Comment by runarberg

Too late to edit, but there is a mistake s/document.elements/myForm.elements/r :

    -   const inputField = document.elements.namedItem("number");
    -   const outputField = document.elements.namedItem("result");
    +   const inputField = myForm.elements.namedItem("number");
    +   const outputField = myform.elements.namedItem("result");