滚动后改变导航条颜色？

小开

This can be done using jQuery.

Here is a link to a fiddle.

When the window scrolls, the distance between the top of the window and the height of the window is compared. When the if statement is true, the background color is set to transparent. And when you scroll back to the top the color comes back to white.

$(document).ready(function(){
$(window).scroll(function(){
if($(window).scrollTop() > $(window).height()){
$(".menu").css({"background-color":"transparent"});
}
else{
$(".menu").css({"background-color":"white"});
}


})
})

小开

Here is a jsfiddle example. Using Jquery to change the background color based on scroll pixel position.

Here is a fiddle using bootstrap

$(document).ready(function(){
var scroll_start = 0;
var startchange = $('#startchange');
var offset = startchange.offset();
if (startchange.length){
$(document).scroll(function() {
scroll_start = $(this).scrollTop();
if(scroll_start > offset.top) {
$(".navbar-default").css('background-color', '#f0f0f0');
} else {
$('.navbar-default').css('background-color', 'transparent');
}
});
}
});

小开

<script>
$(document).ready(function(){
$(window).scroll(function() { // check if scroll event happened
if ($(document).scrollTop() > 50) { // check if user scrolled more than 50 from top of the browser window
$(".navbar-fixed-top").css("background-color", "#f8f8f8"); // if yes, then change the color of class "navbar-fixed-top" to white (#f8f8f8)
} else {
$(".navbar-fixed-top").css("background-color", "transparent"); // if not, change it back to transparent
}
});
});
</script>

小开

this is a simple pure javascript

var myNav = document.getElementById('mynav');
window.onscroll = function () {
if (document.body.scrollTop >= 200 ) {
myNav.classList.add("nav-colored");
myNav.classList.remove("nav-transparent");
}
else {
myNav.classList.add("nav-transparent");
myNav.classList.remove("nav-colored");
}
};

in some chrome versions there is a bug with:

document.body.scrollTop

so you may add a condition like this:

 if (document.body.scrollTop >= 200 || document.documentElement.scrollTop >= 200 )

sure you must have 2 classes

.nav-colored { background-color:#000; }
.nav-transparent { background-color:transparent;}

小开

window.addEventListener('scroll', function (e) {
var nav = document.getElementById('nav');
if (document.documentElement.scrollTop || document.body.scrollTop > window.innerHeight) {
nav.classList.add('nav-colored');
nav.classList.remove('nav-transparent');
} else {
nav.classList.add('nav-transparent');
nav.classList.remove('nav-colored');
}
});

best approach to use event listener. especially for Firefox browser, check this doc Scroll-linked effects and Firefox is no longer support document.body.scrollTop and alternative to use document.documentElement.scrollTop. This is completes the answer from Yahya Essam

小开

Here is simplest way how to change navbar color after window scroll:

Add follow JS to head:

$(function () {
$(document).scroll(function () {
var $nav = $(".navbar-fixed-top");
$nav.toggleClass('scrolled', $(this).scrollTop() > $nav.height());
});
});

and this CSS code

.navbar-fixed-top.scrolled {
background-color: #fff !important;
transition: background-color 200ms linear;
}

Background color of fixed navbar will be change to white when scroll exceeds height of navbar.

See follow JsFiddle

小开

Slight variation to the above answers, but with Vanilla JS:

var nav = document.querySelector('nav'); // Identify target


window.addEventListener('scroll', function(event) { // To listen for event
event.preventDefault();


if (window.scrollY <= 150) { // Just an example
nav.style.backgroundColor = '#000'; // or default color
} else {
nav.style.backgroundColor = 'transparent';
}
});

小开

So I'm using querySelector to get the navbar
I added a scroll event to the window to track the scrollY property
I check if it's higher than 50 then I add the active class to the navbar, else if it contains it already, I simply remove it and I'm pretty sure the conditions can be more currated and simplified.

I made this codepen to help you out!

const navbar = document.querySelector('#nav')


window.addEventListener('scroll', function(e) {
const lastPosition = window.scrollY
if (lastPosition > 50 ) {
navbar.classList.add('active')
} else if (navbar.classList.contains('active')) {
navbar.classList.remove('active')
} else {
navbar.classList.remove('active')
}
})

小开

I use WordPress which comes with Underscore. So when you register your theme scripts, you would use 'jquery' and 'underscore' as the handle for the array of the dependancies. If you are not using WordPress, then make sure that you load both the jQuery framework and Underscore before your scripts.

