INTERVIEW Question : Local Storage - Access between Tabs

Question:
"I have a website (for example, Netflix) where I have stored some data in local storage using the browser. If I open a new tab and navigate to Facebook, can I access the local storage data from Netflix in the Facebook application?"

ANSWERE:

No, you won't be able to access Netflix's local storage data from Facebook or any other website. This is due to the Same-Origin Policy (SOP) enforced by web browsers, which restricts how documents and scripts from one origin can interact with resources from another origin. The Same-Origin Policy is a critical security measure to prevent malicious activities such as cross-site scripting (XSS) and cross-site request forgery (CSRF).

Understanding the Same-Origin Policy

The Same-Origin Policy is based on the origin, which is defined by the scheme (protocol), host (domain), and port of the URL:

• Scheme: http, https, ftp, etc.
• Host: The domain name like www.netflix.com or www.facebook.com.
• Port: Usually 80 for http and 443 for https, but can be specified explicitly in the URL.

For two URLs to have the same origin, all three components must be identical.

Example:

https://www.netflix.com and https://www.facebook.com are different origins.
https://www.netflix.com and https://blog.netflix.com are different origins because the subdomains are different.
https://www.netflix.com and http://www.netflix.com are different origins because the schemes are different.

Local Storage and Same-Origin Policy

Local Storage is scoped by origin, which means that:

Data stored by https://www.netflix.com is only accessible to pages under https://www.netflix.com.
Data stored by https://www.facebook.com is only accessible to pages under https://www.facebook.com.

This prevents any website from accessing the local storage data of another website, thereby protecting user data from being exposed to unauthorized sites.

Example Code

If you store data in local storage from Netflix:

// On Netflix's site
localStorage.setItem('userPreferences', JSON.stringify({ theme: 'dark', language: 'en' }));

And then try to access it from Facebook:

// On Facebook's site
const data = localStorage.getItem('userPreferences');
console.log(data); // This will return null because Facebook can't access Netflix's local storage

Why This Is Important

Security: Prevents unauthorized access to sensitive user data.
Privacy: Ensures that websites cannot track or share data about users without consent.
Data Integrity: Keeps data compartmentalized, ensuring one website's data doesn't interfere with another's.

Conclusion

Due to the Same-Origin Policy, local storage data is isolated to the origin that created it. This means that you cannot access local storage data from Netflix while you are on Facebook, or any other website. This is a crucial security feature designed to protect user data and privacy.