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;
}

button.active {
  color: white;
  background: black;
}

React

class TodoApp extends React.Component {
  state = { ids: [0,1,2,3,4], active: 0 }
  
  setActive = id => { this.setState({ active: id }) }
  
  render() {
    return (
      <div>
      { 
        this.state.ids.map(id => (
          <button 
            key={id}
            onClick={() => this.setActive(id)}
            className={`${this.state.active === id ? "active" : ''}`}
          >
            {id}
          </button>
        ))
      }
      </div>
    )
  }
}

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