👨‍💼 Discriminated unions (also called "tagged unions" or "algebraic data types") are a pattern where each type in a union has a common "discriminant" property with a literal type value.

type Circle = { kind: 'circle'; radius: number }
type Rectangle = { kind: 'rectangle'; width: number; height: number }
type Shape = Circle | Rectangle

The kind property lets TypeScript narrow the type automatically. This is how you make invalid states unrepresentable—the type system ensures you can only create valid combinations.

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📜 TypeScript Handbook - Discriminated Unions