Permanent Solution for "No 'Access-Control-Allow-Origin' header is present on the requested resource"

I have seen this problem quite a few times and it pops up time and again. This is a CORS(i.e. Cross origin request issue).The exact error I got this time is as follows:

XMLHttpRequest cannot load

https://myURL/myappdomain.subdomain.qual1/$count. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access. The response had HTTP status code 401.

Following are the possible solutions I have worked out in past. But they dont always work. They are URL specific solutions:

1) Having CORS plugin on chrome installed

2) Disabling web security from command line "--disable-web-security"

3) using 'jsonp' as format instead of 'json'

4) toggling cross-origin to "true" or "false".

Questions I need answer for

1) Why do we get this error? Is it something that the Server is imposing on the client pages?

2) What is the safest way to solve this? i.e. The method in which there is not security vulnerability and a reliable method.

3) Why cors is never an issue for API calls made from within nodeJS code?

via nihal