Toggle className with vanilla JavaScript

<!DOCTYPE html>
<html>

  <head>
    <link rel="stylesheet" href="style.css">
  </head>

  <body>
    <h1>Hello Plunker!</h1>
    <script src="script.js"></script>
  </body>

</html>

const toggleClass = (element, className) => {
  let classNames = element.className.split(' ');
  let index = classNames.indexOf(className);
  if (index === -1) {
    classNames.push(className);
  } else {
    classNames.splice(index, 1);
  }
  element.className = classNames.filter(item => item !== '').join(' ');
}

// Usage:
let body = document.getElementsByTagName('body')[0];
toggleClass(body, 'ready');

/* Styles go here */