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Your Deploy Changed the App. localStorage Kept the Old Schema.

Ferid Brković

4 min read11 hours ago

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Handle yesterday’s saved preferences without silently resetting them — or overwriting a format you don’t understand.

The new theme picker works on a clean browser. Returning users open the same release and lose their dark-mode preference.

The old app saved { "version": 1, "darkMode": true } . The new app expects { "version": 2, "theme": "dark" } .

Both objects are valid JSON. Only one matches the new code.

This is a small compatibility problem hiding inside a frontend feature. Your deploy replaced the application, but it did not replace the data already stored in someone’s browser. A successful release needs an answer for that older record.

Start with a format you can identify

localStorage stores strings for an origin and retains data across browser sessions. It does not assign a schema to your application’s values.…