Show element when in viewport on scroll

Question

I've been trying to show an element on scroll when it's in viewport and when no, hide it again. But no matter what I try, I can't make it work.

This is what I have so far, but the function is running just once, when the page is loaded and not when it's scrolled, so it doesn't update the value.

$(window).scroll(function() {
    var top_of_element = $("#cont_quote blockquote").offset().top;
    var bottom_of_element = $("#cont_quote blockquote").offset().top + $("#cont_quote blockquote").outerHeight();
    var bottom_of_screen = $(window).scrollTop() + window.innerHeight;
    var top_of_screen = $(window).scrollTop();

    if((bottom_of_screen > top_of_element) && (top_of_screen < bottom_of_element)){
       $('#cont_quote blockquote').animate({'opacity':'1'},1000);
    }
    else {
       $('#cont_quote blockquote').animate({'opacity':'0'},1000);
    }
});



<section id="cont_quote">
    <article class="cont_q">
        <blockquote>Lorem ipsum</blockquote>
    </article>
</section>

Answer 1

In pure javascript, you could do something like this, which uses a lot less resources than a full on jQuery approach:

function inViewport( element ){
  
  // Get the elements position relative to the viewport

  var bb = element.getBoundingClientRect();
  
  // Check if the element is outside the viewport
  // Then invert the returned value because you want to know the opposite

  return !(bb.top > innerHeight || bb.bottom < 0);
  
}

var myElement = document.querySelector( 'div' );

// Listen for the scroll event

document.addEventListener( 'scroll', event => {
 
  // Check the viewport status

  if( inViewport( myElement ) ){
    
    myElement.style.background = 'red';
    
  } else {
    
    myElement.style.background = '';
    
  }
  
})

body {
  
  height: 400vh;
  
}

div {
  
  width: 50vw;
  height: 50vh;
  position: absolute;
  top: 125vh;
  left: 25vw;
  transition: background 4s;
  border: 1px solid red;
  
}

<p>Scroll Down</p>
<div></div>

Here is a snippet with the opacity change:

function inViewport( element ){
  
  // Get the elements position relative to the viewport

  var bb = element.getBoundingClientRect();
  
  // Check if the element is outside the viewport
  // Then invert the returned value because you want to know the opposite

  return !(bb.top > innerHeight || bb.bottom < 0);
  
}

var myElement = document.querySelector( 'div' );

// Listen for the scroll event

document.addEventListener( 'scroll', event => {
 
  // Check the viewport status

  if( inViewport( myElement ) ){
    
    myElement.style.opacity = 1;
    
  } else {
    
    myElement.style.opacity = '';
    
  }
  
})

body {
  
  height: 400vh;
  
}

div {
  
  width: 50vw;
  height: 50vh;
  position: absolute;
  top: 125vh;
  left: 25vw;
  transition: opacity 1s;
  opacity: .2;
  background: blue;
  
}

<p>Scroll Down</p>
<div></div>

And here is a snippet showing you how to define where in the viewport it triggers, I just changed the innerHeight and 0 values to an object where you define the amount of pixels from the top it should be and the amount of pixels from the bottom. Don't forget to also add an event listener for resize, as these pixel based values will change if your viewport changes, so your myViewport object would need to be updated accordingly:

function inViewport( element, viewport = { top: 0, bottom: innerHeight } ){
  
  // Get the elements position relative to the viewport

  var bb = element.getBoundingClientRect();
  
  // Check if the element is outside the viewport
  // Then invert the returned value because you want to know the opposite

  return !(bb.top > viewport.bottom || bb.bottom < viewport.top);
  
}

var myViewport = { top: innerHeight * .4, bottom: innerHeight * .6 };
var myElement = document.querySelector( 'div' );

// Listen for the scroll event

document.addEventListener( 'scroll', event => {
 
  // Check the viewport status

  if( inViewport( myElement, myViewport ) ){
    
    myElement.style.opacity = 1;
    
  } else {
    
    myElement.style.opacity = '';
    
  }
  
})

window.addEventListener( 'resize', event => {
 
  // Update your viewport values

  myViewport.top = innerHeight * .4;
  myViewport.bottom = innerHeight * .6;
  
})

body {
  
  height: 400vh;
  
}

div {
  
  width: 50vw;
  height: 50vh;
  position: absolute;
  top: 125vh;
  left: 25vw;
  transition: opacity 1s;
  opacity: .2;
  background: blue;
  
}

<p>Scroll Down</p>
<div></div>

Answer 2

i tried to solve your problem by your code only. its working fine for me now. plz try this and let me know. also open your browser console to see if there is any js error.

$(window).scroll(function() {
  var top_of_element = $("#cont_quote blockquote").offset().top;
  var bottom_of_element = $("#cont_quote blockquote").offset().top + $("#cont_quote blockquote").outerHeight();
  var bottom_of_screen = $(window).scrollTop() + window.innerHeight;
  var top_of_screen = $(window).scrollTop();

  if((bottom_of_screen > top_of_element) && (top_of_screen < bottom_of_element)){
    $('#cont_quote blockquote').fadeIn(1000);
    console.log('if cond');
  } else {      
    $('#cont_quote blockquote').fadeOut(1000);
    console.log('else cond');
  }
});

Answer 3

Try using:

$(window).on('scroll mousewheel', function() {

And surround your function with:

$(document).ready(function(){
});