Promise.all 和 .map 返回 null

Question

const returnPostData = async (req, res, initialPostsQueryArray) => {
    const promises = initialPostsQueryArray.map((post) => {

        let postObject;

        voteCount = sumValues(post.voting_options);
        pool.query('SELECT voting_option FROM votes WHERE user_id = $1 AND post_id = $2', 
            [req.user.userId, post.post_id], 
            (error, results) => {
                if (error) {
                    console.log(error);
                    return res.json({'Error': error.detail});
                }
                userVoteOption = results.rows[0].voting_option;
        });
        postObject.voteCount = voteCount;
        postObject.userVoteOption = userVoteOption;
        return postObject;
    });

    return Promise.all(promises).then(postData => {
        return res.json(postData);
    })
}

我正在尝试为每个帖子返回一个 postObjects 数组。出于某种原因，它继续为这些对象打印 null ，因为返回的 res.json 甚至在承诺完成之前就以某种方式运行。任何帮助表示赞赏。

我之前遇到过这个问题，使用了相同的代码，但由于某种原因它对这个问题不起作用。

Answer 1

多个问题：

• 你return postObjectbefore userVoteOption is assigned
• 你实际上并没有use promises with node-postgres
• ...因此map 回调的返回值不是一个承诺
• 你从不初始化postObject
• 您将函数标记为async，但从未使用await
• 您单独处理循环中的每个错误（这会导致res.json 在出现多个错误时被多次调用）

要解决这些问题，请使用

async function returnPostData(req, res, initialPostsQueryArray) {
    try {
        const promises = initialPostsQueryArray.map(async (post) => {
            const voteCount = sumValues(post.voting_options);
            const results = await pool.query(
                'SELECT voting_option FROM votes WHERE user_id = $1 AND post_id = $2', 
                [req.user.userId, post.post_id]
            );
            const userVoteOption = results.rows[0].voting_option;
            const postObject = { voteCount, userVoteOption };
            return postObject;
        });
        const postData = await Promise.all(promises);
        res.json(postData);
    } catch(error) {
        console.log(error);
        res.json({'Error': error.detail});
    }
}

此外，您实际上根本不应该在这里使用循环。一次从 postgres 查询多行！使用this approach 提供ID：

async function returnPostData(req, res, initialPostsQueryArray) {
    try {
        const voteCounts = new Map();
        const ids = [];
        for (const post of initialPostsQueryArray) {
            ids.push(post.post_id);
            voteCounts.set(post.post_id, sumValues(post.voting_options));
        }
        const {rows} = await pool.query(
            'SELECT post_id, voting_option FROM votes WHERE user_id = $1 AND post_id = ANY($2::int[])', 
            [req.user.userId, ids]
        );
        const postData = rows.map(row => {
            const postObject = {
                voteCount: voteCounts.get(row.post_id),
                userVoteOption: row.voting_option,
            };
            return postObject;
        });
        const postData = await Promise.all(promises);
        res.json(postData);
    } catch(error) {
        console.log(error);
        res.json({'Error': error.detail});
    }
}