A:端口3000

import React, { useEffect } from 'react';

function App() {
  useEffect(() => {
    const childWindow = document.getElementById('child').contentWindow;

    const sendMessageToChild = () => {
      childWindow.postMessage("主页面消息", "http://localhost:4000");
    };

    const receiveMessageFromChild = (event) => {
      if (event.origin === "http://localhost:4000" && typeof event.data === "string") {
        console.log(event);
        const messageElement = document.getElementById('message');
        if (messageElement) {
          messageElement.innerHTML = `收到 ${event.origin} 消息：${event.data}`;
        }
        event.source.postMessage("主页面收到消息并回复", event.origin);
      }
    };

    window.addEventListener('message', receiveMessageFromChild, false);

    sendMessageToChild();

    return () => {
      window.removeEventListener('message', receiveMessageFromChild);
    };
  }, []);

  return (
    <div className="App">
      <iframe id="child" src="http://localhost:4000"></iframe>
      <div>
        <h2>主页面跨域接收消息区域</h2>
        <div id="message"></div>
      </div>
    </div>
  );
}

export default App;

B:端口4000

import React, { useEffect } from 'react';

function App() {
  useEffect(() => {
    const parentWindow = window.parent;

    const sendMessageToParent = () => {
      parentWindow.postMessage("子页面消息收到", 'http://localhost:3000');
    };

    const receiveMessageFromParent = (event) => {
      if (event.origin === "http://localhost:3000" && typeof event.data === "string") {
        console.log(event);
        const messageElement = document.getElementById('message');
        if (messageElement) {
          messageElement.innerHTML = `收到 ${event.origin} 消息：${event.data}`;
        }
        event.source.postMessage("子页面收到消息并回复", event.origin);
      }
    };

    window.addEventListener('message', receiveMessageFromParent, false);

    sendMessageToParent();

    return () => {
      window.removeEventListener('message', receiveMessageFromParent);
    };
  }, []);

  return (
    <div className="App">
      hello world
      <div id="message"></div>
    </div>
  );
}

export default App;