W3Schools Tryit Editor

"use strict";
// @ts-check
// Type Inference: Const Assertions (JS demo with JSDoc to simulate const behavior)

// Regular type inference (widens to string)
let personName = "Alice"; // type: string
console.log(typeof personName);

// "Const assertion" idea (narrows to a literal). In JS we simulate by treating it as a constant value.
/** @type {"Alice"} */
const nameConst = "Alice";
console.log(nameConst);

// With objects: simulate TS `as const` using JSDoc literal types and Object.freeze
/**
 * @typedef {{ id: 1; name: "Alice"; roles: readonly ["admin", "user"] }} UserConst
 */

/** @type {UserConst} */
const user = /** @type {any} */ ({
  id: 1,
  name: "Alice",
  roles: ["admin", "user"]
});

// Freeze to enforce runtime immutability
Object.freeze(user);
Object.freeze(user.roles);

console.log(user);

// Attempting to modify readonly properties will throw at runtime; we print friendly messages
try {
  user.name = "Bob";
  console.log("Modified name:", user.name);
} catch (e) {
  console.log("Error: Cannot modify readonly property 'name'");
}

try {
  user.roles.push("guest");
  console.log("Modified roles:", user.roles);
} catch (e) {
  console.log("Error: Cannot modify readonly roles");
}

string
Alice
{ id: 1, name: 'Alice', roles: [ 'admin', 'user' ] }
Error: Cannot modify readonly property 'name'
Error: Cannot modify readonly roles