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The Ultimate Guide for Error handling with async/await
2018-12-10
With async/await handling errors got as simple as possible
writing
learning
errors
async
await
promises
try-catch

Many developers getting familiar with async/await these days, as this feature is getting even more stable and performant in Node.js, and it's recommended to use them instead of Promises, or callbacks.

But I've seen the question appear from several sources: "Okay okay, we got async/await, but how we handle errors?" This post aims to be an ultimate guide for those, who seek the answer for this. Don't worry, it won't be long, or complex!

Let it throw

{% image_tag "pull-left" "let-it-throw.gif" "Elsa handling async errors properly" %}

Yup, that's it!

For illustration, we could use this small scene from "Frozen".

Just let your async operations throw an error, these errors will travel up the async call stack until a point where you actually care about them.

You can handle these errors there, log them, and let the user know about the error in some way.

Explanation

In JavaScript a really clean way to code is to stick to this simple rule: Operations (functions, methods, etc) should finish their work by either

  • return a value (also, undefined or void), or
  • throw an error

Think about it: if a function returns an error, what would that mean? The result of a process is an error? Can we store it to a database, pass it along to the user? Sure, we can type check for errors all the time, but that sounds tedious, and thankfully the somewhat-similar error-first approach is slowly behind us.

Your code should be clean and never hide its intent. Looking at it should be as simple to comprehend as possible so the next developer after you, or yourself after a year (or week, or only days) can read, understand, debug or improve, or just add new functionality as easy as possible.

Async/await helps you to achieve this. The code can be simple, show clear intent. The errors, well they just happen, but now you can care about them elsewhere.

This is because thrown errors in JS travel up the stack. A function throws an error, the calling context receives it, and if not handled (caught in a try/catch block, or .catch() by Promises) it will be thrown again upwards, to the next context. This happens over and over again until it gets caught or we reach the end of the call stack. This s the case when the error or exception is not handled, and cause an additional Unhandled Exception error, and our program terminates.

To handle errors like this, coming from async functions, you can use try/catch blocks, or if you treat your async functions as little Promise factories (which I would not advise), you can use their .catch() methods.

"But isn't try/catch slow? And I've heard even V8 discourages using them!"

That was the case back in the days before V8 v5.9 or v6 - meaning before Node.js 8.3.0 or Chrome 59 and the August of 2017 - that's when the new compiler of V8 was released to production!

V8's old interpreter and compiler called Crankshaft was performing really poor on these fields, but then, to prepare for the modern JS, ES2015 and further releases of the language, Crankshaft was replaced by Ignition and TurboFan, which were designed and prepared for tasks like these.

{% blockquote 'Benedikt Meurer @bmeurer' 'https://v8.dev/blog/high-performance-es2015' %} Fortunately, Ignition and TurboFan (V8’s new interpreter and compiler pipeline), were designed to support the entire JavaScript language from the beginning, including advanced control flow, exception handling, and most recently for-of and destructuring from ES2015. The tight integration of the architecture of Ignition and TurboFan make it possible to quickly add new features and to optimize them fast and incrementally. {% endblockquote %}

Example time!

I've prepared two small examples using popular web frameworks: Express and Hapi

Express

Express has a nice default error handler middleware prepared to catch any errors in route handlers. So that will be the point, where you care about errors, and show them to your users.

Unfortunately, Express does not handle errors thrown in async route handlers, you need to use their next() function argument in your route handler. To keep code clean, you can implement a small wrapper around your route handlers, that will catch the errors, and pass them to the next callback - like in this great example by Gergely Nemeth

Hapi

Hapi 17 was rewritten to utilize async/await everywhere, so you can throw your exceptions, or even better your Boom errors at Hapi, and it will handle them nicely, and resolve the requests accordingly, no need for special middlewares, unless you have some use case for that.

Conclusion

Embrace async/await, it's simple to use, makes your code more readable, and you can handle your errors may be easier than before. If you still have concerns, check out my examples in this git repo.

Remember, just let it throw :)