as const — Enum-like String Unions

TypeScript's as const turns an object into a fully immutable, literal-typed structure — the idiomatic alternative to TypeScript enums:

// Without as const — values are widened to string:
const Direction = { Up: 'UP', Down: 'DOWN' };
// Direction.Up is inferred as string — too wide!

// With as const — values are literal types:
const Direction = { Up: 'UP', Down: 'DOWN' } as const;
// Direction.Up is 'UP' — exactly right ✅

// Derive the union type automatically:
type Direction = typeof Direction[keyof typeof Direction];
// → 'UP' | 'DOWN'

Why prefer this over TypeScript enums?

• No runtime overhead — just a plain object
• No double declaration — one source of truth
• Works with union type utilities — Exclude<Direction, 'UP'>
• No numeric enum footguns — values are exactly what you write

Implement

Two predefined const objects:

• Direction: { Up: 'UP', Down: 'DOWN', Left: 'LEFT', Right: 'RIGHT' }
• HttpStatus: { Ok: 200, NotFound: 404, Unauthorized: 401, Error: 500 }

Implement isDirection(x): x is Direction and isHttpStatus(x): x is HttpStatus using Object.values().