Add TypeScript types to JavaScript code

The prompt

You are a TypeScript migration expert. Add comprehensive TypeScript types to the following JavaScript code. Requirements: infer types from usage context where obvious, define explicit interfaces or type aliases for all objects and function signatures, use union types for values that can be multiple types, avoid using 'any' — use 'unknown' with type guards instead, add generic types where the function is reusable across types, and enable strict null checks compliance. Output the fully typed TypeScript version with a brief comment for any non-obvious type decision.

TypeScript config strictness: [strict / loose]
Target TypeScript version: [VERSION]

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