15 — Discriminated Unions & Exhaustive Checks

Core Concepts

• Discriminated unions — union of objects sharing a literal discriminant field
• Switch narrowing — switch(shape.kind) narrows to specific variant
• Exhaustive checks — never type ensures all cases handled
• Pattern matching — type-safe branching on union members
• State machines — model application states with discriminated unions

Why Discriminated Unions

• The single most important pattern in TypeScript
• Models data that can be "one of several shapes"
• Compiler ensures you handle every case
• Eliminates impossible states by design