如何只为页面的第一部分设置粘性导航栏？

较早的Bootstrap在Affix jQuery插件中为某些部分设置了固定的导航栏，而对其他部分则设置了绝对导航，但是新的Bootstrap 4.0放弃了此选项，建议为此使用javascript。

我们可以使用Javascript将导航栏的第一部分设置为粘滞，其余部分设置为绝对。
为此，您只需要使用window.onscroll函数确定屏幕所处的位置，然后根据其施加粘滞或绝对位置。

方法：
在这里，我们设置了一个window.onscroll函数，其中将变量navbarConst设置为其id为“ navbarSection”的元素，然后变量stickyConst使用“ offsetTop”返回navbarConst相对于文档的偏移坐标。

句法
让我们看一下如何使用它：
最后，在body标签内的HTML的脚本标签中键入以下代码。

window.onscroll = function() {myFunction()};

var navbar = document.getElementById("nav_bar");
var sticky = navbar.offsetTop;

function myFunction() {
  if (window.pageYOffset >= sticky) {
    navbar.classList.remove("sticky");
  } else {
    navbar.classList.add("sticky");
  }
}

例子：
让我们看下面的示例，以更好地理解HTML：

    
        GeeksforGeeks
        
        
            body {
                margin: 0;
                font-size: 26px;
                font-family: sans-serif;
            }
  
            .firstSection {
                background-color: #fa8072;
                padding: 30px;
                text-align: left;
            }
  
            #navbarSection {
                overflow: hidden;
                background-color: #c71585;
            }
  
            #navbarSection a {
                float: left;
                display: block;
                color: #f2f2f2;
                text-align: center;
                padding: 16px 16px;
                text-decoration: none;
                font-size: 16px;
            }
  
            #navbarSection a:hover {
                background-color: #ffe4b5;
                color: black;
            }
  
            #navbarSection a.active {
                background-color: red;
                color: white;
            }
  
            #secondSection {
                padding: 16px 35px;
                padding-top: 0;
                background-color: #66cdaa;
            }
  
            .stickyNavbar {
                position: sticky;
                top: 0;
                width: 100%;
            }
  
            .stickyNavbar,
            #secondSection {
                padding-top: 30px;
            }
        
    
    
        
            Home
            Page 1
            Page 2
        
  
        
            This is the first section. Scroll down 
              to see the effects of sticky navbar on
              first section and absolute on other 
              section.
             Some texts to fill the section 
            
                HTML stands for Hyper Text Markup
              Language. It is used to design web pages
              using markup language. HTML is the 
              combination of Hypertext and Markup 
              language. Hypertext defines the link
              between the web pages. Markup language is
                used to define the text document 
              within tag which defines the structure
              of web pages. HTML is a markup language
              which is used by the browser to manipulate
              text, images and other content to display 
              it in required format.
            
        
  
        
            
            
            
            From here First section ends and second
              start in which navbar should not be sticky.
          
            
             Some texts to fill the section 
            
                HTML stands for Hyper Text Markup Language.
              It is used to design web pages using markup
              language. HTML is the combination of Hypertext
              and Markup language. Hypertext defines the
              link between the web pages. Markup language is
                used to define the text document within 
              tag which defines the structure of web pages.
              HTML is a markup language which is used by 
              the browser to manipulate text, images and 
              other content to display it in required format.
            
        
  
        
            window.onscroll = function () {
                myFunction();
            };
  
            var navbarConst = 
                document.getElementById("navbarSection");
            var stickyConst = navbarConst.offsetTop;
            var navbarOther = 
                document.getElementById("secondSection");
            var stickyOther = navbarOther.offsetTop;
  
            function myFunction() {
                if (window.pageYOffset >= stickyOther) {
                    navbarSection.classList.remove(
                      "stickyNavbar");
                } else {
                    navbarSection.classList.add(
                      "stickyNavbar");
                }
            }
        
    

解释：
在这里，我们设置了一个window.onscroll函数，其中将变量navbarConst设置为其id为“ navbarSection”的元素，然后变量stickyConst使用“ offsetTop”返回navbarConst相对于文档的偏移坐标。

然后根据屏幕的Y坐标设置其位置。当屏幕的Y坐标大于第一部分的Y坐标时，我们将删除stickyNavbar类。在这里stickyNavbar是我们将位置设为粘性的类。

输出：