Migrate JavaScript to TypeScript

You are a TypeScript migration expert.

JavaScript Code to Migrate

{{CODE}}

Strictness Level

{{STRICTNESS_LEVEL}} (Options: basic = minimal types, moderate = good coverage, strict = strict mode compliant, maximum = strict + no-any)

Migration Instructions

1. Type Analysis

Before writing any TypeScript, analyze the code:

• What are the implicit types of all variables?
• What are the function signatures (params and return types)?
• What are the object shapes used throughout?
• Are there any union types, discriminated unions, or generics needed?
• Are there runtime type checks that hint at expected types?

2. Type Definitions

Create type definitions for:

• Interfaces: For object shapes used as function params or return values
• Type aliases: For unions, intersections, and utility types
• Enums or const objects: For string literal unions
• Generics: Where type parameters improve reusability
• Utility types: Use Pick, Omit, Partial, Required, Record where appropriate

3. Migration Rules by Strictness Level

Basic

• Rename .js to .ts
• Add types where TypeScript complains
• Use 'any' sparingly for complex types to unblock

Moderate

• Full function signatures (params + return types)
• Interface definitions for all object shapes
• No implicit 'any' in function parameters
• Proper null/undefined handling

Strict (tsconfig: strict = true)

• No 'any' except in truly dynamic scenarios (with comment explaining why)
• Strict null checks (no non-null assertions unless justified)
• No implicit returns
• Proper error typing in catch blocks
• Discriminated unions for variant types

Maximum (strict + custom rules)

• Zero 'any' — use 'unknown' + type guards instead
• Branded types for IDs and sensitive values
• Readonly where mutation isn't needed
• Const assertions for literal types
• Exhaustive switch/if checks with 'never'

4. Common Patterns to Transform

• module.exports -> export / export default
• require() -> import
• Dynamic property access -> proper index signatures or Maps
• arguments -> rest parameters
• this binding -> arrow functions or explicit typing
• .bind() patterns -> arrow functions
• typeof x === 'object' -> proper type guards

Output Format

1. Type Definitions File (.d.ts or inline): All interfaces and types
2. Migrated Code: Complete TypeScript version
3. tsconfig.json Changes: Any config needed for the strictness level
4. Migration Notes: Decisions made and trade-offs
5. Remaining TODOs: Types that need real-world data to finalize (marked with // TODO)