TypeScript 4.3 for JavaScript users

export class Thing {
  #size = 0;

  /** @returns {number} */
  get size() {
    return this.#size;
  }

  /** @param {string | number | boolean} value */
  set size(value) {
    const n = Number(value);
    this.#size = Number.isFinite(n) ? n : 0;
  }
}

const thing = new Thing();
thing.size = "42";
/** @type {number} */
export const size = thing.size;
thing.size = null;error TS2322: Type 'null' is not assignable to type 'string | number | boolean'.

export class Shape {
  /** @returns {number} */
  area() {
    return 0;
  }
}

export class Square extends Shape {
  side = 2;

  area() {error TS4119: This member must have a JSDoc comment with an '@override' tag because it overrides a member in the base class 'Shape'.
    return this.side ** 2;
  }
}

/**
 * A shopping cart. Add lines with {@link Cart#add}, read the total with
 * {@link cartTotal | the cartTotal function}, and see
 * {@link https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes classes on MDN}.
 */
export class Cart {
  /** @type {{ price: number, quantity: number }[]} */
  lines = [];

  /**
   * @param {number} price
   * @param {number} quantity
   */
  add(price, quantity) {
    this.lines.push({ price, quantity });
  }
}

/**
 * Same as {@linkcode Cart} lines summed. See also {@linkplain Cart}.
 * @param {Cart} cart
 */
export function cartTotal(cart) {
  return cart.lines.reduce((sum, line) => sum + line.price * line.quantity, 0);
}

/** @returns {Promise<boolean>} */
async function isAdmin() {
  return false;
}

export async function greeting() {
  const admin = isAdmin();
  if (admin) {error TS2801: This condition will always return true since this 'Promise<boolean>' is always defined.
    return "Welcome back";
  }
  return "Hello";
}

export class Ticket {
  static #issued = 0;

  /** @param {string} title */
  constructor(title) {
    Ticket.#issued += 1;
    this.label = this.#format(title);
  }

  /** @param {string} title */
  #format(title) {
    return `#${Ticket.#issued} ${title.trim()}`;
  }
}

const ticket = new Ticket("Broken link");
ticket.#format("again");error TS18013: Property '#format' is not accessible outside class 'Ticket' because it has a private identifier.