Literal Preservation with as const

Literal Preservation with as const: working example

Run this standalone example after compiling with TypeScript 5.4+ in strict mode, targeting ES2022. Expected result: The program prints State: running; the commented operations are rejected.

const states = ["idle", "running", "done"] as const;
type State = typeof states[number];
function report(state: State): string { return `State: ${state}`; }
console.log(report(states[1]));
// states.push("failed"); // Type error: readonly tuple.
// report("missing"); // Type error: not an allowed literal.

1. Expected result: The program prints State: running; the commented operations are rejected.
2. The tuple preserves each element’s literal type.
3. Indexing its type with number derives the allowed state union from one source of truth.