js__typescript__pluralsight1/app/app.ts

function startGame() {
  // Starting a new game.
  let playerName: string | undefined = getInputValue('playername');
  logPlayer(playerName);

  postScore(100, playerName);
}

function getInputValue(elementId: string): string | undefined {
  // Type assertions applies to the whole document.get...., not to document.
  // We know that we're only going to pass valid IDs, so will never get a null,
  // so we can assert the result will always be an input element.
  const inputElement: HTMLInputElement = <HTMLInputElement>document.getElementById(elementId);
  if (inputElement.value === '') {
    return undefined;
  }
  else {
    return inputElement.value;
  }
}

function postScore(score: number, playerName: string = 'MultiMath player'): void {
  let logger: (value: string) => void;

  if (score < 0) {
    logger = logError;
  }
  else {
    logger = logMessage;
  }

  let scoreElement: HTMLElement | null = document.getElementById('postedScores');
  scoreElement!.innerText = `${score} - ${playerName}`;
  logger(`Score: ${score}`);
}

function logPlayer(name: string = 'MultiMath player'): void {
  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);

const logMessage = (message: string) => console.log(message);

function logError(message: string): void {
  console.error(message);
}