Mongoose Cannot set headers after they are sent to the client

Question

I have this Mongoose/Express code that lets you update a document from the logged in user. I am getting an error: node:events:368 throw er; // Unhandled 'error' event ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

This is my code.

router.post("/update/:id", checkAuth, (req, res) => {
        // check if the snippet was created by the user who is logged in and update the snippet with the new data from the request body
        Snippet.findOne({
            _id: req.params.id,
            userId: req.userData.userId,
        })
            .then((snippet) => {
                if (!snippet) {
                    return res.status(400).json({
                        message: "You don't have a snippet with this ID",
                    });
                }
    
                if (req.body.title) {
                    console.log("if statement");
                    Snippet.findOne(
                        {
                            title: req.body.title,
                            userId: req.userData.userId,
                        },
                        (titleCheckErr, titleCheckSnippet) => {
                            if (titleCheckErr) {
                                return res.status(500).json({
                                    error: titleCheckErr,
                                });
                            }
                            if (titleCheckSnippet) {
                                return res.status(400).json({
                                    message:
                                        "You already have a snippet with that title",
                                });
                            }
                        }
                    );
    
                    snippet.title = req.body.title;
                }
                snippet.description = req.body.description;
                snippet.code = req.body.code;
                snippet.language = req.body.language;
    
                // save the snippet
                snippet.save((saveSnippetErr, saveSnippet) => {
                    if (saveSnippetErr) {
                        return res.status(500).json({
                            error: saveSnippetErr,
                        });
                    }
                    return res.status(201).json({
                        message: "Snippet updated",
                        snippet: saveSnippet,
                    });
                });
            })
            .catch((err) => {
                return res.status(500).json({
                    error: err,
                });
            });
    });

Answer 1

I figured it out. I was making it too complicated. I simply changed it from using mongoose to check if the title exists, and made it a simple if statement that kept the title the same from db if there was no request body title. Here's the code.

router.post("/update/:id", checkAuth, (req, res) => {
    // check if snippet was created by the user who is logged in, and update the snippet with new data from the request body
    Snippet.findOne(
        { _id: req.params.id, userId: req.userData.userId },
        (err, snippet) => {
            if (err) {
                return res.status(500).json({
                    error: err,
                });
            }
            if (!snippet) {
                return res.status(400).json({
                    message: "Snippet not found",
                });
            }
            if (req.body.title) {
                snippet.title = req.body.title;
            } else {
                snippet.title = snippet.title;
            }
            snippet.description = req.body.description;
            snippet.code = req.body.code;
            snippet.language = req.body.language;

            // save the snippet
            snippet.save((saveErr, saveSnippet) => {
                if (saveErr) {
                    return res.status(500).json({
                        error: saveErr,
                    });
                }
                console.log("updated");
                return res.status(201).json({
                    message: "Snippet updated",
                    snippet: saveSnippet,
                });
            });
        }
    );
});