Keep getting “cannot read property style of null” error
Question
I am working on my own website and not good with codes yet. When I am scrolling down I want to appear another content of the navbar and when I am on the top, original navbar is appearing. I want this to be done in pure JavaScript with no libraries or framewokrs. Please see codes below and I know that codes are not organised. I will do that later on.
var nav = document.querySelector("nav");
var hide = document.querySelector(".hide");
var appear = document.querySelector(".appear")
window.onscroll = function(){
if(document.body.scrollTop > 70){
hide.style.display = "block";
appear.style.display = "none"
} else {
hide.style.display = "none";
appear.style.display = "block"
}
}
nav{
list-style-type: none;
text-align: center;
background-color: #3FA9A5;
position: sticky;
top: 0;
}
.hide{
font-size: 70px;
font-family: 'Long Cang', cursive;
display: block;
}
.appear{
height: 70px;
display: none;
}
.appear img{
width: 210px;
}
ul{
margin: 0 auto;
padding: 0;
}
body{
margin: 0;
}
.container{
max-width: 1080px;
width: 95%;
margin: 10px auto;
display: grid;
grid-template-columns: 25% 50% 25%;
}
.text{
text-align: center;
}
.profile {
border: 1px solid black;
padding: 0 10px 20px 10px;
}
#main{
width: 100%;
}
.post{
margin-left: 4.165%;
}
#image{
width: 100%;
}
#post-divide{
margin-left: 10px;
margin-right: 10px;
}
.comments{
width: 100%;
margin-top: 68.5px;
padding-bottom: 293.5px;
border: 1px solid black;
}
h2{
text-align: center;
}
.center{
grid-column: 2;
}
<link rel="stylesheet" type="text/css" href="test.css">
<link href="https://fonts.googleapis.com/css?family=Indie+Flower|Long+Cang&display=swap" rel="stylesheet">
<title>test</title>
</head>
<body>
<nav>
<ul>
<li class="hide">Unknown</li>
<li class="appear"><img src="cat.png"></li>
</ul>
</nav>
<div class="container">
<div class="col-1">
<div class="profile text">
<img id="main" src="https://data.whicdn.com/images/86629641/superthumb.jpg?t=1384568664">
<hr>
<p>12 posts</p>
<p>instagram</p>
<button>Subscribe!</button>
</div>
</div>
<div class="col-1">
<div class="post">
<h2>TITLE</h2>
<div>
<img id="image" src="https://i.pinimg.com/originals/76/d4/8c/76d48cb2928845dfcfab697ac7cbcf1c.jpg">
</div>
<hr id="post-divide">
</div>
</div>
<div class="col-1">
<div class="comments text"></div>
</div>
<div class="col-1 center">
<div class="post">
<h2>TITLE</h2>
<div>
<img id="image" src="https://i.pinimg.com/originals/76/d4/8c/76d48cb2928845dfcfab697ac7cbcf1c.jpg">
</div>
<hr id="post-divide">
</div>
</div>
<div class="col-1">
<div class="comments text"></div>
</div>
</div>
I think I should add something to the JS code but don't know why Would be thankful if you would advise me how could I write HTML/CSS code so I do not have to create 2 navbars if it is possible
4 answers
solution1
2 2019-10-28 11:44:58
The following instruction:
document.querySelector("hide");
Will query for elements like:
<hide></hide>
Since plain selectors without prefix ( div , header , span ) will query for the whole element tags, not for classes or attrbitues.
Maybe you meant to query for the class, using the . :
document.querySelector(".hide");
solution2
0 ACCPTED 2019-10-28 11:44:18
var hide = document.querySelector(".hide");
var appear = document.querySelector(".appear")
So you should use class selector
solution3
0 2019-10-28 11:47:39
You are using "hide" and "appear" as selectors but they do not exist in your HTML.
Use ".hide" and ".appear" in your querySelector instead.
var hide = document.querySelector(".hide");
var appear = document.querySelector(".appear");
solution4
0 2019-10-28 12:20:03
Since both navbars have a static data, I would suggest to keep both of them and follow with answers of guys, that suggested to update querySelector param. Or you can hide/show the data inside of navbar (in your case it's only ul element) and leave the whole navbar always visible. So you can put classes appear/hide on ul element instea of navbar and then in JS get them with document.querySelector('.navbar.hide') and document.querySelector('.navbar.appear') .
Using framework/library will definitely simplify it. However, if you still want to have only one navbar in pure js/html/css (or it's data just dynamic) I would probably do like this:
HTML:
<nav class="navbar">
<ul>
<li><img src="cat.png"></li>
</ul>
</nav>
somewhere in JS:
var navbarUl = document.querySelector('.navbar ul');
window.onscroll = function() {
if (document.body.scrollTop > 70) {
navbarUl.innerHtml = '';
navbarUl.appendChild(getTopNavbarHTML);
} else {
navbarUl.innerHtml = '';
navbarUl.appendChild(getNavbarHTML);
}
}
getNavbarHTML and getTopNavbarHTML - will return documentFragment with li elements, see for details https://www.w3schools.com/jsref/met_document_createdocumentfragment.asp
But changing DOM during a scroll event can drastically decrease performance of a web app
solution5
0 2019-10-28 13:23:18
Thanks for help to all of you. Now everything works and i changed code a little. Now I have other problem that when I go back to the Top, first navbar doesn't shows back and it still stays on second navbar style. Also when I start scrolling down, second navbar style shows up as soon as I scroll down, but I want it to appear when first is totally invisible which height is 70px
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