未捕获的类型错误：无法读取 null 的属性“offsetTop”

我正在使用 HTML、CSS 和 JavaScript 创建一个带有粘性和响应式导航栏的网页。我创建了响应式导航栏，并试图使其具有粘性。问题是它不粘并且显示错误： 未捕获的类型错误：无法读取 null 的属性“offsetTop”

HTML 代码：

<div class="topnav" id="myTopnav">
<a href="#home" class="active">Home</a>
<a href="#career">Careers</a>
<a href="#fellowship">About Us</a>
<a href="javascript:void(0);" class="icon" onclick="myFunctionForResponsive()">
    <i class="fas fa-bars"></i>
</a>
</div>

JavaScript 代码：

// When the user scrolls the page, execute myFunction 
window.onscroll = function() {myFunctionForSticky()};

// Get the navbar
var navbar = document.getElementById("myTopnav");

// Get the offset position of the navbar
var sticky = navbar.offsetTop;

// Add the sticky class to the navbar when you reach its scroll position. 
Remove "sticky" when you leave the scroll position
function myFunctionForSticky() {
    if(window.pageYOffset >= sticky){
    console.log("window.pageYOffset >= sticky");
}
else{
    console.log("Not window.pageYOffset >= sticky");
}
 if (window.pageYOffset >= sticky) {
    navbar.classList.add("sticky");
  } else {
    navbar.classList.remove("sticky");  
  }
}

/*Toggle between adding and removing the "responsive" class to topnav
when the user clicks on the icon*/

function myFunctionForResponsive() {
    var x = document.getElementById("myTopnav");
    if (x.className === "topnav") {
        x.className += " responsive";
    } else {
        x.className = "topnav";
    }
}

如果我正在上课而不是 id 那么它会显示错误： 未捕获的类型错误：无法读取未定义的属性“删除”

想要访问 DOM 的代码需要包装在一个监听的事件监听器中DOMContentLoaded.

当前您正在尝试访问具有 id 的元素myTopnav当浏览器还没有解析 HTML 时，这意味着你的document.getElementById("myTopnav");回报null。在下一行代码中，您尝试读取offsetTop的财产null那你的document.getElementById("myTopnav")返回，导致Cannot read property 'offsetTop' of null.

https://developer.mozilla.org/en-US/docs/Web/Events/DOMContentLoaded

document.addEventListener('DOMContentLoaded', function() {
  // When the event DOMContentLoaded occurs, it is safe to access the DOM

  // When the user scrolls the page, execute myFunction 
  window.addEventListener('scroll', myFunctionForSticky);

  // Get the navbar
  var navbar = document.getElementById("myTopnav");

  // Get the offset position of the navbar
  var sticky = navbar.offsetTop;

  // Add the sticky class to the navbar when you reach its scroll position. 
  // Remove "sticky" when you leave the scroll position

  function myFunctionForSticky() {
    if (window.pageYOffset >= sticky) {
      console.log("window.pageYOffset >= sticky");
    } else {
      console.log("Not window.pageYOffset >= sticky");
    }
    if (window.pageYOffset >= sticky) {
      navbar.classList.add("sticky");
    } else {
      navbar.classList.remove("sticky");
    }
  }

  /*Toggle between adding and removing the "responsive" class to topnav
  when the user clicks on the icon*/

  function myFunctionForResponsive() {
    navbar.classList.toggle('responsive');
  }
})