React.cloneElement not appending className

Question

How can I append or change the className prop with React.cloneElement?

When I use React.cloneElement I'm unable to change or append the className prop. I've searched for hours but I found nothing. React.Children.only or removing the spread don't change the behavior. It appear to be a bug, or a performance optimization feature?.

Expect html: <div class="parent"><div class="child other-class">testing...</div></div>

Result html: <div class="parent"><div class="child">testing...</div></div>

Class example:

class Parent extends React.Component {
  render() {
    return (
      <div className={"parent"}>
        {React.cloneElement(React.Children.only(this.props.children), {
          ...this.props.children.props,
          className: `${this.props.children.props.className} other-class`,
        })}
      </div>
    );
  }
}

class Child extends React.Component {
  render() {
    return <div className={"child"}>{"testing..."}</div>;
  }
}

Functional component example:

const Parent = ({ children }) => (
  <div className={"parent"}>
    {React.cloneElement(React.Children.only(children), {
      ...children.props,
      className: `${children.props.className} other-class`,
    })}
  </div>
);

const Child = () => <div className={"child"}>{"testing..."}</div>;

 const Parent = ({ children }) => ( <div className={"parent"}> {React.cloneElement(React.Children.only(children), {...children.props, className: `${children.props.className} other-class`, })} </div> ); const Child = () => <div className={"child"}>{"testing2..."}</div>; ReactDOM.render( <React.StrictMode> <Parent> <Child /> </Parent> </React.StrictMode>, document.getElementById("root") );

 <script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script> <div id="root"></div>

Answer 1

The problem is that you're never using className in Child , which is what you're manipulating in Parent . Child puts a className on a div , but that isn't Child 's className , it's just a hardcoded one that Child puts on the div .

If you want Child to put that class on the div , you have to write the code to do that. Also, you don't need the spread, the props are merged. Finally, to get the original className , I'd use the result of calling Children.only , rather than going back to this.props.children (though that will work because only would throw if there weren't only one).

See comments:

 class Parent extends React.Component { render() { // Get the `className` from the child after verifying there's only one const child = React.Children.only(this.props.children); const className = `${child.props.className} other-class`; return ( <div className={"parent"}> {React.cloneElement(child, { // No need to spread previous props here className, })} </div> ); } } class Child extends React.Component { render() { // Use `className` from `Child`'s props const className = (this.props.className || "") + " child"; return <div className={className}>{"testing..."}</div>; } } // Note the `classname` on `Child`, to show that your code using // `this.props.children.props.className` ReactDOM.render(<Parent><Child className="original"/></Parent>, document.getElementById("root"));

 .child { color: red; }.child.other-class { color: green; }.original { font-style: italic; }

 <div id="root"></div> <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script>