老师你好，我用http-server启动了两个窗口。
A窗口（http://127.0.0.1:8000/a）代码如下：

 <div class="A">A</div>
  <iframe src="http://127.0.0.1:3000/b" frameborder="0"></iframe>
  <script>
    var BWindow = document.querySelector('iframe').contentWindow
    BWindow.postMessage('helloB',' http://127.0.0.1:3000/b')
    window.addEventListener('message', function(){
      console.log(111)
    })

B窗口（http://127.0.0.1:3000/b）代码如下：

 <div class="B">B</div>
    <div class="e"></div>
    <script>
      window.addEventListener('message', function(e) {
        e.source.postMessage('hello a ', e.origin)
      })
    </script>

最后执行报错，请问是为什么呢
Failed to execute ‘postMessage’ on ‘DOMWindow’: The target origin provided (‘http://127.0.0.1:3000’) does not match the recipient window’s origin (‘http://127.0.0.1:8000’).