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

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>