Empty body in fetch post request

The issue is

mode: 'no-cors'

From the documentation...

Prevents the method from being anything other than HEAD, GET or POST, and the headers from being anything other than simple headers

The simple content-type header restriction allows

• text/plain,
• application/x-www-form-urlencoded, and
• multipart/form-data

This causes your nicely crafted Content-Type: application/json header to become content-type: text/plain (at least when tested through Chrome).

Since your Express server is expecting JSON, it won't parse this request.

I recommend omitting the mode config. This uses the default "cors" option instead.

Since your request is not simple, you'll probably want to add some CORS middleware to your Express server.

Another (slightly hacky) option is to tell Express to parse text/plain requests as JSON. This allows you to send JSON strings as simple requests which can also avoid a pre-flight OPTIONS request, thus lowering the overall network traffic...

app.use(express.json({  type: ['application/json', 'text/plain']}))

EDIT: Added ending parenthesis to app.use final code block.

Please see document mode, no-cors mode only send simple headers. So the content-Type is changed to text/html; instead of application/json so your server can't recognize body format and return empty.

Remove mode: 'no-cors', and you should be fine.

To complement the excellent answers that mentioned removing the mode: 'no-cors' option, and if you don't feel like adding a CORS middleware to Express, you may just want to handle "pre-flight" requests under an OPTIONS block:

app.options('*', (req, res) => {  res.setHeader('Access-Control-Allow-Origin', '*');  res.setHeader('Access-Control-Allow-Headers', 'Content-Type'); // Add other headers here  res.setHeader('Access-Control-Allow-Methods', 'POST'); // Add other methods here  res.send();});

Another nice answer here : https://www.vinaygopinath.me/blog/tech/enable-cors-with-pre-flight/

Hope this helps!