JavaScript에서 비동기 함수 완료를 어떻게 보장할 수 있나요?

<code class="javascript">function firstFunction(_callback) {
  // Perform asynchronous work here...
  _callback();
}

function secondFunction() {
  firstFunction(() => {
    console.log('First function completed');
  });
}</code>

<code class="javascript">firstFunction(() => console.log('First function completed'));</code>

<code class="javascript">const firstFunctionPromise = () => {
  // Return a Promise object...
};

secondFunction()
  .then(result => {
    console.log('First function completed using Promise');
  })
  .catch(error => {
    console.log('Error occurred: ' + error);
  });</code>

<code class="javascript">const secondFunction = async () => {
  const result = await firstFunction();

  console.log('First function completed using async/await');
};</code>