CodePen: https://codepen.io/carasmo/pen/ZmQQYy

To make this demo (remember it requires both jQuery and Underscore).

HTML:

<header class="site-header">
<div class="logo">
</div>
<nav>navigation</nav>
</header>


<article>
Content with a forced height for scrolling.  Content with a forced height for scrolling. Content with a forced height for scrolling. Content with a forced height for scrolling. Content with a forced height for scrolling. Content with a forced height for scrolling. Content with a forced height for scrolling
</article>

CSS:

body,
html {
margin: 0;
padding: 0;
font: 100%/180% sans-serif;
background: #eee;
}


html {
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
}


*,
*::before,
*::after {
box-sizing: inherit;
}


article {
height: 2000px;
padding: 5%;
background: #fff;
margin: 2% auto;
max-width: 900px;
box-shadow: 0px 0px 30px 0px rgba(0, 0, 0, 0.10);
}




.site-header {
background: #fff;
padding: 20px 5%;
box-shadow: 0px 0px 12px 0px rgba(0, 0, 0, 0.23);
transition: all .5s ease-in-out;
-web-kit-position: sticky;
position: sticky;
top: 0;
display: -webkit-box;
display: -ms-flexbox;
display: flex;
-webkit-box-orient: horizontal;
-webkit-box-direction: normal;
-ms-flex-direction: row;
flex-direction: row;
-webkit-box-align: center;
-ms-flex-align: center;
align-items: center;
-webkit-box-pack: center;
-ms-flex-pack: center;
justify-content: center;


}


.logo {
background-image: url('the-path-to-the-logo.svg');
background-repeat: no-repeat;
background-position: center center;
width: 200px;
height: 60px;
background-size: contain;
transition: width .5s ease-in-out, height .5s ease-in-out;
}


.site-header nav {
text-align: right;
-webkit-box-flex: 1;
-ms-flex-positive: 1;
flex-grow: 1;
-ms-flex-preferred-size: 0;
flex-basis: 0;
}


.site-header.is-scrolling {
opacity: .8;
background: tomato;
padding: 10px 5%;
}


.site-header.is-scrolling .logo {
height: 40px;
width: 100px;
}

jQuery:

( function( window, $, undefined ) {


'use strict';


////////////// Begin jQuery and grab the $ ////////////////////////////////////////


$(document).ready(function() {


function is_scrolling() {


var $element = $('.site-header'),
$nav_height = $element.outerHeight( true );


if ($(this).scrollTop() >= $nav_height ) { //if scrolling is equal to or greater than the nav height add a class


$element.addClass( 'is-scrolling');


} else { //is back at the top again, remove the class


$element.removeClass( 'is-scrolling');
}


}//end is_scrolling();


$(window).scroll(_.throttle(is_scrolling, 200));




}); //* end ready




})(this, jQuery);

小开

$(window).on('activate.bs.scrollspy', function (e,obj) {


if ((window.innerHeight + window.pageYOffset) >= document.body.offsetHeight) {
return;
}
  

var isBGLight = $(obj.relatedTarget).hasClass('nav_white');
var isBGDark = $(obj.relatedTarget).hasClass('nav_blue');
$('.menu').removeClass('nav_white');
$('.menu').removeClass('nav_blue');
if(isBGDark)
{
$('.menu').addClass('nav_white');
}else if(isBGLight)
{
$('.menu').addClass('nav_blue');
}
/*var isScrolled = $(document).scrollTop() > 1;
$('.menu').toggleClass('scrolled', isScrolled);
$(".demos").toggleClass("demo");
$(".demos").toggleClass("demo1");
var posicionActual = $(document).scrollTop();
$.each($('.nav_transparent'),function(){
if ($(this).position().top < posicionActual){
$("nav.menu").removeClass("nav_white");
$("nav.menu").removeClass("nav_blue");
$("nav.menu").addClass("nav_transparent");
$(".demos").removeClass("demo");
$(".demos").addClass("demo1");
$(".cls").removeClass("cls2");
$(".cls").addClass("cls1");
$(".cl").removeClass("cl2");
$(".cl").addClass("cl1");
$(".hamb-bottom").css({"background-color": "#fff"});
$(".hamb-middle").css({"background-color": "#fff"});
$(".hamb-top").css({"background-color": "#fff"});
}
});
$.each($('.nav_blue'),function(){
if ($(this).position().top <= posicionActual){
$("nav.menu").removeClass("nav_transparent");
$("nav.menu").removeClass("nav_white");
$("nav.menu").addClass("nav_blue");
$(".demos").removeClass("demo1");
$(".demos").addClass("demo");
$(".cls").removeClass("cls2");
$(".cls").addClass("cls1");
$(".cl").removeClass("cl2");
$(".cl").addClass("cl1");
$(".hamb-bottom").css({"background-color": "#fff"});
$(".hamb-middle").css({"background-color": "#fff"});
$(".hamb-top").css({"background-color": "#fff"});
}
});
$.each($('.nav_white'),function(){
if ($(this).position().top <= posicionActual){
$("nav.menu").removeClass("nav_blue");
$("nav.menu").removeClass("nav_transparent");
$("nav.menu").addClass("nav_white");
$(".demos").removeClass("demo");
$(".demos").addClass("demo1");
$(".cls").removeClass("cls1");
$(".cls").addClass("cls2");
$(".cl").removeClass("cl1");
$(".cl").addClass("cl2");
$(".hamb-bottom").css({"background-color": "#4285f4"});
$(".hamb-middle").css({"background-color": "#4285f4"});
$(".hamb-top").css({"background-color": "#4285f4"});
}
});*/
});
$(window).on("scroll", function(){
if($(document).scrollTop() < 10)
{
$('.nav').removeClass('nav_white');
$('.nav').removeClass('nav_blue');
$('.nav').removeClass('nav_transparent');
$('.nav').addClass('nav_transparent');
}
});

