fetch post json请求报错

Question

fetch('http://localhost:3000/books', {
                method: 'post',
                body: JSON.stringify({
                    uname: 'zhangsan',
                    pwd: '456'
                }),
                headers: {
                    'Content-Type': 'application/json'
                }
            })
            .then(function(data) {
                // return data.text();
                return response.json();
            }).then(function(data) {
                console.log(data);

            })

报错提示：

Access to fetch at 'http://localhost:3000/books/123' from origin 'null' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

我前面用x-www-form-urlencoded方式传递参数能正常返回，而且二者请求的是相同的接口，但是换成json方式就报上面的错误了

fetch('http://localhost:3000/books', {
                method: 'post',
                body: 'uname=lisi&pwd=123',
                headers: {
                    'Content-Type': 'application/x-www-form-urlencoded'
                }
            })
            .then(function(data) {
                return data.text();
            }).then(function(data) {
                console.log(data);

            })

后台代码：

app.all('*', function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header('Access-Control-Allow-Methods', 'PUT, GET, POST, DELETE, OPTIONS');
    res.header("Access-Control-Allow-Headers", "X-Requested-With");
    res.header('Access-Control-Allow-Headers', 'Content-Type');
    res.header('Access-Control-Allow-Headers', 'mytoken');
    next();
});

app.post('/books', (req, res) => {
    res.send('POST请求传递参数！' + req.body.uname + '---' + req.body.pwd)
})

上网查了下多数都是说跨域请求问题，试过一遍也没有解决，如果是跨域请求问题，问什么有的报错，有的不报错呢

Answer 1

    res.header("Access-Control-Allow-Headers", "X-Requested-With");
    res.header('Access-Control-Allow-Headers', 'Content-Type');
    res.header('Access-Control-Allow-Headers', 'mytoken');

合并到一行

可以试试 https://www.lilnong.top/cors/1010000022879873 这个接口