小编you*_*hwc的帖子

const button = document.createElement("button")
const subject = document.querySelector(".bg-backdrop")

button.textContent = "Register"
subject.insertAdjacentElement("beforebegin",button)

button.addEventListener("click", word)

function word() {
    console.log("It works!")
}

.wrapper {
  position: relative;
  z-index: 10;
}

.bg-backdrop {
  position: fixed;
  top: 0px;
  right: 0px;
  bottom: 0px;
  left: 0px;
  background-color: black;
  opacity: 0.2;
}

<div class="wrapper">
 <div class="bg-backdrop"></div>
</div>