Asynchronous JS, Async & Await

So far, everything you’ve written has been synchronous — JavaScript runs your code line by line, top to bottom, waiting for each line to finish before moving to the next.

Imagine ordering food at a fast-food counter. In a synchronous world, the cashier takes your order, then stands frozen at the register until your burger is cooked — everyone behind you waits forever.

In an asynchronous world, the cashier takes your order, gives you an order number, and immediately takes the next customer’s order while the kitchen prepares your food in the background. When it’s ready, they call your number.

JavaScript works the same way — it can start a long task (like fetching data from an API) and keep running the rest of your code without freezing the page.

async and await

The async keyword

Placing async in front of a function means the function will always return a Promise.

async function getGreeting() {
  return "Hello, Students!";
}

getGreeting().then((msg) => console.log(msg)); // "Hello, Students!"

The await keyword

await pauses execution inside an async function until a Promise finishes — without freezing the rest of your code.

async function fetchUserData() {
  await new Promise((resolve) => setTimeout(resolve, 2000));
  return { username: "ojasvi" };
}

async function displayUser() {
  console.log("Fetching user data..."); // prints immediately

  const user = await fetchUserData(); // waits ~2 seconds here

  console.log("User received:", user.username);
}

displayUser();

console.log("This logs immediately, because async JS doesn't block!");

Handling errors with try...catch

Asynchronous tasks can fail (bad network, server down). Handle errors with try...catch.

async function getWeatherData() {
  try {
    const response = await fetch("https://api.weather.com/invalid-endpoint");
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error("Oops! Something went wrong:", error);
  }
}

getWeatherData();

Try it yourself

1. Given a function checkInventory() that returns a Promise, write an async function called orderItem() that uses await to wait for it and logs "Item ordered successfully!".
2. Wrap orderItem()’s logic in a try...catch so that if the inventory check fails, it logs "Out of stock!" instead of crashing.