cannot set body with fetch api code example

Example 1: js fetch 'post' json

//Obj of data to send in future like a dummyDb
const data = { username: 'example' };

//POST request with body equal on data in JSON format
fetch('https://example.com/profile', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify(data),
})
.then((response) => response.json())
//Then with the data from the response in JSON...
.then((data) => {
  console.log('Success:', data);
})
//Then with the error genereted...
.catch((error) => {
  console.error('Error:', error);
});

//																		Yeah

Example 2: .fetch method

fetch('http://example.com/data.json')
  .then(data => data);
  .catch(err => console.log(err));