Const assertions

TypeScript · Reference cheat sheet

📋 Overview

as const asserts a value is deeply readonly with the narrowest literal types. Use it for config objects, route tables, and tuple literals so keyof / indexed access stay precise. Pairs well with satisfies.

🔧 Core concepts

• Literals — "ok" as const → type "ok" (not string).
• Arrays → readonly tuples — [1, "a"] as const → readonly [1, "a"].
• Objects — all props readonly; nested values narrowed.
• Enum alternative — as const object + (typeof O)[keyof typeof O].
• Mutable copy — spread or typed mutable alias when mutation is required.

💡 Examples

const status = "success" as const; // "success"

const point = [10, 20] as const;
// readonly [10, 20]
point[0]; // 10

const config = {
  host: "localhost",
  port: 5432,
  features: ["auth", "cache"],
} as const;

type Config = typeof config;
type Feature = (typeof config.features)[number]; // "auth" | "cache"

const Routes = {
  home: "/",
  user: "/users/:id",
} as const;
type RouteKey = keyof typeof Routes;
type RoutePath = (typeof Routes)[RouteKey];

// With satisfies — narrow + validate
type Theme = { primary: string; radius: number };
const theme = {
  primary: "#336699",
  radius: 8,
} as const satisfies Theme;

// Function args
function move(dir: "up" | "down") {}
const dirs = ["up", "down"] as const;
move(dirs[0]);

⚠️ Pitfalls

• as const makes structures readonly — assigning to props fails.
• Widening returns if you annotate : string[] instead of using as const.
• as const on mutable class fields doesn’t freeze at runtime — only types.
• Large const objects can slow the checker — split modules if needed.
• Runtime still mutable unless you also Object.freeze (shallow).

🔗 Related

• satisfies.md
• literal_types.md
• readonly.md
• keyof_typeof.md
• tuple_types.md
• enums.md