function formToJson(form) {
if ((typeof form === 'object') && (form.nodeName === 'FORM')) {
let entries = new FormData(form).entries();
return JSON.stringify(Object.assign(...Array.from(entries, ([x,y]) => ({[x]:y}))));
}
return null;
}
Пример использования:
console.log(formToJson(document.getElementById('MyForm')));