CODEWITHSUNDEEP
node.jsexpressnode-modules
Jake Walker
Jake Walker

Reputation: 305

Why can't I use res.json() twice in one post request?

I've got an chatbot app where I want to send one message e.g. res.json("Hello") from express, then another message later e.g. res.json("How are you doing"), but want to process some code between the two.

My code seems to have some problems with this, because when I delete the first res.json() then the second one works fine and doesn't cause any problems.

Looking in my heroku logs, I get lots of gobbledy gook response from the server, with an IncomingMessage = {}, containing readableState and Server objects when I include both of these res.json() functions.

Any help would be much appreciated.

Upvotes: 0

Views: 2970

Answers (2)

jfriend00
jfriend00

Reputation: 707786

HTTP is request/response. Client sends a request, server sends ONE response. Your first res.json() is your ONE response. You can't send another response to that same request. If it's just a matter of collecting all the data before sending the one response, you can rethink your code to collect all the data before sending the one response.

But, what you appear to be looking for is "server push" where the server can send data to the client continually whenever it wants to. The usual solution for that is a webSocket connection (or socket.io which is built on top of webSocket and adds more features).

In the webSocket/socket.io architecture, the client makes a connection the server and the connection is kept open indefinitely. Then either side of the connection can send messages to the other end. This is most useful when the server wants to "push" data to the client at any time. In this case, the client establishes the connection, then the server can send data to the client over that connection at any time. The client registers a listener for incoming messages and will be notified anytime the server sends it some data.

Both webSocket and socket.io are fully supported in modern browsers and in node.js. I would personally recommend using socket.io because some of the features it adds (a messaging layer, auto-reconnect, etc...) are very useful.

To use a continuously connected socket like this, you will have to make sure your hosting infrastructure is properly configured to allow it.

Upvotes: 1

Robert Moskal
Robert Moskal

Reputation: 22553

res.json() always sends the response to the client immediately (calling it again will cause an error). If you need to gradually build up a response then you can progressively decorate a plain old javascript object; for example, appending items to an array. When you are done call res.json() with the constructed response.

But you should post your code so we can see what's happening.

Upvotes: 1

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