Express middleware "TypeError: Converting circular structure to JSON" when sending a non-c-CodePudding

I am using the following function as middleware and want to send the length of the array (req.session.songArray.length) as a JSON however any JSON I attempt to send through res gives the error below. I understand that circular objects cannot be JSON stringified but just trying to use { num: 20 } gives this error and I am not sure why. When I remove the res.status.json line the code runs without errors. I am using npm packages genius-lyrics and express-session for insight.

async function initSongArray(req, res, next){
    const searches = await Client.songs.search(req.params.artist);
    let songNum = 0
    while((searches[songNum].artist.name.includes('&')) && req.params.and === 0){
        console.log("& detected")
        songNum  ;
    }
    req.session.artist = searches[songNum].artist;
    req.session.songArray = await req.session.artist.songs({sort: 'popularity', perPage: 20, page: 1});
    req.session.currPage = 2
    console.log(req.session.songArray.length)
    res.status(200).json( { num: 20 } )
}

app.get('/artist/:artist/:and', initSongArray)

C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express-session\index.js:598 var str = JSON.stringify(sess, function (key, val) { ^

TypeError: Converting circular structure to JSON --> starting at object with constructor 'Client' | property 'songs' -> object with constructor 'SongsClient' --- property 'client' closes the circle at JSON.stringify () at hash (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express-session\index.js:598:18) at isModified (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express-session\index.js:426:57) at shouldSave (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express-session\index.js:448:11) at ServerResponse.end (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express-session\index.js:334:11) at ServerResponse.send (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express\lib\response.js:232:10) at ServerResponse.json (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\node_modules\express\lib\response.js:278:15) at initSongArray (C:\Users\bgrie\Desktop\a\compsci.websites\typesiteNode\index.js:70:21) at processTicksAndRejections (node:internal/process/task_queues:96:5)

CodePudding user response：

The session is trying to serialize its data into JSON in order to save it to the session store. So, you're trying to put something in the session that has a circular reference in it and thus can't be serialized that way. So, apparently, searches[songNum].artist is not just a simple object. It must have references to other objects that then create a circular reference.

I'd suggest you create a new object and assign just a few simple properties from searches[songNum].artist that you actually need in the session.

Probably searches[songNum].artist has some database stuff in it. If your database has a .toObject() method or something like that, it may be useful to turn it into a plain object.