const formSignin = document.querySelector('#formSignin');

formSignin.addEventListener("submit", event =>  {
event.preventDefault();
const formDataSignin = new FormData(formSignin);

fetch('/signin', {method: 'post',    body: formDataSignin})
      .then(res => res.json())
      .then(data =>{  //Alguna respuesta  });

});

console.log(req.body.user);