the solutions, maybe

小开

I've recently done it slightly different to some of the examples above so thought I'd share, albeit very late!

Firstly the HTML, note there is only one class within the header:

<body>


<header class="GreyHeader">
</header>




</body>

And the CSS:

body {
height: 3000px;
}


.GreyHeader{
height: 200px;
background-color: rgba(107,107,107,0.66);
position: fixed;
top:200;
width: 100%;


}


.FireBrickRed {
height: 100px;
background-color: #b22222;
position: fixed;
top:200;
width: 100%;
}


.transition {
-webkit-transition: height 2s; /* For Safari 3.1 to 6.0 */
transition: height 2s;
}

The html uses only the class .greyHeader but within the CSS I have created another class to call once the scroll has reached a certain point from the top:

$(function() {
var header = $(".GreyHeader");
$(window).scroll(function() {
var scroll = $(window).scrollTop();


if (scroll >= 500) {
header.removeClass('GreyHeader').addClass("FireBrickRed ");
header.addClass("transition");
} else {
header.removeClass("FireBrickRed ").addClass('GreyHeader');
header.addClass("transition");
}
});
});

check this fiddle: https://jsfiddle.net/29y64L7d/1/

小开

Today I've gone through the same question, how to change navbar background-color as scrolling. And I was seeking for a solution using CSS only, no jquery, no bootstrap nor javascript. But then it turned out couldn't be done with CSS only yet (as of today Dec 2019). And have to choose, I'll stick with the core technology - javascript instead of jquery or bootstrap unless it's far more complicated using js than the others. But luckily it's not.

Here's the code:
- It uses onscroll/ scroll event of window to trigger the event listener.
- In the event listener, use pageYOffset/ scrollY of window to check the scroll status.

Browser support are seemingly the same between both:
- https://caniuse.com/#search=pageYOffset
- https://caniuse.com/#search=scrollY

var navbar = document.querySelector('nav')


window.onscroll = function() {


// pageYOffset or scrollY
if (window.pageYOffset > 0) {
navbar.classList.add('scrolled')
} else {
navbar.classList.remove('scrolled')
}
}

body {
margin: 0;
padding: 0;
background: url(https://occ-0-325-395.1.nflxso.net/dnm/api/v6/6AYY37jfdO6hpXcMjf9Yu5cnmO0/AAAABaKr-dQAdVTt7fuGCgzntgBBrFce2DMW72dF86eO7EnXbFZvzmX2TPnQAg3HwAsvt7ZnDnP0nwuHOtPwpWGGOE22fXq2.webp?r=847) top/contain no-repeat;
}


nav {
position: -webkit-sticky;
position: sticky;
/* sticky or fixed are fine */
position: fixed;
top: 0;
height: 69px;
width: 100%;
background: linear-gradient(to bottom, #000, #0003 70%,#0000); /* background when scroll is in the top */
transition: background .5s; /* control how smooth the background changes */
}


nav.scrolled {
background: #0a0a0a;
}


main {
height: 200vh;
}

<nav></nav>
<main></main>

小开

First you make an id named nav (can change whatever you want) inside the nav div (exp: id="nav") Then at the bottom where body tag had been finished. You add this code

