控制台报的错误是：

Access to XMLHttpRequest at 'http://localhost:3000/error' from origin 'null' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.
GET http://localhost:3000/error net::ERR_FAILED

 

而不是400状态码

所以我们需要在app.js文件中加入

app.all('*', function (req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "X-Requested-With");
    res.header("Access-Control-Allow-Methods", "PUT,POST,GET,DELETE,OPTIONS");
    res.header("X-Powered-By", ' 3.2.1')
    res.header("Content-Type", "application/json;charset=utf-8");
    next();
});

app.get('/error', (req, res) => {
    res.status(400).send('not ok');

})

 

这样就解决了这个问题