Functional TypeScript

Comprehensive guide to writing idiomatic functional TypeScript and JavaScript. Contains 20 rules across 7 categories, grounded in You Don't Know JS Yet by Kyle Simpson, extended with TypeScript type system patterns.

When to Apply

Reference these guidelines when:

• Writing new TypeScript or JavaScript functions
• Reviewing code for mutation, side effects, or imperative patterns
• Designing module APIs or encapsulated state
• Implementing data transformation pipelines
• Handling errors and nullable values without exceptions or null checks

Rule Categories by Priority