<script>
$(document).ready(function()
$(window).scroll(function(){
var scroll = $(window).scrollTop();
if(scroll>50){
$("#nav").css("background", "#555");
}
else {
$("#nav").css("background", "transparent");
}
})
})
</script>

小开

$(document).ready(function(){
$(window).scroll(function() {
if ($(document).scrollTop() >1290 ) {
$(".navbar-fixed-top").css("background-color", "rgb(255, 160, 160)");
}else if ($(document).scrollTop() >850) {
$(".navbar-fixed-top").css("background-color", "black");
}else if ($(document).scrollTop() >350) {
$(".navbar-fixed-top").css("background-color", "rgba(47, 73, 158, 0.514)");
}
else {
$(".navbar-fixed-top").css("background-color", "red");
}
});
});

@import url(https://fonts.googleapis.com/css?family=Roboto+Slab:400,700|Open+Sans);
body {
font-family: "Roboto Slab", sans-serif;
position: relative;
}


h1,
h2,
h3,
h4 {
font-family: "Open Sans", sans-serif;
}


.main {
padding-top: 50px;
}


#home {
padding-top: 20%;
background-image: linear-gradient( rgba(0, 0, 0, 0.5), rgba(0, 0, 0, 0.5)), url('https://s31.postimg.org/l5q32ptln/coffee_cup_mug_apple.jpg');
background-attachment: fixed;
background-repeat: no-repeat;
background-position: center center;
position: relative;
height: 100vh;
}


#home h2 {
color: white;
font-size: 4em;
}


#home h4 {
color: white;
font-size: 2em;
}


#home ul {
list-style-type: none;
text-align: center;
}


#home li {
padding-bottom: 12px;
padding-right: 12px;
display: inline;
}


#home li:last-child {
padding: 0;
}


@media (max-width: 710px) {
#home li {
display: block;
}
}


.img-style {
height: 200px;
width: 200px;
margin-top: 50px;
}


#about {
height: 100vh;
padding-top: 10%;
background: linear-gradient( rgba(0, 0, 0, 0.5), rgba(0, 0, 0, 0.5)), url('https://s32.postimg.org/sm6o6617p/photo_1432821596592_e2c18b78144f.jpg');
background-attachment: fixed;
background-repeat: no-repeat;
background-position: center center;
position: relative;
color: white;
}


#about p,
li {
font-size: 17px;
}


.navbar.color-yellow {
background-color: yellow;
height: 50px;
color: yellow;
}


.navbar.color-change {
background-color: #f45b69;
height: 50px;
color: rgba(255, 254, 255, 0.8);
}


#portfolio {
padding-top: 30px;
rgba(226,
226,
226,
1);
background: linear-gradient(to bottom, rgba(226, 226, 226, 1) 0%, rgba(209, 209, 209, 1) 25%, rgba(219, 219, 219, 1) 57%, rgba(254, 254, 254, 1) 100%);
height: 100vh;
}


#portfolio .block .portfolio-contant ul li {
float: left;
width: 32.22%;
overflow: hidden;
margin: 6px;
position: relative;
}


#portfolio .block .portfolio-contant ul li:hover .overly {
opacity: 1;
}


#portfolio .block .portfolio-contant ul li:hover .position-center {
position: absolute;
top: 50%;
-webkit-transform: translate(0%, -50%);
-moz-transform: translate(0%, -50%);
-ms-transform: translate(0%, -50%);
transform: translate(0%, -50%);
}


#portfolio .block .portfolio-contant ul li a {
display: block;
color: #fff;
}


#portfolio .block .portfolio-contant ul li a h2 {
font-size: 22px;
text-transform: uppercase;
letter-spacing: 1px;
}


#portfolio .block .portfolio-contant ul li a p {
font-size: 15px;
}


#portfolio .block .portfolio-contant ul li a span {
font-style: italic;
font-size: 13px;
color: #655E7A;
}


#portfolio .block .portfolio-contant ul img {
width: 100%;
height: auto;
}


#portfolio .block .portfolio-contant .overly {
position: absolute;
top: 0;
bottom: 0;
right: 0;
left: 0;
background: rgba(0, 0, 0, 0.9);
opacity: 0;
-webkit-transition: .3s all;
-o-transition: .3s all;
transition: .3s all;
text-align: center;
}


