Distinguish local progress convenience from real purchase entitlement
The practical problem
Browser localStorage is writable by the user and may be cleared automatically in private browsing. It is suitable for game progress but not for charging customers.
Implementation protocol
Validate persisted objects, clamp unlocked stages to documented bounds and catch storage exceptions. Keep paid downloads behind independent server-side authorization.
Worked scenario
If a stored level changes from 8 to 999999, the game must clamp or reset safely. Editing a flag must not unlock a paid file.
What could go wrong
A browser-side 'paid=true' value is never reliable proof of payment.
Acceptance criteria
Test spoofed storage, quota failures and browser data clearing.
Source material and evidence boundaries
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