate chunk; the web server uses chunked transfer encoding by default so all you have to do is wrap an array around the items to make it a valid JSON object.
Roughly (untested):
app.get('/users/:email/messages/unread', function(req, res, next) {
var firstItem=true, query=MessageInfo.find(/*...*/);
res.writeHead(200, {'Content-Type': 'application/json'});
query.each(function(docs) {
// Start the JSON array or separate the next element.
res.write(firstItem ? (firstItem=false,'[') : ',');
res.write(JSON.stringify({ msgId: msg.fileName }));
});
res.end(']'); // End the JSON array and response.
});
Alternatively, as you mention, you can simply send the array contents as-is. In this case the response body will be buffered and sent immediately, which may consume a large amount of additional memory (above what is required to store the results themselves) for large result sets. For example:
// ...
var query = MessageInfo.find(/*...*/);
res.writeHead(200, {'Content-Type': 'application/json'});
res.end(JSON.stringify(query.map(function(x){ return x.fileName })));