JSFiddle - React, Tailwind, and code Playground

HTML

<form>
  <fieldset>
    <input type="text" placeholder="John Doe" id="name"><br>
    <input type="email" placeholder="john@example.com" id="email"><br>
    <textarea placeholder="Lorem ipsum dolor sit amet." id="message"></textarea>
  </fieldset>

  <input type="submit" value="Submit" disabled>
</form>

JavaScript

const form = document.forms[0];
const inputs = form.querySelectorAll("fieldset > input, fieldset > textarea");
const submit = form.querySelector("[type=submit]");

for (i in inputs) {
	var _input = inputs[i];

	if (typeof _input == 'object') {
    _input.addEventListener("change", () => {
      submit.disabled = false;

      for (i in inputs) {
        var _input = inputs[i];
				
        if (typeof _input == 'object') {
          if (_input.value == '') {
            submit.disabled = true;
            break;
          }
        }
      }
    });
  }
}