Exhaustive switches & never checks

type Circle = { kind: "circle"; radius: number }
type Square = { kind: "square"; size: number }
type Shape = Circle | Square

function area(s: Shape): number {
  switch (s.kind) {
    case "circle":
      return Math.PI * s.radius * s.radius
    case "square":
      return s.size * s.size
  }
}

type Triangle = { kind: "triangle"; base: number; height: number }
// If we extend Shape but forget to add case, we want a compile-time error
// type Shape = Circle | Square | Triangle

function assertNever(x: never): never {
  throw new Error(`Unhandled case: ${String(x)}`)
}

function areaSafe(s: Shape): number {
  switch (s.kind) {
    case "circle":
      return Math.PI * s.radius * s.radius
    case "square":
      return s.size * s.size
    default:
      return assertNever(s) // if Shape grows, compiler errors here
  }
}

type Status = "idle" | "loading" | "success" | "error"

function toIcon(s: Status) {
  switch (s) {
    case "idle": return "⏸"
    case "loading": return "⏳"
    case "success": return "✅"
    case "error": return "⚠️"
    default: return assertNever(s)
  }
}

// Alternative: const check
const _check: never = ((): Status => "idle")() as never // force compile-time check with care

// Tips
// - Enable noFallthroughCasesInSwitch in tsconfig for safer switches
// - Prefer discriminated unions over boolean flags
// - Keep default branch only to call assertNever; avoid swallowing cases