#portfolio .block .portfolio-contant .position-center {
position: absolute;
top: 50%;
left: 10%;
-webkit-transform: translate(0%, 50%);
-moz-transform: translate(0%, 50%);
-ms-transform: translate(0%, 50%);
transform: translate(0%, 50%);
-webkit-transition: .5s all;
-o-transition: .5s all;
transition: .5s all;
}


#contact {
height: 100vh;
padding-top: 10%;
background: linear-gradient( rgba(0, 0, 0, 0.5), rgba(0, 0, 0, 0.5)), url("https://s32.postimg.org/ex6q1qxkl/pexels_photo.jpg");
background-attachment: fixed;
background-size: cover;
background-repeat: no-repeat;
background-position: center center;
position: relative;
}


#contact h3 {
color: white;
}


footer ul {
list-style-type: none;
text-align: center;
}


footer li {
display: inline;
padding-right: 10px;
}


footer li:last-child {
padding: 0;
}


footer p {
color: grey;
font-size: 11px;
}

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.4.0/css/font-awesome.min.css" rel="stylesheet" />
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
<!DOCTYPE html>
<html lang="en">


<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
</head>


<body>
<header>
<nav class="navbar navbar-default navbar-fixed-top" role="navigation">
<div class="container">
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#collapse" aria-expanded="false">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#featured">Portfolio</a>
</div>
<!-- navbar-header -->
<div class="collapse navbar-collapse" id="collapse">
<ul class="nav navbar-nav navbar-right">
<li class="active"><a href="#home">Home</a></li>
<li><a href="#about">About</a></li>
<li><a href="#portfolio">Portfolio</a></li>
<li><a href="#contact">Contact</a></li>
</ul>
</div>
<!-- collapse navbar-collapse -->


</div>
<!-- container -->
</nav>
</header>


<div class="main">
<div class="row " id="home" data-speed="2" data-type="background">
<div class="container">
<h2 class="text-center">Welcome to my portfolio</h2>
<h4 class="text-center">Where awesomeness is crafted</h4>
<hr>
<ul>
<li><a href="https://github.com/vamshi121" class="btn btn-primary"><i class="fa fa-github"></i> GitHub</a></li>
<li><a href="https://tn.linkedin.com/in/mannemvamshi" class="btn btn-primary"><i class="fa fa-linkedin"></i> LinkedIn</a></li>
<li><a href="https://freecodecamp.com/" class="btn btn-primary"><i class="fa fa-fire"></i> FreeCodeCamp</a></li>


</ul>
</div>
<!--container-->


</div>
<!--home-->
<div class="row" id="about" data-speed="2" data-type="background">
<div class="container">
<div class="row">
<div class="col-md-5 col-md-offset-1">
<h2>About me</h2>
<p>Courteous and Enthusiastic,I'm interested in IT and around in its globe. I began to be fascinated by web programming i.e, building websites and developing cross-platform apps.I'm always looking for new ventures where i can learn evolve and
expertise.
</marquee>
</div>
</p>
<p>My skills are:
<ul>
<li> Front-end : HTML5, CSS3 , jQuery, Bootstrap, AngularJS</li>
<li>Back-end: Ruby on Rails</li>
<li>Methodology: Agile, TDD</li>
</ul>
</p>
</div>
<!--col-md-5-->
<div class="col-md-4 col-md-offset-1">
<img class="img-circle img-style hidden-xs" src="https://www.triketech.site/images/log.png" />
</div>
</div>
<!--row-->
</div>
<!--container-->
</div>
<!--about-->
<div class="row" id="portfolio" data-speed="2" data-type="background">
<div class="container">
<h2 class="text-center">Portfolio projects</h2>
<div class="row">
<div class="col-md-12">
<div class="block">
<div class="portfolio-contant">
<ul id="portfolio-contant-active">
<li class="mix Branding">
<a href="#">
<img src="#" alt="">
<div class="overly">
<div class="position-center">
<h2>Local Support</h2>




</div>
</div>
<!--overly-->
</a>
</li>
</ul>
</div>
<!--portfolio-contant-->
</div>
<!--block-->
</div>
<!--col-->
</div>
<!--row-->
</div>
<!--container-->
</div>
<!--portfolio-->
<div class="row" id="contact" data-speed="2" data-type="background">
<div class="container">
<div class="col-md-4 col-md-offset-1">
<h3>Contact me at:</h3>
<h3>thegreatvamshi@triketech.com</h3>
</div>
<!--col-md-4-->
</div>
<!--container-->
</div>
<!--contact-->
</div>
<!--main-->
<footer>
<ul>
<li><a href="#home">Home</a> </li>
<li><a href="#about">About</a> </li>
<li><a href="#portfolio">Portfolio</a> </li>
<li><a href="#contact">Contact</a> </li>
</ul>
<p class="text-center">Copyright &copy; - All Rights Reserved. </p>
</footer>


