Valentino Gagliardi

Posted on Sep 14, 2020 • Originally published at valentinog.com

A mostly complete guide to error handling in JavaScript

#beginners #javascript

What is an error in programming?

Things don't go always well in our programs.

In particular, there are situations where we may want to stop the program or inform the user if something bad happens.

For example:

the program tried to open a non-existent file.
the network connection is broken.
the user entered invalid input.

In all these cases we as programmers, create errors, or we let the programming engine create some for us.

After creating the error we can inform the user with a message, or we can halt the execution altogether.

What is an error in JavaScript?

An error in JavaScript is an object, which is later thrown to halt the program.

To create a new error in JavaScript we call the appropriate constructor function. For example, to create a new, generic error we can do:

const err = new Error("Something bad happened!");

When creating an error object it's also possible to omit the new keyword:

const err = Error("Something bad happened!");

Once created, the error object presents three properties:

message: a string with the error message.
name: the error's type.
stack: a stack trace of functions execution.

For example, if we create a new TypeError object with the appropriate message, the message will carry the actual error string, while name will be "TypeError":

const wrongType = TypeError("Wrong type given, expected number");

wrongType.message; // "Wrong type given, expected number"
wrongType.name; // "TypeError"

Firefox also implements a bunch on non-standard property like columnNumber, filename, and lineNumber.

Many types of errors in JavaScript

There are many types of errors in JavaScript, namely:

Error
EvalError
InternalError
RangeError
ReferenceError
SyntaxError
TypeError
URIError

Remember, all these error types are actual constructor functions meant to return a new error object.

In your code you'll mostly use Error and TypeError, two of the most common types, to create your own error object.

Most of the times, the majority of errors will come directly from the JavaScript engine, like InternalError or SyntaxError.

An example of TypeError occurs when you try to reassign const:

const name = "Jules";
name = "Caty";

// TypeError: Assignment to constant variable.

An example of SyntaxError is when you misspell language keywords:

va x = '33';
// SyntaxError: Unexpected identifier

Or when you use reserved keywords in wrong places, like await outside of an async function:

function wrong(){
    await 99;
}

wrong();

// SyntaxError: await is only valid in async function

Another example of TypeError occurs when we select non-existent HTML elements in the page:

Uncaught TypeError: button is null

In addition to these traditional error objects, an AggregateError object is going to land soon in JavaScript. AggregateError is convenient for wrapping multiple errors together, as we'll see later.

Besides these built-in errors, in the browser we can find also:

DOMException.
DOMError, deprecated and no longer used today.

DOMException is a family of errors related to Web APIs. They are thrown when we do silly things in the browser, like:

document.body.appendChild(document.cloneNode(true));

The result:

Uncaught DOMException: Node.appendChild: May not add a Document as a child

For a complete list see this page on MDN.

What is an exception?

Most developers think that error and exceptions are the same thing. In reality, an error object becomes an exception only when it's thrown.

To throw an exception in JavaScript we use throw, followed by the error object:

const wrongType = TypeError("Wrong type given, expected number");

throw wrongType;

The short form is more common, in most code bases you'll find:

throw TypeError("Wrong type given, expected number");

throw new TypeError("Wrong type given, expected number");

It's unlikely to throw exceptions outside of a function or a conditional block. Instead, consider the following example:

function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

Here we check if the function argument is a string. If it's not we throw an exception.

Technically, you could throw anything in JavaScript, not only error objects:

throw Symbol();
throw 33;
throw "Error!";
throw null;

However, it's better to avoid these things: always throw proper error objects, not primitives.

By doing so you keep error handling consistent through the codebase. Other team members can always expect to access error.message or error.stack on the error object.

What happens when we throw an exception?

Exceptions are like an elevator going up: once you throw one, it bubbles up in the program stack, unless it is caught somewhere.

Consider the following code:

function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

toUppercase(4);

If you run this code in a browser or in Node.js, the program stops and reports the error:

Uncaught TypeError: Wrong type given, expected a string
    toUppercase http://localhost:5000/index.js:3
    <anonymous> http://localhost:5000/index.js:9

In addition, you can see the exact line where the error happened.

This report is a stack trace, and it's helpful for tracking down problems in your code.

The stack trace goes from bottom to top. So here:

    toUppercase http://localhost:5000/index.js:3
    <anonymous> http://localhost:5000/index.js:9

We can say:

something in the program at line 9 called toUppercase
toUppercase blew up at line 3

In addition to seeing this stack trace in the browser's console, you can access it on the stack property of the error object.

If the exception is uncaught, that is, nothing is done by the programmer to catch it, the program will crash.

When, and where you catch an exception in your code depends on the specific use case.

For example you may want to propagate an exception up in the stack to crash the program altogether. This could happen for fatal errors, when it's safer to stop the program rather than working with invalid data.

