源代码：

request({
             url: 'https://api.weixin.qq.com/wxa/getwxacodeunlimit?access_token=' + token,
                    method: "POST",
                    json: true,
                    headers: {
                        "content-type": "application/json",
                    },
                    body: {
                        "path": "pages/index/index",
                        "scene": 'a=1'
                    },
                    responseType: 'arraybuffer'
                }, function(error, response, body) {
                    if (!error && response.statusCode == 200) {
                         console.log(body);
                        // let tempo = new Buffer.from(body, 'binary')
                        // tempo = tempo.toString('base64');
                        base64 = 'data:image/png;base64,' + body.toString('base64');
                       // console.log(base64);
                        fs.writeFileSync('./图片base.txt', base64, 'utf-8');
                        fs.writeFileSync('./图片.jpg', body);
                        res.send(base64)
                    }
                });
            }

结果

乱码

问题原因

查看request官方文档发现默认使用utf-8编码

(Note: if you expect binary data, you should set encoding: null.) 注意：如果你需要二进制数据（图片就属于这一类别），你应该将编码格式设成null。
然后，按照官网文档的建议，我设置了下，奇迹发生了

request({
    url: 'https://api.weixin.qq.com/wxa/getwxacodeunlimit?access_token=' + token,
    method: "POST",
    json: true,
    headers: {
                        "content-type": "application/json",
               },
   encoding: null,
   body: {
                        "path": "pages/index/index",
                        "scene": 'a=1'
           },
   responseType: 'arraybuffer'
           }, function(error, response, body) {
   if (!error && response.statusCode == 200) {
             console.log(body);
             // let tempo = new Buffer.from(body, 'binary')
              // tempo = tempo.toString('base64');
              base64 = 'data:image/png;base64,' +   body.toString('base64');
               // console.log(base64);
            fs.writeFileSync('./图片base.txt', base64, 'utf-8');
            fs.writeFileSync('./图片.jpg', body);
            res.send(base64)
                    }
                });