How to Share localStorage Across Subdomains

Many websites store data using localStorage instead of cookies, as it provides better performance and security. However, localStorage can present a challenge when dealing with subdomains, as each subdomain has its own separate localStorage object. This can become an issue when a user accesses a website using both the main domain (e.g., site.example) and a subdomain (e.g., www.site.example), as their data will be stored in different localStorage objects. This can lead to a problem if the user expects their data to be accessible regardless of the subdomain that they are using.

One possible solution to this issue is to use an iframe from the main domain to communicate with the subdomains. By using postMessage to send messages between the iframe and the subdomain, the subdomain can access the main domain's localStorage. To implement this solution, the following steps can be taken:

1. Create an iframe in the main domain (parent.example)
2. In each child domain (child.example), send a postMessage to the iframe on the main domain
3. Establish a protocol for interpreting postMessage messages to facilitate communication between the subdomain and the main domain's iframe.

By following this process, all subdomains will be able to share the same localStorage as the main domain, allowing the user's data to be accessible regardless of the subdomain that they are using.