event listener

custom.js

'use strict'

const el = document.querySelector('h1')
const btn = document.getElementById('show')
const height = el.scrollHeight

el.style.cssText = 'overflow: hidden; transition: 1s; max-height: 0;'

// Listens for click event before triggering script
btn.addEventListener('click', () => {
  el.style.cssText = `overflow: hidden; transition: 1s; max-height: ${height}px;`
})

index.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>JavaScript Event Listener</title>
  <meta name="robots" content="noindex, nofollow">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="styles.css">
</head>

<body>
  <header>
    <h1>JavaScript Event Listener</h1>
  </header>
  <div class="wrapper">
    <div class="container">
      <button id="show">Click</button>
    </div>
    <!-- .container-->
  </div>
  <!-- .wrapper-->
  <footer>
    <small><a href="https://opensource.org/licenses/MIT">MIT</a></small>
  </footer>
  <script src="custom.js"></script>
</body>

</html>

styles.css

body {
  background: #eeeeee;
  font-size: 16px;
  font-family: "Trebuchet MS", Helvetica, sans-serif;
  margin: 0;
}

a {
  color: crimson;
  text-decoration: none;
}

a:hover {
  color: red;
}


/* Layout
------------------------------------------*/

header,
.container,
footer {
  margin: 0 auto;
  width: 88%;
  max-width: 960px;
}

.wrapper {
  padding: 1em 0;
  background: #ffffff;
}