Call JavaScript methods in parent page/child page in iframe

Ming: Suppose there are 2 pages, index.html and inner.html. There is an iframe in index.html, and the src of this iframe points to inner.html.
What we have to do now is:
1. Call a js method on inner.html in index.html
2. Call a js method on index.html in inner.html

The implementation code is as follows:

index.html

1.
2.
3.
8.
9.
10.

11.This is index page.
12.
13.
14.

15.
16.
17.

inner.html

1.
2.
3.
8.
9.
10.

11.This is inner page.
12.
13.
14.

15.
16.