</body>


</html>

<script>
$(document).ready(function(){
$(window).scroll(function() {
if ($(document).scrollTop() >1920 ) {
$(".navbar-fixed-top").css("background-color", "rgb(255, 160, 160)");
} else if ($(document).scrollTop() >1580) {
$(".navbar-fixed-top").css("background-color", "black");
} else if ($(document).scrollTop() >620) {
$(".navbar-fixed-top").css("background-color", "rgba(47, 73, 158, 0.514)");
} else {
$(".navbar-fixed-top").css("background-color", "transparent");
}
});
});
</script>

小开

i think this solution is shorter and simpler than older answers. This is Js Code:

const navbar = document.querySelector('.nav-fixed');
window.onscroll = () => {
if (window.scrollY > 300) {
navbar.classList.add('nav-active');
} else {
navbar.classList.remove('nav-active');
}
};

And my css:

header.nav-fixed {
width: 100%;
position: fixed;
transition: 0.3s ease-in-out;
}


.nav-active {
background-color:#fff;
box-shadow: 5px -1px 12px -5px grey;
}

小开

How about the Intersection Observer API? This avoids the potential sluggishness from using the scroll event.

HTML

<nav class="navbar-fixed-top">Navbar</nav>
<main>
<div class="content">Some content</div>
</main>

CSS

.navbar-fixed-top--scrolled changes the nav bar background color. It's added to the nav bar when the content div is no longer 100% visible as we scroll down.

.navbar-fixed-top {
position: sticky;
top: 0;
height: 60px;
}


.navbar-fixed-top--scrolled {
/* change background-color to whatever you want */
background-color: grey;
}

JS

Create the observer to determine when the content div fully intersects with the browser viewport.

The callback function is called:

the first time the observer is initially asked to watch the target element
when content div is no longer fully visible (due to threshold: 1)
when content div becomes fully visible (due to threshold: 1)

isIntersecting indicates whether the content div (the target element) is fully intersecting with the observer's root (the browser viewport by default).

// callback function to be run whenever threshold is crossed in one direction or the other
const callback = (entries, observer) => {
const entry = entries[0];
  

// toggle class depending on if content div intersects with viewport
const navBar = document.querySelector('.navbar-fixed-top');
navBar.classList.toggle('navbar-fixed-top--scrolled', !entry.isIntersecting);
}


// options controls circumstances under which the observer's callback is invoked
const options = {
// no root provided - by default browser viewport used to check target visibility
// only detect if target element is fully visible or not
threshold: [1]
};


const io = new IntersectionObserver(callback, options);


// observe content div
const target = document.querySelector('.content');
io.observe(target);

IntersectionObserver options

The nav bar currently changes background color when the content div starts moving off the screen.

If we want the background to change as soon as the user scrolls, we can use the rootMargin property (top, right, bottom, left) and set the top margin to negative the height of the nav bar (60px in our case).

const options = {
rootMargin: "-60px 0px 0px 0px",
threshold: [1]
};

You can see all the above in action on CodePen. Kevin Powell also has a good explanation on this (Github & YouTube).

小开

First things first, you have to include Jquery into your HTML file or whatever file you are using.
create a script code space in the head part of your file and include the code below.

$(document).ready(function(){
$(window).scroll(function(){
if($(window).scrollTop() > 100){
$(".navbar").css({"background-color":"black"});
}
else{
$(".navbar").css({"background-color":""});
}


})
})

With this where the code says ($(window).scrollTop() > 100) 100 is in "px" so you can specify the height at which the function is called.

This line of code $(".navbar").css({"background-color":"black"}); is where you replace your class name of the Nav element. This is just directly accessing the CSS and then editing the CSS.

HTML Code Below

<header class="hero">
<nav class="navbar ">
<div class="container">
<a class="navbar-brand" href="#">
<img src="" alt=""> </a>
<button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav me-auto mb-2 mb-lg-0">
<li class="nav-item">
<a class="nav-link active" aria-current="page" href="#">Home</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">TV shows</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Movies</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">News and Popular</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">My List</a>
</li>
                 

</ul>
                

                

</div>
</div>
</nav>
</header>