TypeError: form.author is undefined

Hello
When the user logged in to the site author name and email fields removed from the output page. in the output, JS validator need to check field exists before getting value

This error display in the console tab of Firefox browser
TypeError: form.author is undefined

Fixed code for Example:

var x = form.author;
if (x != undefined && x.value === "") {
    d1.innerHTML = "Enter your Name";
    flag = false;
} else {
    d1.innerHTML = "";
}