Ionic iframe - [ts] Property 'contentWindow' does not exist on type 'HTMLElement'.

最近在ionic项目中,html中嵌入iframe标签,在ts引用时出现以下错误

[ts] Property ‘contentWindow’ does not exist on type ‘HTMLElement’.

我的html代码如下

<ion-content scroll="true" overflow-scroll="true">
  <iframe  name="mainFrame" id="mainframe" class="width-100 height-100" style="position: absolute; min-width: 100%; min-height: 100%; z-index: 99;">
  </iframe>
</ion-content>

我一开始的ts引用代码如下

document.getElementById("mainframe").contentWindow.postMessage("getPicValue:" + imageData, "*");

最后的解决方案是

let iframe = document.getElementById("mainframe");
var iWindow = (<HTMLIFrameElement> iframe).contentWindow;
iWindow.postMessage("getPicValue:" + imageData, "*");