Answers for "body.addclass"

add classnames to body

openModal = (event) => {
  document.body.classList.add('modal-open');
  this.setState({ showModal: true });
}
hideModal = (event) => {
  document.body.classList.remove('modal-open');
  this.setState({ showModal: false });
}

add class to body javascript

var body = document.body;

body.classList.add("body_class");