Как включить автозаполнение в форме с помощью JS?

function billingFunction () {
  if (document.getElementById('same').checked) {
    var shipinfo = document.getElementById('shippingName','shippingZip').value;
    var billinfo = document.getElementById('billingName','billingZip').value;
    shipinfo = billinfo;
  } else {
    document.getElementById('billingName','billingZip').value = '';
  }   

<h1>JavaScript</h1>
    <p> Whenever the checkbox is checked...</p>

<form>
        <fieldset>
            <legend>Shipping Information</legend>
            <label for ="shippingName">Name:</label>
            <input type = "text" name = "shipName" id = "shippingName" required><br/>
            <label for = "shippingZip">Zip code:</label>
            <input type = "text" name = "shipZip" id = "shippingZip" pattern = "[0-9]{5}" required><br/>
        </fieldset>
        <input type="checkbox" id="same" name="same" onchange= "billingFunction()"/>
        <label for = "same">Is the Billing Information the Same?</label>
                
        <fieldset> 
            <legend>Billing Information</legend>
            <label for ="billingName">Name:</label>
            <input type = "text" name = "billName" id = "billingName" required><br/>
            <label for = "billingZip">Zip code:</label>
            <input type = "text" name = "billZip" id = "billingZip" pattern = "[0-9]{5}" required><br/>
        </fieldset>
            <input type = "submit" value = "Verify"/>
        </form>