Having introduced the basics let's now turn our attention to error and exception handling in both synchronous and asynchronous JavaScript code.

Synchronous error handling

Synchronous code is most of the times straightforward, and so its error handling.

Error handling for regular functions

Synchronous code is executed in the same order in which is written. Let's take again the previous example:

function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

toUppercase(4);

Here the engine calls and executes toUppercase. All happens synchronously. To catch an exception originating by such synchronous function we can use try/catch/finally:

try {
  toUppercase(4);
} catch (error) {
  console.error(error.message);
  // or log remotely
} finally {
  // clean up
}

Usually, try deals with the happy path, or with the function call that could potentially throw.

catch instead, captures the actual exception. It receives the error object, which we can inspect (and send remotely to some logger in production).

The finally statement on the other hand runs regardless of the function's outcome: whether it failed or succeeded, any code inside finally will run.

Remember: try/catch/finally is a synchronous construct: it has now way to catch exceptions coming from asynchronous code.

Error handling for generator functions

A generator function in JavaScript is a special type of function.

It can be paused and resumed at will, other than providing a bi-directional communication channel between its inner scope and the consumer.

To create a generator function we put a star * after the function keyword:

function* generate() {
//
}

Once inside the function we can use yield to return values:

function* generate() {
  yield 33;
  yield 99;
}

The return value from a generator function is an iterator object. To pull values out a generator we can use two approaches:

calling next() on the iterator object.
iteration with for...of.

If we take our example, to get values from the generator we can do:

function* generate() {
  yield 33;
  yield 99;
}

const go = generate();

Here go becomes our iterator object when we call the generator function.

From now on we can call go.next() to advance the execution:

function* generate() {
  yield 33;
  yield 99;
}

const go = generate();

const firstStep = go.next().value; // 33
const secondStep = go.next().value; // 99

Generators also work the other way around: they can accept values and exceptions back from the caller.

In addition to next(), iterator objects returned from generators have a throw() method.

With this method we can halt the program by injecting an exception into the generator:

function* generate() {
  yield 33;
  yield 99;
}

const go = generate();

const firstStep = go.next().value; // 33

go.throw(Error("Tired of iterating!"));

const secondStep = go.next().value; // never reached

To catch such error you would wrap your code inside the generator with try/catch (and finally if needed):

function* generate() {
  try {
    yield 33;
    yield 99;
  } catch (error) {
    console.error(error.message);
  }
}

Generator functions can also throw exceptions to the outside. The mechanism for catching these exceptions is the same for catching synchronous exceptions: try/catch/finally.

Here's an example of a generator function consumed from the outside with for...of:

function* generate() {
  yield 33;
  yield 99;
  throw Error("Tired of iterating!");
}

try {
  for (const value of generate()) {
    console.log(value);
  }
} catch (error) {
  console.error(error.message);
}

/* Output:
33
99
Tired of iterating!
*/

Here we iterate the happy path inside a try block. If any exceptions occurs we stop it with catch.

Asynchronous error handling

JavaScript is synchronous by nature, being a single-threaded language.

Host environments like browsers engines augment JavaScript with a number of Web API for interacting with external systems, and for dealing with I/O bound operations.

Examples of asynchronicity in the browser are timeouts, events, Promise.

Error handling in the asynchronous world is distinct from its synchronous counterpart.

Let's see some examples.

Error handling for timers

In the beginning of your explorations with JavaScript, after learning about try/catch/finally, you might be tempted to put it around any block of code.

Consider the following snippet:

function failAfterOneSecond() {
  setTimeout(() => {
    throw Error("Something went wrong!");
  }, 1000);
}

This function throws after roughly 1 second. What's the right way to handle this exception?

The following example does not work:

function failAfterOneSecond() {
  setTimeout(() => {
    throw Error("Something went wrong!");
  }, 1000);
}

try {
  failAfterOneSecond();
} catch (error) {
  console.error(error.message);
}

As we said, try/catch is synchronous. On the other hand we have setTimeout, a browser API for timers.

By the time the callback passed to setTimeout runs, our try/catch is long gone. The program will crash because we failed to capture the exception.

They travel on two different tracks:

Track A: --> try/catch
Track B: --> setTimeout --> callback --> throw

If we don't want to crash the program, to handle the error correctly we must move try/catch inside the callback for setTimeout.

But, this approach doesn't make much sense most of the times. As we'll see later, asynchronous error handling with Promises provides a better ergonomic.

Error handling for events

HTML nodes in the Document Object Model are connected to EventTarget, the common ancestor for any event emitter in the browser.

That means we can listen for events on any HTML element in the page.

(Node.js will support EventTarget in a future release).

The error handling mechanics for DOM events follows the same scheme of any asynchronous Web API.

