var numberr =document.getElementById("myNumber").value;


function myFunctionVar() {
  document.getElementById("myNumber").value = numberr;
}

let myNumber;
  let input = document.querySelector('#myNumber')
  input.addEventListener('input', function(e){
    myNumber = e.target.value
    console.log(myNumber)
  })

<form onsubmit="myFunctionVar();">
<input type="number" id="myNumber" value="9">
<input type="submit">

</form>