Answer by Philipp Claßen for How do I return the response from an asynchronous call?

Originally, callbacks were used for asynchronous operations (e.g., in the XMLHttpRequest API). Now promise-based APIs like the browser's Fetch API have become the default solution and the nicer async/await syntax is supported by all modern browsers and on Node.js (server side).

A common scenario - fetching JSON data from the server - can look like this:

async function fetchResource(url) {  const res = await fetch(url);  if (!res.ok) {    throw new Error(res.statusText);  }  return res.json();}

To use it in another function:

async function doSomething() {  try {    const data = await fetchResource("https://example.test/resource/1");    // ...  } catch (e) {    // Handle error    ...  }}

If you design a modern API, it is strongly recommended to prefer promise-based style over callbacks. If you inherited an API that relies on callbacks, it is possible to wrap it as a promise:

function sleep(timeout) {  return new Promise((resolve) => {    setTimeout(() => {      resolve();    }, timeout);  });}async function fetchAfterTwoSeconds(url) {  await sleep(2000);  return fetchResource(url);}

In Node.js, which historically relied exclusively on callbacks, that technique is so common that they added a helper function called util.promisify.