formtojson.js

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')));