How can I call a function inside an iframe?

In main.html I want to call function a in the following iframe.

...
<iframe src="/viewer/index.do?mapNo=20&amp;viewMode=ifrm">
<script>
function a() {
     console.log('hi');
</script>
</iframe>
...

I wonder how to call function a inside iframe when such code exists.

Best Regards!