为什么我的＆＃34;回到顶部＆＃34;按钮仅在我位于页面底部时才起作用？

Question

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// When the user scrolls down 20px from the top of the document, 
//  show the button.
window.onscroll = function () {
  scrollFunction()
};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    document.getElementById("Button").style.textAlign = "center".display =
      "block";
  } else {
    document.getElementById("Button").style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document

function topFunction() {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
}

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