错误 [ERR_HTTP_HEADERS_SENT]：在向客户端发出请求时无法设置标头，当向 API 发出请求时

Question

I keep getting this error in my terminal after I make a request to the API

[ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (_http_outgoing.js:518:11)
    at ServerResponse.header (D:\GitHub\Obsidion-Dashboard\node_modules\express\lib\response.js:771:10)
    at ServerResponse.send (D:\GitHub\Obsidion-Dashboard\node_modules\express\lib\response.js:170:12)
    at ServerResponse.json (D:\GitHub\Obsidion-Dashboard\node_modules\express\lib\response.js:267:15)
    at D:\GitHub\Obsidion-Dashboard\api\database.js:169:36
    at processTicksAndRejections (internal/process/task_queues.js:97:5) {
  code: 'ERR_HTTP_HEADERS_SENT'
}

My router looks like this

router.get('/prefix', (req, res) => {
    if (!req.query.token) throw new Error('NoTokenProvided');
    if (!req.query.guildid) throw new Error('NoGuildIDProvided');
    const token = req.query.token;
    const guildid = req.query.guildid;

    var sql = `SELECT token FROM auth WHERE token='${token}'`;

    db.all(sql, [], (err, rows) => {
        if (err) {
            throw new Error("ErrorWithDatabase");
        }

        if (rows.length === 0) {
            res.json({code: 401, "message": "That is not a valid token."})
        }

        rows.forEach((row) => {
            if (row.token === token) {
                pool.query(`SELECT prefix FROM guild WHERE id='${guildid}'`)
                .then(response => {
                    // console.log(response.rows[0].prefix)
                    if (!response.rows[0]) {
                        return res.json({code: 200, "message": "User has not setup a custom prefix."})
                    } else {
                        return res.json({code: 200, "prefix": response.rows[0].prefix})
                    }
                })
                .catch(e => console.error(e))
            } else {
                res.json({code: 401, "message": "That is not a valid token."})
            }
        })

    });
})

It shouldn't be trying to send headers to the router after I have returned res.json. It is connecting to a postgresql database, and seems to be connecting to it just fine.

Answer 1

You are attempting to send a response inside a .forEach() which means you will be trying to send more than one response per request. Can't do that. You get to send one response for each incoming request.

If you want to just send the first response that matches, then change your .forEach() to a regular for loop and then when you return, it will terminate the loop.

If you want to send multiple responses, then what you need to do is to collect all the results into a local array and then send them all at the end. And, since you're using asynchronous operations inside the loop, you will either have to use await to pause the loop or use Promise.all() to collect all the asynchronous results and to know when they are all done.

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FYI, in these days of asynchronous programming and block scoped variables, I would just about consider .forEach() to be obsolete because a for loop is superior in every way. It has the ability to use await and actually suspend the loop and it has superior flow control since you can break , continue or return - none of which you can do with .forEach() .