fade in fade out

custom.js

'use strict'

const box = document.getElementById('box')
const open = document.getElementById('open')
const close = document.getElementById('close')

let opacity

box.style.opacity = 0
// Fade in function
function fadeIn(el) {
  opacity = 0

  function fade() {
    if (opacity < .9) {
      opacity += .1
    }

    el.style.opacity = opacity
    console.log(el.style.opacity)
    if (opacity < .9) {
      requestAnimationFrame(fade)
    } else {
      cancelAnimationFrame(fade)
    }

  }
  requestAnimationFrame(fade)
}

// Fade out function
function fadeOut(el) {
  opacity = 1

  function fade() {
    if (opacity > .1) {
      opacity -= .1
    }

    el.style.opacity = opacity
    console.log(el.style.opacity)
    if (opacity > .1) {
      requestAnimationFrame(fade)
    } else {
      cancelAnimationFrame(fade)
    }

  }
  requestAnimationFrame(fade)
}

open.addEventListener('click', () => {
  fadeIn(box)
})

close.addEventListener('click', () => {
  fadeOut(box)
})

index.html

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

<head>
  <meta charset="UTF-8">
  <title>JavaScript Fade In or Fade Out (rAF)</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 Fade In or Fade Out (rAF)</h1>
  </header>
  <div class="wrapper">
    <div class="container">
      <div id="box"></div>
      <button id="open">Open</button>
      <button id="close">Close</button>
    </div>
  </div>
  <footer>
    <small><a href="https://opensource.org/licenses/MIT">MIT</a></small>
  </footer>
  <script src="custom.js"></script>
</body>

</html>

styles.css

body {
  background: #eee;
  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 {
  background: #ffffff;
  padding: 1em 0;
}


/* Animated box
------------------------------------------*/

#box {
  width: 400px;
  height: 400px;
  background: pink;
}