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;
}

.active {
  color: red;
}

div {
  cursor: pointer;
}

React

class App extends React.Component {
	state = {
    	users: [
      	{ name: "John" },
        { name: "Sarah" },
        { name: "Siri" },
        { name: "Jim" },
        { name: "Simon" },
      ],
      activeIndex: 0,
    }
  
  render() {
  	const { users, activeIndex } = this.state;

    return (
      <div>
        {users.map((u, i) => (
          <div
            className={i === activeIndex ? 'active' : ''}
            onClick={() => this.setState({ activeIndex: i })}
            key={u.name}
          >
            {u.name}
          </div>
        ))}
      </div>
    )
  }
}

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