react-transition-group 從 v1 升級到 v4 比較重要的有幾點：

1. 升級後 CSSTransitionGroup 元件被移除了
2. leave 變成 exit
3. transitionName 變成 classNames
4. transitionEnterTimeout 之類的變成 timeout 物件
5. transition{Appear, Enter, Leave} 變成 appear, enter, exit

如果在元件中有用到下列方法的就比較麻煩一點了，要自己拔掉：

• componentWillAppear
• componentDidAppear
• componentWillEnter
• componentDidEnter
• componentWillLeave
• componentDidLeave

沒有什麼奇怪的 hack 的話，也可以考慮用 jscodeshift 轉移。如果元件已經肥大到沒辦法輕易 migrate 的話，可以建立一個 wrapper 把原本的 props 包裝一下：

javascript
/**
 * NOTE:
 * This is temporary wrapper component for Transition.js,
 * It provides the same API as `CSSTransitionGroup` in v1
 */
import * as React from 'react';
import { TransitionGroup, CSSTransition } from 'react-transition-group';
type Props = {
  component?: React.ReactNode;
  transitionName: string;
  transitionEnterTimeout?: number;
  transitionExitTimeout?: number;
  transitionEnter?: boolean;
  transitionExit?: boolean;
  children: React.ReactNode,
};
const CSSTransitionGroup = ({
  transitionEnterTimeout= 0,
  transitionExitTimeout= 0,
  transitionEnter= true,
  transitionExit= true,
  transitionName,
  children,
  component,
}: Props) => {
  const transitionChildrenProps = {
    classNames: transitionName,
    timeout: { enter: transitionEnterTimeout, exit: transitionExitTimeout }
  };
  const transitions = React.Children.map(children, (child: any) => (
    <CSSTransition key={`transition-${child.key}`} {...transitionChildrenProps}>{child}</CSSTransition>
  ));
  return (
    <TransitionGroup
			component={component}
			enter={transitionEnter}
			exit={transitionExit}
 		>
      {transitions}
    </TransitionGroup>
  );
};
export default CSSTransitionGroup

這樣就可以無痛轉移了。不過為了方便維護著想，還是要慢慢拔掉啊。