iframe求助

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以下为一个内嵌iframe页面的html:

<!DOCTYPE html>
<html lang="zh">

<head>
<meta charset="UTF-8">
<title>ThingJS的 Iframe演示</title>

</head>

<body>

<div width="800px">
      <button style="position: absolute;top: 15px;left: 15px;width: 75px;" onclick="flyToCar()">飞到小车</button>
      <button style="position: absolute;top: 45px;left: 15px;width: 75px;"
         onclick="flyToBack()">返        回</button>
      <iframe id="testMsg" style="width: 600px;height: 400px;"
         src='https://www.thingjs.com/s/aa25e09eae2f73e6ce080d73?params=105b0f77fd24654d4eebc434e9'></iframe>

</div>
<script>
      function flyToCar() {
         var a = document.getElementById("testMsg");
         var msg = {
            funcName: 'test'
         }
         a.contentWindow.postMessage(msg, "*");
      }

      function flyToBack() {
         var a = document.getElementById("testMsg");
         var msg = {
            funcName: 'test2',
            param: "2"
         }
         a.contentWindow.postMessage(msg, "*");
      }
</script>
</body>

</html>

请问如果使用活字格的JS命令模拟调用function?如flyToCar()