Disable the buttons in html by adding a disabled attribute. Write an on-change handler for the inputs where the buttons are enabled only if there is value.

Ref: https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/change_event

const input1 = document.querySelector('#num1');
const input2 = document.querySelector('#num2');

input1.addEventListener('change', handleButton);
input2.addEventListener('change', handleButton);

function handleButton() {
  if(input1.value && input2.value) {
    document.querySelector(".button").removeAttribute("disabled");
  } else {
    document.querySelector(".button").setAttribute("disabled", "disabled");
  }
}

You can target the buttons by the classname which you have used. I have used .button in the answer.