JSFiddle - React, Tailwind, and code Playground

HTML

<form class="haha">
    name <input type="text" name="user_name" />
</form>
<form class="haha">
    zzz <input type="text" name="zzz" value="123" />
</form>
<form class="haha">
    zzz <input type="text" name="zzz" value="456" />
</form>

<form class="haha">
  email <input type="email" name="user_email" />
</form>

<pre id="out"></pre>

JavaScript

const formSelector = '.haha';
window.addEventListener('submit', submitHandler);

function submitHandler(evt) {
  if (event.target.matches(formSelector)) {
    evt.preventDefault();

    const forms = Array.from(document.querySelectorAll(formSelector));
    const formDataAll = forms.reduce(function collectFormsData(fd, form) {
      for (const [key,val] of new FormData(form).entries()) {
        fd.append(`${key}[]`, val);
      }
      return fd;
    }, new FormData());

    debugPrint(formDataAll); // for test only
    
    // add your AJAX send here !!!
  }
}







// for test only
function debugPrint(formDataAll) {
  const data = [];
  for(const [k,v] of formDataAll.entries()) {
    data.push([k, v]);
  }

  out.textContent = JSON.stringify(data, null, 4);
  console.table(data);
}