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 Promises, then you only have to start awaiting 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

As 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.