JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
      <div id="image"></div>
      <div id="my_form">

        <div class="sign_in modal">

          <h1>Get Started!</h1>

          <form action="" method="get">

            <input type="text" name="fname" placeholder="Name" required>
            <input type="text" name="email" placeholder="Email" required>
            <input type="password" name="password" placeholder="Password" required>
            <input type="submit" value="Submit" class="submit">

          </form>

          <h3><a href="#" id="switchLogin">Been Here Before?</a></h3>
        </div>

         <div class="log_in modal">

          <h1>Welcome Back!</h1>

          <form action="" method="get">

            <input type="text" name="uname" placeholder="User Name" required>
            <input type="password" name="password" placeholder="Password" required>
            <input type="submit" value="Submit" class="submit">

          </form>

          <h3><a href="#" id="switchSignin">First Time?</a></h3>
        </div>
      </div>
    </div>

CSS

.slideOut{
  -webkit-animation: slideOut 1s linear;
  -moz-animation:    slideOut 1s linear;
  -o-animation:      slideOut 1s linear;
  animation:         slideOut 1s linear;
}

@-webkit-keyframes slideOut {
  0%   { opacity: 1;  display:block;}
  100% { opacity: 0; transform: translateX(-1000px);}
}
@-moz-keyframes slideOut {
  0%   { opacity: 1;  display:block;}
  100% { opacity: 0; transform: translateX(-1000px);}
}
@-o-keyframes slideOut {
  0%   { opacity: 1;  display:block;}
  100% { opacity: 0; transform: translateX(-1000px);}
}
@keyframes slideOut {
  0%   { opacity: 1;  display:block;}
  100% { opacity: 0; transform: translateX(-1000px);}
}

JavaScript

var body = document.getElementsByTagName('body'),
    signIn = document.getElementsByClassName('sign_in modal'),
    logIn = document.getElementsByClassName('log_in'),
    switchLogIn = document.getElementById('switchLogin'),
    switchSignIn = document.getElementById('switchSignin'),
    link = document.querySelector('a');

link.addEventListener("click", function(e){
    if ((e.target || e.srcElement).id == 'switchLogin'){
        console.log(signIn);
        console.log(signIn[0].classList);
        link.style.color="red";
    }else{
           console.log("nope");
    }
})