方法一：ES6 模板字符串

className={`title ${index === this.state.active ? 'active' : ''}`}

方法二：join("")

className={["title", index === this.state.active?"active":null].join(' ')}

方法三：classnames (需要下載classnames)

var classNames = require('classnames');
 
var Button = React.createClass({
  // ...
  render () {
    var btnClass = classNames({
      btn: true,
      'btn-pressed': this.state.isPressed,
      'btn-over': !this.state.isPressed && this.state.isHovered
    });
    return <button className={btnClass}>{this.props.label}</button>;
  }
});