Javascript doesnt seem to be working on Sliding Signup/Sign In form but works on codepen

I am creating a sign up and sign in form and I am taking a tutorial on it. However, the button used to slide the container does not seem to be working. This is a link to the codepen of how it works. https://codepen.io/47David/pen/ZEXxRdd It works perfectly fine on codepen, yet it somehow does not work on my browser nor any browser I use. I figure it might be what I am using to run it. I am using Xampp to run this.

const signUpButton = document.getElementById('signUp');
const signInButton = document.getElementById('signIn');
const container = document.getElementById('container');

signUpButton.addEventListener('click', () => {
  container.classList.add('right-panel-active');
});

signInButton.addEventListener('click', () => {
  container.classList.remove('right-panel-active');
});

@import url('https://fonts.googleapis.com/css?family=Montserrat:400,800');
* {
  box-sizing: border-box;
}

body {
  background: #f6f5f7;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  font-family: 'Montserrat', sans-serif;
  height: 100vh;
  margin: -20px 0 50px;
}

h1 {
  font-weight: bold;
  margin: 0;
}

h2 {
  text-align: center;
}

p {
  font-size: 14px;
  font-weight: 100;
  line-height: 20px;
  letter-spacing: 0.5px;
  margin: 20px 0 30px;
}

span {
  font-size: 12px;
}

a {
  color: #333;
  font-size: 14px;
  text-decoration: none;
  margin: 15px 0;
}

button {
  border-radius: 20px;
  border: 1px solid #FF4B2B;
  background-color: #FF4B2B;
  color: #FFFFFF;
  font-size: 12px;
  font-weight: bold;
  padding: 12px 45px;
  letter-spacing: 1px;
  text-transform: uppercase;
  transition: transform 80ms ease-in;
}

button:active {
  transform: scale(0.95);
}

button:focus {
  outline: none;
}

button.ghost {
  background-color: transparent;
  border-color: #FFFFFF;
}

form {
  background-color: #FFFFFF;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  padding: 0 50px;
  height: 100%;
  text-align: center;
}

input {
  background-color: #eee;
  border: none;
  padding: 12px 15px;
  margin: 8px 0;
  width: 100%;
}

.container {
  background-color: #fff;
  border-radius: 10px;
  box-shadow: 0 14px 28px rgba(0, 0, 0, 0.25), 0 10px 10px rgba(0, 0, 0, 0.22);
  position: relative;
  overflow: hidden;
  width: 768px;
  max-width: 100%;
  min-height: 480px;
}

.form-container {
  position: absolute;
  top: 0;
  height: 100%;
  transition: all 0.6s ease-in-out;
}

.sign-in-container {
  left: 0;
  width: 50%;
  z-index: 2;
}

.container.right-panel-active .sign-in-container {
  transform: translateX(100%);
}

.sign-up-container {
  left: 0;
  width: 50%;
  opacity: 0;
  z-index: 1;
}

.container.right-panel-active .sign-up-container {
  transform: translateX(100%);
  opacity: 1;
  z-index: 5;
  animation: show 0.6s;
}

@keyframes show {
  0%,
  49.99% {
    opacity: 0;
    z-index: 1;
  }
  50%,
  100% {
    opacity: 1;
    z-index: 5;
  }
}

.overlay-container {
  position: absolute;
  top: 0;
  left: 50%;
  width: 50%;
  height: 100%;
  overflow: hidden;
  transition: transform 0.6s ease-in-out;
  z-index: 100;
}

.container.right-panel-active .overlay-container {
  transform: translateX(-100%);
}

.overlay {
  background: #FF416C;
  background: -webkit-linear-gradient(to right, #FF4B2B, #FF416C);
  background: linear-gradient(to right, #FF4B2B, #FF416C);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: 0 0;
  color: #FFFFFF;
  position: relative;
  left: -100%;
  height: 100%;
  width: 200%;
  transform: translateX(0);
  transition: transform 0.6s ease-in-out;
}

