Sharing localStorage Across Subdomains

When migrating from cookies to localStorage, cross-domain data accessibility can become a challenge. In scenarios like the one described in the question, where different subdomains (e.g., site.example and www.site.example) isolate their localStorage objects, data sharing becomes problematic.

An innovative solution proposed in the answer involves utilizing an iframe from the main domain, parent.example. By leveraging postMessage communication between child subdomains (child.example) and the parent domain iframe, a custom protocol can be established to facilitate data exchange.

Through this approach, the parent domain iframe acts as a central repository forlocalStorage data, which is then made accessible to all child subdomains. By sending postMessage messages with specific instructions to the iframe, child domains can retrieve or store data in the shared localStorage object, overcoming the limitations imposed by subdomain isolation.