<form>
  <label>
    Username
    <input type="text" name="username" value="nguyen van ti">
  </label>
  <label>
    Full Name
    <input type="text" name="fullname">
  </label>
  <div hidden>
    <button>Save</button>
    <button>Discard</button>
  </div>
</form>

form > label {
  display: block;
}

const form = document.querySelector("form");
const div = document.querySelector("div");
const initialValues = {};

Array.from(form.elements).forEach(elem => {
  initialValues[elem.name] = elem.value;
});

form.addEventListener("input", e => {
  const { name, value } = e.target;
  div.hidden = initialValues[name] === value;
});