async/await

async and await are two JavaScript keywords that make it possible to:

1. initiate asynchronous requests (such as HTTP requests with fetch) prefixed with await
2. go back into the JavaScript event loop
3. wait for and process other asynchronous events in the order that they occur
4. and resume execution of the line after await as soon as the previous awaited request finishes (with either resolve or reject)

Let's take a look at an example. Define an async function that calls other async functions and awaits on them before proceeding.

async function getData() { // any function that uses `await` must be declared with `async` prefix
  const url = "https://someurl.com/data.json";
  try {
    const response = await fetch(url); // make async request that takes time
    /*
     * this is where control disappears into the event loop
     * and resumes below only after request above completes with `resolve`
     */
    if(response.ok) { // ok is true only for successful HTTP status codes (such as 200)
      const data = await response.json(); // another async request
      return data; // resumes only after all data from response has been collected and parsed as JSON
    } else {
      return Promise.reject(response);
    }
  } catch(e) { // resume here only after request above completes with `reject`
    // error handling
    return Promise.reject(e);
  }
}

Now call the function above (using Promise API)

getData().then(console.log).catch(console.error).finally(() => console.log("DONE"));

Or a nicer way to do the same using async/await, and IIFE

(async function() {
  try {
    const result = await getData();
    console.log(result);
  } catch(e) {
    console.error(e);
  } finally {
    console.log("DONE");
  }
})();

If something doesn't make sense above, then see the Promise API module also, along with the references.

References:

1. MDN: Fetch API