.container.right-panel-active .overlay {
  transform: translateX(50%);
}

.overlay-panel {
  position: absolute;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  padding: 0 40px;
  text-align: center;
  top: 0;
  height: 100%;
  width: 50%;
  transform: translateX(0);
  transition: transform 0.6s ease-in-out;
}

.overlay-left {
  transform: translateX(-20%);
}

.container.right-panel-active .overlay-left {
  transform: translateX(0);
}

.overlay-right {
  right: 0;
  transform: translateX(0);
}

.container.right-panel-active .overlay-right {
  transform: translateX(20%);
}

.social-container {
  margin: 20px 0;
}

.social-container a {
  border: 1px solid #DDDDDD;
  border-radius: 50%;
  display: inline-flex;
  justify-content: center;
  align-items: center;
  margin: 0 5px;
  height: 40px;
  width: 40px;
}

footer {
  background-color: #222;
  color: #fff;
  font-size: 14px;
  bottom: 0;
  position: fixed;
  left: 0;
  right: 0;
  text-align: center;
  z-index: 999;
}

footer p {
  margin: 10px 0;
}

footer i {
  color: red;
}

footer a {
  color: #3c97bf;
  text-decoration: none;
}


/* Animations */


/* Move signin to the right*/

.container.right-panel-active .sign-in-container {
  transform: translateX(100%);
}


/* Move overlay to the left*/

.container.right-panel-active .overlay-container {
  transform: translateX(-100%);
}

.container.right-panel-active .sign-up-container {
  transform: translateX(100%);
}

<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.8.1/css/all.css" crossorigin="anonymus">
<h2>Welcome to JLancer!</h2>
<div class="container" id="container">
  <div class="form-container sign-up-container">
    <form action="#">
      <h1>Create Account</h1>
      <div class="social-container">
        <a href="#" class="social"><i class="fab fa-facebook-f"></i></a>
        <a href="#" class="social"><i class="fab fa-google-plus-g"></i></a>
        <a href="#" class="social"><i class="fab fa-linkedin-in"></i></a>
      </div>
      <span>or use your email for registration</span>
      <input type="text" placeholder="Name" />
      <input type="email" placeholder="Email" />
      <input type="password" placeholder="Password" />
      <button>Sign Up</button>
    </form>
  </div>
  <div class="form-container sign-in-container">
    <form action="#">
      <h1>Sign in</h1>
      <div class="social-container">
        <a href="#" class="social"><i class="fab fa-facebook-f"></i></a>
        <a href="#" class="social"><i class="fab fa-google-plus-g"></i></a>
        <a href="#" class="social"><i class="fab fa-linkedin-in"></i></a>
      </div>
      <span>or use your account</span>
      <input type="email" placeholder="Email" />
      <input type="password" placeholder="Password" />
      <a href="#">Forgot your password?</a>
      <button>Sign In</button>
    </form>
  </div>
  <div class="overlay-container">
    <div class="overlay">
      <div class="overlay-panel overlay-left">
        <h1>Welcome Back!</h1>
        <p>To keep connected with us please login with your personal info</p>
        <button class="ghost" id="signIn">Sign In</button>
      </div>
      <div class="overlay-panel overlay-right">
        <h1>Hello, Friend!</h1>
        <p>Enter your personal details and start journey with us</p>
        <button class="ghost" id="signUp">Sign Up</button>
      </div>
    </div>
  </div>
</div>

<footer>
  <p>
    This was created <i class="fa fa-heart"></i> by
    <a target="_blank" href="#">David Babalola</a> - This is a beta, if you have any problems click here
    <a target="_blank" href="#">here</a>.
  </p>
</footer>

Solution

Make sure CSS and JS file paths are pointing to the correct location. If you don't know you have to paste CSS and JS into respective files and point them using these lines.

<link rel="stylesheet" type="text/css" href="styles.css"/>
<script src=script.js></script>

I ran it on my PC it worked fine, maybe it didn't work on your localhost because you didn't place the JS file in its location.