node报错Request header field Content-Type is not allowed by

When I cross-domain request a post in the front console error Request header field Content-Type is not allowed by Access-Control-Allow-Header. This is because when I set up cross-domain request type of request is not required in the back-end, as shown:   app.all('*', function (req, res, next) {   // header specifies whether the resource in response to the response are allowed to share with a given origin. * Represents all domains are accessible, and can be specified instead of the * url, it means that only the specified url can access to resources      res.header("Access-Control-Allow-Origin", "*");     res.header("Access-Control-Allow-Headers",  "X-Requested-With");     // a manner that allows the requested resource     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();   });   Therefore what is provided directly from the code fragment:   app.all('*', function (req, res, next) {   // header specifies whether the resource in response to the response are allowed to share with a given origin. * Represents all domains are accessible, and can be specified instead of the * url, it means that only the specified url can access to resources      res.header("Access-Control-Allow-Origin", "*");     res.header("Access-Control-Allow-Headers",  " Origin, X-Requested-With, Content-Type, Accept");     // a manner that allows the requested resource     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();   });