Read more link with pure JavaScript

Question

I am trying to create a simple Read More example. It consists of a paragraph and a button with half of the paragraph enclosed in a span tag which is initially set to hidden. When user clicks on Read More button the hidden span shows up. I have got the working code but just want to do a fade in effect like JQuery but with pure Javascript. Anyone please help.

var span = document.getElementsByTagName('span')[0];
var hideshow = document.getElementById('hideshow');

span.style.display = 'none';

hideshow.onclick = function() {
  span.style.display = 'block';
};

<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ipsa maiores dolore earum ducimus molestiae, aut. <span>Quisquam consequuntur, maiores et, doloremque atque provident similique consequatur totam voluptas vitae veniam, molestiae laborum.</span></p>

<button id="hideshow">Read More</button>

Answer 1

Starting here

span.style.opacity = 0;

You'll need to gradually transition the opacity to here.

span.style.opacity = 1;

You'll need to use an asynchronous construct (setTimeout/setInterval/requestAnimationFrame) for iterating, because a synchronous one (while/for/for-in/forEach) will block the main thread, preventing the browser from actually rendering the element with the updated opacity.

function fadeIn(element) {

  function transition() {
    if(element.style.opacity < 1) {
      requestAnimationFrame(transition);
      element.style.opacity = Number(element.style.opacity) + 0.05;
    }
  }

  transition();
}

Alternatively, if you're happy to use CSS (rather than pure JS) you can do this with classes and transitions.

.out {
  opacity: 0;
  transition-duration: 0.5s;
}

.in {
  opacity: 1;
  transition-duration: 0.5s;
}

Make sure that the element has the out class when it arrives in the DOM, then when you're ready to fade it in, swap it for the in class and the browser will handle the animation for you.

Answer 2

One approach is to use a CSS3 transition in order to transition the element's opacity.

In the example below, the class fade-in is added to the child span element when clicking the button.

var button = document.querySelector('.read-more');

button.addEventListener('click', function(event) {
  var span = event.target.previousElementSibling.querySelector('span');
  span.classList.add('fade-in');
});

.show-more span {
  display: inline-block;
  height: 0;
  overflow: hidden;
  transition: opacity 2s;
  opacity: 0;
}
.show-more span.fade-in {
  height: auto;
  opacity: 1;
}

<p class="show-more">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ipsa maiores dolore earum ducimus molestiae, aut. <span>Quisquam consequuntur, maiores et, doloremque atque provident similique consequatur totam voluptas vitae veniam, molestiae laborum.</span></p>
<button class="read-more">Read More</button>

If you want an approach that works for multiple elements, you could also use the following:

var buttons = document.querySelectorAll('.read-more');

for (var i = 0; i < buttons.length; i++) {
  buttons[i].addEventListener('click', function(event) {
    var span = event.target.previousElementSibling.querySelector('span');
    span.classList.add('fade-in');
  });
}

.show-more span {
  display: inline-block;
  height: 0;
  overflow: hidden;
  transition: opacity 2s;
  opacity: 0;
}
.show-more span.fade-in {
  height: auto;
  opacity: 1;
}

<p class="show-more">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ipsa maiores dolore earum ducimus molestiae, aut. <span>Quisquam consequuntur, maiores et, doloremque atque provident similique consequatur totam voluptas vitae veniam, molestiae laborum.</span></p>
<button class="read-more">Read More</button>

<p class="show-more">Another shorter paragraph. <span>Quisquam consequuntur, maiores et, doloremque atque provident similique consequatur totam voluptas vitae veniam, molestiae laborum.</span></p>
<button class="read-more">Read More</button>

Answer 3

var duration = 2000; // msecs

document.getElementById('hideshow').onclick = () => {
   requestAnimationFrame((start_time) => {
      var anim = (time) => {
         var p = (time - start_time) / duration;
         (p < 1) && requestAnimationFrame(anim);
         span.style.opacity = 1 - p;
      }
      anim(start_time);
   })
}