function startGame() {
  // Starting a new game.
  let playerName: string = 'Audrey';
  logPlayer(playerName);

  const messagesElement: HTMLElement | null = document.getElementById('messages');
  // We know messagesElement cannot be null because we control the markup, so
  // we can use a non-null type assertion
  const element = messagesElement!;
  element.innerText = 'Welcome to MultiMath! Starting a new game';
  console.log('Starting new game');
}

function logPlayer(name: string) {
  console.log(`New game starting for player: ${name}.`);
}

function arm(doc: HTMLDocument) {
  // We know getElementById() cannot be null because we control the markup, so
  // we can use a non-null type assertion on the function call result.
  doc.getElementById('startGame')!.addEventListener('click', startGame);
}

arm(document);

// Type assertions (not casting, more like in Go).
let value: any = 42;
let fixedString1: string = (<number>value).toFixed(2);
let fixedString2: string = (value as number).toFixed(2);
console.log(value, fixedString1, fixedString2);

/**
 * y is now optional, while x is not.
 */
function foo(x: number, y?: string): string {
  return x.constructor.toString() + y;
}

foo(2, "3");
foo(2);

function bar(...args: number[]) {
  console.log(args.reduce((accu: number, curr: number) => accu + curr, 0));
}

bar(1, 2, 3, 4, 5); // (5+6)/2 = 15

// Rejected because of noImplicitAny.
// function qux(x, y) { return x + y; }
// qux('a', 'b');