Javascript cross-origin summary_javascript skills

JS cross-origin summary mainly solves the problem of cross-origin access in javascript. There are two cross-origin javascript cases:
1. subdomains based on the same parent domain, such as a.c.com and B .c.com
2. Based on different parent domains, such as www.a.com and www. B .com
3. different ports, such as www.a.com: 8080 and www.a.com: 8088
4. Different protocols, such as: http://www.a.com and https://www.a.com
For situations 3 and 4, you must use the backend proxy. The specific method is as follows:
A. Create a proxy program in the initiator's domain
B. The initiator's js calls the proxy program in the current domain
C. The proxy sends the request to the receiver and obtains the corresponding data.
D. The proxy returns the obtained data to the initiator's js
The initiator Page code is as follows:

The Code is as follows: