Three TypeScript Patterns for Building Rule-Dependent Date Tools in Frontend

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A developer on DEV Community has outlined three TypeScript design patterns for building frontend date tools that depend on changing policy rules. The first pattern replaces boolean eligibility flags with discriminated union types, so incomplete inputs cannot silently resolve to false negatives. The second introduces versioned rule objects with effective dates and source metadata, ensuring old results remain traceable when rules are updated. The third pattern bundles evaluation output with route details, rule IDs, input warnings, and source links, making the reasoning behind any date result visible in the UI. The author notes these patterns apply broadly to tax-residency calculators, insurance waiting periods, and any frontend where business rules evolve over time.

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