如何在Post请求中随json参数一起发送表单数据？

var formData = {
  username: 'John',
  password: '123456'
};

var jsonData = JSON.stringify(formData);

axios.post('/api/endpoint', jsonData)
  .then(function(response) {
    console.log(response);
  })
  .catch(function(error) {
    console.log(error);
  });

fetch('/api/endpoint', {
  method: 'POST',
  body: jsonData,
  headers: {
    'Content-Type': 'application/json'
  }
})
  .then(function(response) {
    console.log(response);
  })
  .catch(function(error) {
    console.log(error);
  });

$.ajax({
  url: '/api/endpoint',
  type: 'POST',
  data: jsonData,
  contentType: 'application/json',
  success: function(response) {
    console.log(response);
  },
  error: function(error) {
    console.log(error);
  }
});