虽然我是从首页来到表单页面，但是没有出现referrer页面。我该如何解决这个问题？
document.referrer == ""
许多站点重定向到 example.com/thank-you 页面，所以我想获取引用它的页面。
example.com/form redirects to example.com/thank-you
example.com/form1 redirects to example.com/thank-you
example.com/form2 redirects to example.com/thank-you
example.com/form3 redirects to example.com/thank-you
example.com/form4 redirects to example.com/thank-you
example.com/form5 redirects to example.com/thank-you
