HTMLOutputElement: свойство defaultValue

<fieldset>
  <legend>Add two numbers</legend>
  <p>
    <input type="number" id="operand1" value="5" aria-label="First number" />
    +
    <input type="number" id="operand2" value="7" aria-label="Second number" />
    =
    <output
      id="result"
      for="operand1 operand2"
      aria-live="polite"
      aria-controls="output"
      >12</output
    >
  </p>
</fieldset>
<pre id="logs" aria-live="polite"></pre>

const logs = document.getElementById("logs");
const operand1 = document.getElementById("operand1");
const operand2 = document.getElementById("operand2");
const result = document.getElementById("result");

function updateResult() {
  result.value = operand1.valueAsNumber + operand2.valueAsNumber;
  logs.innerText = `result.defaultValue: ${result.defaultValue}\nresult.value: ${result.value}`;
}

operand1.addEventListener("input", updateResult);
operand2.addEventListener("input", updateResult);
updateResult();