<label test="initial value"></label>

<button>Click me to set test to <code>"a new value"</code>…</button>

<p>
  Current value of the <code>test</code> attribute:
  <output id="result">None.</output>
</p>

const element = document.querySelector("label");
const button = document.querySelector("button");
const result = document.querySelector("#result");

const attribute = element.attributes[0];
result.value = attribute.value;

button.addEventListener("click", () => {
  attribute.value = "a new value";
  result.value = attribute.value;
});