Async/Await: Introduction

Callbacks and promises are great when it comes to performing async operations. Promises improved over callbacks and provide a flat code syntax, especially when it comes to chaining promises. The operators on promises like allSettled, any, then and catch make it easier to write complex async operations.

Async/Await was introduced in ES7 to promote a cleaner syntax to promises. Under the hood, async/await are promises; they provide a nice abstraction layer under those keywords.

Async

async keyword could be used in front of any function (declaration, expressions, callbacks or literally anywhere). All it means is that the function will always return a promise. Any return values other than a promise will be wrapped in a resolved promise.

async function foo() {
    return "Parwinder" // returning a string but `async` will ensure it is wrapped in a promise
}

foo().then((data) => { // we can safely use then because async function foo returns a promise
    console.log(data); // Parwinder
})

We could return a promise in function foo, and it will still work. It will be unnecessary, though.

async function foo() {
    return Promise.resolve("Parwinder")
}

foo().then((data) => {
    console.log(data); // Parwinder
})

Await

await keyword makes JavaScript wait until the promise settles and returns its result. It can only be used inside an async function.

async function foo() {
    const myPromise = new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve("Parwinder"); // resolves with "Parwinder" after 2 seconds
        }, 2000);
    });

    // will not move to the next line until myPromise resolves/rejects
    const name = await myPromise;
    // the execution pauses (or awaits) for the promise

    console.log(name); // Parwinder
}

foo();

As you can see in the above example await provides a cleaner syntax compared to Promise.then.