Why Do I Get a 'Can't set headers after they are sent' Error in Node.js/Express?

Error: "Can't set headers after they are sent" in Node.js/Express

This error occurs when attempting to set headers after the response has already been sent to the client. In Node.js and Express, there are certain rules regarding the order in which response methods can be called:

Head State: Before res.writeHead() is called, only res.setHeader() and similar functions can be used to set headers.

Body State: After res.writeHead() is called, the headers can no longer be modified. Only res.write() and res.end() can be used to write data to the response body.

In the provided code, the error occurs because a redirect is attempted after the response headers have already been sent by res.writeHead(). Specifically, the following code causes the issue:

res.redirect("/great");

To fix this, the redirect must be done before res.writeHead() is called. Here's how the code can be modified:

app.get('/auth/facebook', function(req, res) {
  req.authenticate("facebook", function(error, authenticated) {
    if (authenticated) {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.redirect("/great");
      console.log("ok cool.");
      console.log(res['req']['session']);
    }
  });
});

By making this change, the redirect will be attempted before the response headers are sent, preventing the error from occurring.

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