How do I send JSON data to node.js server using fetch?

I have a node.js back-end server. Front-end is running on Angular 10. I want to pass data from front-end to back-end using fetch

Front-end code :

testmariadb($event: MouseEvent) {
    return fetch('/api/customQuery', {
      method: 'POST',
      headers: {'Content-Type': 'application/json'},
      body: JSON.stringify({
        sqlQuery: 'SELECT * FROM software'
      })
    })
  }

Back-end code

app.post('/api/customQuery', async (req, res) => {
  let test = req.body['sqlQuery'] 
  console.log(test)
})

I get this error :

  let test = req.body['sqlQuery'] 
                     ^

TypeError: Cannot read properties of undefined (reading 'sqlQuery')
    at /home/erwan/Documents/GitHub/serverAltertManager/index.js:90:22

What I'm doing wrong ?

Copyright Notice：Content Author:「Lorteau Erwan」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/72854442/how-do-i-send-json-data-to-node-js-server-using-fetch