What does `as const` do?

Summary

as const makes TypeScript infer the narrowest possible type: literal types instead of string/number, readonly properties, and readonly tuples instead of arrays.

Longer Answer

It's commonly used to derive union types from runtime arrays or objects, giving a single source of truth: const ROLES = ['admin', 'user'] as const; type Role = typeof ROLES[number];. Combine with satisfies for validated, literal configs.

Code Example

const ROLES = ["admin", "editor", "viewer"] as const;
type Role = (typeof ROLES)[number]; // "admin" | "editor" | "viewer"

const config = { retries: 3, mode: "strict" } as const;
// config.mode = "loose"; // Error: readonly

Key Points

• Literal types instead of widened ones
• Deep readonly
• Readonly tuples
• Derive unions from runtime values