如何用Express将对象数组正确写入JSON文件？

我用Express后端从React客户端接收对象数组格式的数据，想要写入JSON文件，但生成的结果和预期不符。

后端代码 #

export const addStatements = async (req,res) => {
    console.log(req.body) // form fields
    
    fs.writeFile('./categories.json', JSON.stringify(req.body), err => {
        if (err) throw err;
        console.log("Done writing!")
    })
}

接收的数据 #

后端收到的req.body结构如下：

{
  categoryFilename: '[{"description":"GOOGLE","category":"MISC"},{"description":"AMZN","category":"MERCHANDISE"},{"description":"MCDONALD","category":"DINNING"},{"description":"SAFEWAY","category":"GROCERIES"},{"description":"WISH","category":"MERCHANDISE"},{"description":"99 RANCH","category":"GROCERIES"},{"description":"PARKMOBILE","category":"COLLEGE FEE"},{"description":"CITY OF SAN RAMON","category":"TENNIS"},{"description":"FILA","category":"APPAREL"},{"description":"COMCAST","category":"TELECOM"},{"description":"PEETS","category":"COFFEE"}]'
}

当前生成的JSON文件内容 #

{"categoryFilename":"[{\"description\":\"GOOGLE\",\"category\":\"MISC\"},{\"description\":\"AMZN\",\"category\":\"MERCHANDISE\"},{\"description\":\"MCDONALD\",\"category\":\"DINNING\"},{\"description\":\"SAFEWAY\",\"category\":\"GROCERIES\"},{\"description\":\"WISH\",\"category\":\"MERCHANDISE\"},{\"description\":\"99 RANCH\",\"category\":\"GROCERIES\"},{\"description\":\"PARKMOBILE\",\"category\":\"COLLEGE FEE\"},{\"description\":\"CITY OF SAN RAMON\",\"category\":\"TENNIS\"},{\"description\":\"FILA\",\"category\":\"APPAREL\"},{\"description\":\"COMCAST\",\"category\":\"TELECOM\"},{\"description\":\"PEETS\",\"category\":\"COFFEE\"}]"}

期望的JSON文件格式 #

[{"description":"GOOGLE","category": "MISC"},{"description":"AMZN","category":"MERCHANDISE"},{"description":"MCDONALD","category":"DINNING"},{"description":"SAFEWAY","category":"GROCERIES"},{"description":"WISH","category":"MERCHANDISE"},{"description":"99 RANCH","category":"GROCERIES"},{"description":"PARKMOBILE","category":"COLLEGE FEE"},{"description":"CITY OF SAN RAMON","category":"TENNIS"},{"description":"FILA","category":"APPAREL"},{"description":"COMCAST","category":"TELECOM"},{"description":"PEETS","category":"COFFEE"}]

解决方案 #

问题核心是：你接收的req.body.categoryFilename是已经被JSON序列化的字符串，不是直接的数组。所以需要先把这个字符串解析成JS数组，再写入文件。

修改后的代码：

export const addStatements = async (req,res) => {
    console.log(req.body)
    try {
        // 解析categoryFilename对应的JSON字符串为数组
        const dataArray = JSON.parse(req.body.categoryFilename);
        // 写入文件，第二个参数加null,2是为了生成格式化的JSON，可选
        fs.writeFile('./categories.json', JSON.stringify(dataArray, null, 2), err => {
            if (err) {
                console.error('写入文件出错:', err);
                return res.status(500).send('写入文件失败');
            }
            console.log("Done writing!")
            res.status(200).send('写入成功');
        })
    } catch (parseErr) {
        console.error('解析JSON字符串出错:', parseErr);
        res.status(400).send('数据格式错误');
    }
}

额外建议 #

如果可以调整客户端代码，建议直接发送数组本身，而不是把数组转成字符串后放在categoryFilename字段里。比如客户端发送POST请求时，直接传数组：

// 客户端示例代码
const data = [{"description":"GOOGLE","category":"MISC"}, ...];
fetch('/api/addStatements', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify(data),
});

这样后端可以直接用req.body写入文件，代码更简洁：

export const addStatements = async (req,res) => {
    fs.writeFile('./categories.json', JSON.stringify(req.body, null, 2), err => {
        if (err) {
            console.error(err);
            return res.status(500).send('写入失败');
        }
        console.log("Done writing!")
        res.status(200).send('写入成功');
    })
}

内容的提问来源于stack exchange，提问作者user7087799