October 23, 2017

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Node.js Async Function Best Practices

Since Node.js version 7.6, Node.js ships with a new V8 version that features async functions. As Node.js 8 becomes the active LTS version on October 31, there is no reason for not starting to adopt async functions in your codebase. In this article, I will briefly show you what async functions are, and how they change the way we write Node.js applications.

What are

async

functions?

async

functions let you write

Promise

-based code as if it were synchronous. Once you define a function using the

async

keyword, then you can use the

await

keyword within the function's body. When the

async

function is called, it returns with a

Promise

. When the

async

function returns a value, the

Promise

gets fulfilled, if the

async

function throws an error, it gets rejected.

The

await

keyword can be used to wait for a

Promise

to be resolved and returns the fulfilled value. If the value passed to the

await

keyword is not a Promise, it converts the value to a resolved

Promise

const rp = require("request-promise");
async function main() {
  const result = await rp("https://google.com");
  const twenty = await 20;
  // sleeeeeeeeping  for a second 💤
  await new Promise((resolve) => {
    setTimeout(resolve, 1000);
  });
  return result;
}
main().then(console.log).catch(console.error);

Migrating to

async

functions

If your Node.js applications are already using

Promise

s, then you only have to start

await

ing your Promises, instead of chaining them.

If your applications are built using callbacks, moving to

async

functions should be done gradually. You can start adding new features by using this new technique. If you have to use older parts of the application, you can simply wrap them into Promises.

To do so, you can use the built-in

util.promisify

method:

const util = require("util");
const { readFile } = require("fs");
const readFileAsync = util.promisify(readFile);
async function main() {
  const result = await readFileAsync(".gitignore");
  return result;
}
main().then(console.log).catch(console.error);

Best Practices for

async

functions

Using

async

functions with

express

express

supports Promises out of the box, using

async

functions with express is as simple as:

const express = require("express");
const app = express();
app.get("/", async (request, response) => {
  // awaiting Promises here
  // if you just await a single promise, you could simply return with it,
  // no need to await for it
  const result = await getContent();
  response.send(result);
});
app.listen(process.env.PORT);

Edit 1: as Keith Smith pointed out, the above example has a serious issue - if the Promise gets rejected, the

express

route handler would just hang because there was no error handling there.

To fix this issue, you should wrap your async handlers in a function that handles errors:

const awaitHandlerFactory = (middleware) => {
  return async (req, res, next) => {
    try {
      await middleware(req, res, next);
    } catch (err) {
      next(err);
    }
  };
};
// and use it this way:
app.get(
  "/",
  awaitHandlerFactory(async (request, response) => {
    const result = await getContent();
    response.send(result);
  })
);

Parallel execution

Imagine you are working on something similar, when an operation needs two inputs, one from a database, and one from an external service:

async function main() {
  const user = await Users.fetch(userId);
  const product = await Products.fetch(productId);
  await makePurchase(user, product);
}

In this case, what will happen is the following:

your code will first get the
```
user
```
resource,
then get the
```
product
```
resource,
and finally make the purchase.

As you can see, you can do the first two in parallel, as they have no dependency on each other. For this, you should use the

Promise.all

method:

async function main() {
  const [user, product] = await Promise.all([
    Users.fetch(userId),
    Products.fetch(productId),
  ]);
  await makePurchase(user, product);
}

In some cases, you only need the result of the fastest resolving Promise - in that cases, you can use the

Promise.race

method.

Error handling

Consider the following code example:

async function main() {
  await new Promise((resolve, reject) => {
    reject(new Error("💥"));
  });
}
main().then(console.log);

Once running this snippet, you will get a message in your terminal saying something similar:

(node:69738) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): Error: 💥
(node:69738) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

In the newer versions of Node.js, if Promise rejections won't be handled, it will bring down the whole Node.js process. Because of this, you should use

try-catch

blocks, when necessary:

const util = require("util");
async function main() {
  try {
    await new Promise((resolve, reject) => {
      reject(new Error("💥"));
    });
  } catch (err) {
    // handle error case
    // maybe throwing is okay depending on your use-case
  }
}
main().then(console.log).catch(console.error);

However, with

try-catch

blocks you can hide important exceptions, like system errors, which you want to rethrow. To learn more about when you should rethrow, I strongly recommend to read Eran's post, Learning to Throw Again.

Hi 👋

My name is Gergely, and this is where I write about engineering management and open-source.

Recent

For you

Node.js Async Function Best Practices

What are
async
functions?

Migrating to
async
functions

Best Practices for
async
functions

Using
async
functions with
express

Parallel execution

Error handling