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TypeScript Narrowing Errors: unknown, Unions, Readonly, Excess Properties, Index Access and never Exhaustiveness

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Self-Hosting & Infra intermediate 7 min read $0.01 Updated 2026-10-20

Compiled with TypeScript in strict mode and noUncheckedIndexedAccess: real error codes and messages for each pattern, plus the corrected code using discriminated unions, typeof guards, optional chaining and a never exhaustiveness helper.

Fix the TypeScript errors that come from strict mode: TS18046 on unknown, TS2339 on unions, TS2540 on as const, TS2353 excess properties, TS18048 from noUncheckedIndexedAccess, and exhaustive switch checks with never.

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#typescript#types#narrowing#strict#errors

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