How to check if this checkbox is checked or not without jquery

So my teacher gave us an assignment. Part of it is that if a checkbox is checked when I press a button, it will say whether the 'customer' is signed up for the newsletter or not. She hasn't taught us jquery, so I don't think that's what she wants us to use. Is there a way to do it without jquery?

This is my teacher's code in the body:

        <div class="error"></div>

        <label for="customerFirst">First Name:</label>
        <input type="text" id="customerFirst">

        <label for="customerLast">Last Name:</label>
        <input type="text" id="customerLast">

        <label for="customerEmail">Email:</label>
        <input type="text" id="customerEmail">

        <label for="customerNewsletter" style="font-weight: normal;">Check here to subscribe to the newsletter.</label>
        <input type="checkbox" id="customerNewsletter" checked>

        <button id="submit" onclick=Create()>Create Customer</button>
        <button id="reset" onclick=Reset()>Reset Form</button>

In the script, I wrote this:

        var customer = {
            first: "",
            last: "",
            email: "",};

        var Replace = function() {
            customer.first = document.getElementById('customerFirst').value;
            customer.last = document.getElementById('customerLast').value;
            customer.email = document.getElementById('customerEmail').value;
        }

        var Create = function() {
            Replace();
            document.getElementById('customerList').innerHTML +=(customer.first + " " + customer.last + '\'s email is ' + customer.email +);
            document.getElementById('customerFirst').value = "";
            document.getElementById('customerLast').value = "";
            document.getElementById('customerEmail').value = "";
        }

        var Reset = function() {
            document.getElementById('customerFirst').value = "";
            document.getElementById('customerLast').value = "";
            document.getElementById('customerEmail').value = "";
            document.getElementById('customerList').innerHTML = "";
        }

Knowing how to check the checkbox will get me a lot closer to completing my assignment and make me seem a little less incompetent to my teacher. Thank you for any help.