What are const assertions? | TypeScript Interview Questions

What You'll Learn

as const syntax
What it does
Common use cases

as const

Makes values deeply readonly with literal types.

code.tsTypeScript

// Without as const
const colors = ["red", "green", "blue"];
// Type: string[]

// With as const
const colors = ["red", "green", "blue"] as const;
// Type: readonly ["red", "green", "blue"]

Effects

code.tsTypeScript

const config = {
  host: "localhost",
  port: 3000,
  debug: true
} as const;

// Type:
// {
//   readonly host: "localhost";
//   readonly port: 3000;
//   readonly debug: true;
// }

config.port = 8080; // Error: readonly

Common Use Cases

code.tsTypeScript

// Action types
const ACTIONS = {
  ADD: "ADD",
  REMOVE: "REMOVE"
} as const;

type Action = typeof ACTIONS[keyof typeof ACTIONS];
// "ADD" | "REMOVE"

// Tuple types
function getCoords() {
  return [10, 20] as const;
  // Type: readonly [10, 20]
}

const [x, y] = getCoords();
// x: 10, y: 20 (not number, number)