var formData = new FormData();
formData.append("aa", document.querySelector("#aa").value);
formData.append("bb", document.querySelector("#bb").value);

var data = new URLSearchParams();
for (var pair of formData) {
data.append(pair[0], pair[1]);
}

fetch('/test3.jsp', {
method: 'POST'
, cache: 'no-cache'
, body: data
})
.then((response) => response)
.then((data) => {
console.log(data);
});