How can I share LocalStorage across subdomains in my web application?

Sharing LocalStorage Across Subdomains

Many developers opt to replace cookies with localStorage for browsers that support it. However, one challenge arises: subdomains (such as www.example.com) have their own distinct localStorage objects, leading to data inaccessibility if a user switches between the main domain and a subdomain.

Solution Using iframes and postMessage

To overcome this limitation, one approach involves using iframes and the postMessage API:

1. Create an iframe on the parent domain (parent.example.com).
2. On each child domain (child.example.com), send a postMessage to the parent iframe.
3. Define a protocol to interpret these messages, enabling communication with the parent iframe.

By using this method, all subdomains can share the same localStorage as the main domain, providing a seamless user experience regardless of the URL accessed.

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