React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

input {
  margin-right: 5px;
}

React

class TodoApp extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
    	isMouseOver: false,
    }
  }
  
  render() {
    return (
      <div
        onMouseEnter={() => {
          this.setState({isMouseOver: true})
       	}}
        onMouseLeave={() => {
          this.setState({isMouseOver: false})
       	}}
      >
      
        <h2>Is mouse over? {this.state.isMouseOver.toString()}</h2>
      </div>
    )
  }
}

ReactDOM.render(<TodoApp />, document.querySelector("#app"))