TypeError: Converting circular structure to JSON

[BumpyBill]

Im trying to send the request data, but I get an error message. Any idea on how I can fix this? (This error only occurs when I send JSON)

Here is my server.js:

const express = require("express");
const cors = require("cors");
const bodyParser = require("body-parser");

const app = express();


app.use(cors());

const routes = { post: require("./routes/post") };
app.use("/post", routes.post);

const PORT = process.env.PORT || 8000;


app.listen(PORT, (err) => {
  if (err) console.log(err);
  else console.log(`Listening On Port ${PORT}`);
});

And here is my routes/post.js (where the main issue is I believe):

const router = require("express").Router();

router.post("/example", (req, res) => {
  res.json(req);
});

module.exports = router;

Edit: Here's the error message:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Socket'
    |     property 'parser' -> object with constructor 'HTTPParser'
    --- property 'socket' closes the circle
    at JSON.stringify (<anonymous>)
    at C:\Users\624234\Documents\ExpressTemplate\server\routes\post.js:4:17
    at Layer.handle [as handle_request] (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\layer.js:95:5)
    at C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\index.js:281:22
    at Function.process_params (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\index.js:335:12)
    at next (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\index.js:275:10)
    at Function.handle (C:\Users\624234\Documents\ExpressTemplate\node_modules\express\lib\router\index.js:174:3)

[BumpyBill]

I think the "req" object was too long, but I won't close it because I'm probably incorrect

[ghinks]

Yes there is a circular dependency between req and res and this is not a bug. Try sending particular props from the req object back in the response.

this is from the docs

var express = require('express')
var bodyParser = require('body-parser')

var app = express()

// parse application/x-www-form-urlencoded
app.use(bodyParser.urlencoded({ extended: false }))

// parse application/json
app.use(bodyParser.json())

app.use(function (req, res) {
  res.setHeader('Content-Type', 'text/plain')
  res.write('you posted:\n')
  res.end(JSON.stringify(req.body, null, 2))
})

[davidmashe]

Not a bug. @ghinks gave a good example, circular objects being non-serializable is expected behavior. Closing issue.

[gzwei1]

non-serializable

Hi @davidmashe , does circular in 'circular objects' mean recursive?

[rofrol]

This worked for me:

https://stackoverflow.com/questions/11616630/how-can-i-print-a-circular-structure-in-a-json-like-format/21875464#21875464

I have just copied decycle function from https://github.com/douglascrockford/JSON-js/blob/master/cycle.js and then:

import { writeFileSync } from 'fs';
// ...
writeFileSync('res.json', JSON.stringify(decycle(res, undefined)));

instead of import you can use const fs = require('fs');;

https://stackoverflow.com/questions/43622337/using-import-fs-from-fs