Regex check a user input string in JavaScript

I have a if statement that checks if the user left the input empty and did not enter the correct pattern (which is essentially a student number). However, it seems when I input a random mash of words and number, it still passes, as if it is not checking the regex validation.

    var idInput = formHandle.f__id; //this connects back to my HTML form
    var studentNum = /(n|N)\d{8}/;
    if (idInput.value === "" && !studentNum.test(idInput)) {
        idInput.style.background = 'red';
        idInput.focus();
        return false;
    }

1. Your && should be an ||: If the input is empty or doesn't match the regex. Since the regex won't match an empty string, you don't need the idInput.value === "" check though.
2. !studentNum.test(idInput) should be !studentNum.test(idInput.value)
3. You forgot the anchors ^ (start of string) and $ (end of string) in the regex. Without them, this just searches for "n or N followed by 8 digits" anywhere in the string.

Fixed code:

var idInput = formHandle.f__id; //this connects back to my HTML form
var studentNum = /^(n|N)\d{8}$/;
if (!studentNum.test(idInput.value)) {
    idInput.style.background = 'red';
    idInput.focus();
    return false;
}

(side note, is there any reason to use var instead of let / const?)