How to use Fetch api to post text/plain

I try to use Fetch() API to post text/plain

fetch('/test', {
    method: 'post',
    headers: {
      'Content-Type': 'text/plain'
    },
    body: "this is a test"
  })
 .then(response => response.text())
 .then(body => {
    console.log(body)
 })

This is the router that deal with the request

router.post('/test',(req,res)=>{
    console.log(req.body)
    res.send('OK')
})

However, the output of console.log(req.body) is {}.

I think the problem is the code after body:. I don’t know what is the right way to post text/plain data.