Node / Express: Why can I execute code after res.send?

Question

Node / Express: Why can I execute code after res.send?

I'm wondering what the mechanics behind the behaviour of the following code are:

res.send( 200, { data: 'test data' } );
console.log( 'still here...' );

My understanding is that res.send doesn't return the function, but does close the connection / end the request. This could explain why I can still execute code after a res.send command (I looked through the express source and it doesn't seem to be an asynchronous function).

Is there something else at play that I may be missing?

Thanks,

node.js
express

Answer 1

Sure end ends the HTTP response, but it doesn't do anything special to your code.

You can continue doing other things even after you've ended a response.

What you can't do, however, is do anything useful with res. Since the response is over, you can't write more data to it.

res.send(...);
res.write('more stuff'); // throws an error since res is now closed

This behavior is unlike other traditional frameworks (PHP, ASP, etc) which allocate a thread to a HTTP request and and terminate the thread when the response is over. If you call an equivalent function like ASP's Response.End, the thread terminates and your code stops running. In node, there is no thread to stop. req and res won't fire any more events, but the code in your callbacks is free to continue running (so long as it does not attempt to call methods on res that require a valid response to be open).

Answer 2

I don't have enough reputation to comment but feel this is relevant enough to be noted:

A simple way to stop the execution of the function and send a response at the same time is by doing

return res.send('500', 'Error message here');

This allows you to use short if statements to handle errors such as:

Model.findOne({id: Model_id }, function (err, document) { // Mongoose model

    if (err)
      return res.send('500', 'Error message here');

});

The exact return of the res.send function is an object that seems to contain the entire state of the connection after you ended it (request, status, headers, etc.), but this should be unimportant since you won't be doing anything with it.