<div class="accordion">
  <div class="accordion-panel">
    <h3 class="accordion-header">Panel 1</h3>
    <div class="accordion-body">These are the contents of panel 1</div>
  </div>
  <div class="accordion-panel accordion-expanded">
    <h3 class="accordion-header">Panel 2</h3>
    <div class="accordion-body">These are the contents of panel 2</div>
  </div>
  <div class="accordion-panel">
    <h3 class="accordion-header">Panel 3</h3>
    <div class="accordion-body">These are the contents of panel 3</div>
  </div>
</div>
.accordion-header {
  cursor: pointer;
}

.accordion-panel {
  border: 1px dashed black;
  margin: 2px;
  padding: 5px;
}

.accordion-panel > .accordion-body {
  display: none;
}

.accordion-panel.accordion-expanded > .accordion-body {
  display: block;
}
class Accordions extends React.Component {
  constructor() {
    super();
    
    const active = new Map();
    const accordions = document.querySelectorAll(".accordion");
    for (const accordion of accordions) {
      active.set(accordion, accordion.querySelector(".accordion-expanded"));
    }
    this.state = {
      active
    };
  }
  
  componentDidMount() {  
    const accordions = document.querySelectorAll(".accordion");
    for (const accordion of accordions) {
      const panels = accordion.querySelectorAll(".accordion-panel");
      for (const panel of panels) {
        const head = panel.querySelector(".accordion-header");
        head.addEventListener('click', () => {
          this.setState(state => {
            // clone the map
            const active = new Map(state.active);
            if (active.get(accordion) === panel) {
              active.set(accordion, null);
            } else {
              active.set(accordion, panel);
            }
            return { active };
          });
        });
      }
    }
  }

  componentDidUpdate() {
    for (const accordion of this.state.active.keys()) {
      const active = this.state.active.get(accordion);
      const panels = accordion.querySelectorAll(".accordion-panel");
      for (const panel of panels) {
        panel.classList.toggle("accordion-expanded", panel === active);
      }
    }
  }
  
  render() {
    return null;
  }
}

ReactDOM.render(
  <Accordions/>,
  document.createElement('div') // detached container
)
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