当我们点击复选框时，我试图将送货地址复制到账单中，但它不起作用
这是HTML

<h1>JavaScript Homework</h1>
    <p>Add the JavaScript code needed to enable auto-complete on this form.  Whenever the checkbox is checked, the code should automatically copy the values from Shipping Name and Shipping Zip into the Billing Name and Billing Zip.  If the checkbox is unchecked, the Billing Name and Billing Zip should go blank.</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>

我的JavaScript

function billingFunction(){
 var SN=document.getElementById('shippingName').value;
  var SZ=document.getElementById('shippingZip').value;
  var BN=document.getElementById('billingName').value;
  var BZ=document.getElementById('billingZip').value;
  if (document.getElementById('same').checked==true){ BN=SN;BZ=SZ}
else {BN="";BZ=""}
}

我不知道为什么它不工作。谢谢你的帮助。