Error: Can't set headers after they are sent. (Express 4)

I am getting this when using with express 4 and express-flash.

I used with swig and all worked.

Here is failing when render is called, it return the html and then fails.

Wed, 27 Jan 2016 21:41:57 GMT connect:redis GET "5cg6LbHdXhWuUnKXYWL6kPNRfeN1TN9u"
Wed, 27 Jan 2016 21:41:57 GMT connect:redis SET "yWgxdkWw0rTHBpi-RuzqRkYLbOUQGdYG" {"cookie":{"originalMaxAge":null,"expires":null,"httpOnly":true,"path":"/"},"flash":{}} ttl:86400
_http_outgoing.js:335
    throw new Error('Can\'t set headers after they are sent.');
    ^

Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:335:11)
    at ServerResponse.header (/Users/carlitux/Projects/project/node_modules/express/lib/response.js:718:10)
    at ServerResponse.send (/Users/carlitux/Projects/project/node_modules/express/lib/response.js:163:12)
    at done (/Users/carlitux/Projects/project/node_modules/express/lib/response.js:957:10)
    at /Users/carlitux/Projects/project/node_modules/nunjucks/src/environment.js:22:23
    at RawTask.call (/Users/carlitux/Projects/project/node_modules/asap/asap.js:40:19)
    at flush (/Users/carlitux/Projects/project/node_modules/asap/raw.js:50:29)
    at doNTCallback0 (node.js:419:9)
    at process._tickCallback (node.js:348:13)

[carljm]

I don’t use Express, so I probably won’t be able to be much help here. Usually that kind of error is because something is sending output to the client earlier than expected? I don’t have any idea why Nunjucks would be different from Swig in that regard, though.

[xingbofeng]

because you might send no less than 2 times response.

like my code:

  getAllTables().then((data) => res.send(data)).catch((err) => console.log(err));
  next();

I used Promise.then() and my function next() goes before my res.send(), so I had response more than 1 times to front-end. like me, you can write like this.

  getAllTables().then((data) => {
    res.send(data);
    return next();
  }).catch((err) => console.log(err));