Consider the following example:

const button = document.querySelector("button");

button.addEventListener("click", function() {
  throw Error("Can't touch this button!");
});

Here we throw an exception as soon as the button is clicked. How do we catch it? This pattern does not work, and won't prevent the program from crashing:

const button = document.querySelector("button");

try {
  button.addEventListener("click", function() {
    throw Error("Can't touch this button!");
  });
} catch (error) {
  console.error(error.message);
}

As with the previous example with setTimeout, any callback passed to addEventListener is executed asynchronously:

Track A: --> try/catch
Track B: --> addEventListener --> callback --> throw

If we don't want to crash the program, to handle the error correctly we must move try/catch inside the callback for addEventListener.

But again, there's little of no value in doing this.

As with setTimeout, exception thrown by an asynchronous code path are un-catchable from the outside, and will crash your program.

In the next sections we'll see how Promises and async/await can ease error handling for asynchronous code.

How about onerror?

HTML elements have a number of event handlers like onclick, onmouseenter, onchange to name a few.

There is also onerror, but it has nothing to do with throw and friends.

The onerror event handler fires any time an HTML element like an <img> tag or a <script> hits a non-existent resource.

Consider the following example:

// omitted
<body>
<img src="nowhere-to-be-found.png" alt="So empty!">
</body>
// omitted

When visiting an HTML document with a missing or non-existent resource the browser's console records the error:

GET http://localhost:5000/nowhere-to-be-found.png
[HTTP/1.1 404 Not Found 3ms]

In our JavaScript we have the chance to "catch" this error with the appropriate event handler:

const image = document.querySelector("img");

image.onerror = function(event) {
  console.log(event);
};

Or better:

const image = document.querySelector("img");

image.addEventListener("error", function(event) {
  console.log(event);
});

This pattern is useful for loading alternate resources in place of missing images or scripts.

But remember: onerror, has nothing to do with throw or try/catch.

Error handling with Promise

To illustrate error handling with Promise we'll "promisify" one of our original examples. We tweak the following function:

function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

toUppercase(4);

Instead of returning a simple string, or an exception, we use respectively Promise.reject and Promise.resolve to handle error and success:

function toUppercase(string) {
  if (typeof string !== "string") {
    return Promise.reject(TypeError("Wrong type given, expected a string"));
  }

  const result = string.toUpperCase();

  return Promise.resolve(result);
}

(Technically there's nothing asynchronous in this code, but it serves well to illustrate the point).

Now that the function is "promisified" we can attach then for consuming the result, and catch for handling the rejected Promise:

toUppercase(99)
  .then(result => result)
  .catch(error => console.error(error.message));

This code will log:

Wrong type given, expected a string

In the realm of Promise, catch is the construct for handling errors.

In addition to catch and then we have also finally, similar to the finally in try/catch.

As its synchronous "relative", Promise's finally runs regardless of the Promise outcome:

toUppercase(99)
  .then(result => result)
  .catch(error => console.error(error.message))
  .finally(() => console.log("Run baby, run"));

Always keep in mind that any callback passed to then/catch/finally is handled asynchronously by the Microtask Queue. They're micro tasks with precedence over macro tasks such as events and timers.

Promise, error, and throw

As a best practice when rejecting a Promise it's convenient to provide an error object:

Promise.reject(TypeError("Wrong type given, expected a string"));

By doing so you keep error handling consistent through the codebase. Other team members can always expect to access error.message, and more important you can inspect stack traces.

In addition to Promise.reject, we can exit from a Promise chain by throwing an exception.

Consider the following example:

Promise.resolve("A string").then(value => {
  if (typeof value === "string") {
    throw TypeError("Expected a number!");
  }
});

We resolve a Promise with a string, and then the chain is immediately broken with throw.

To stop the exception propagation we use catch, as usual:

Promise.resolve("A string")
  .then(value => {
    if (typeof value === "string") {
      throw TypeError("Expected a number!");
    }
  })
  .catch(reason => console.log(reason.message));

This pattern is common in fetch, where we check the response object in search for errors:

fetch("https://example-dev/api/")
  .then(response => {
    if (!response.ok) {
      throw Error(response.statusText);
    }

    return response.json();
  })
  .then(json => console.log(json));

Here the exception can be intercepted with catch. If we fail, or decide to not catch it there, the exception is free to bubble up in the stack.

This is not bad per-se, but different environments react differently to uncaught rejections.

Node.js for example in the future will let crash any program where Promise rejections are un-handled:

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.

Better you catch them!

Error handling for "promisified" timers

With timers or events it's not possible to catch exceptions thrown from a callback. We saw an example in the previous section:

function failAfterOneSecond() {
  setTimeout(() => {
    throw Error("Something went wrong!");
  }, 1000);
}

// DOES NOT WORK
try {
  failAfterOneSecond();
} catch (error) {
  console.error(error.message);
}

A solution offered by Promise consists in the "promisification" of our code. Basically, we wrap our timer with a Promise:

function failAfterOneSecond() {
  return new Promise((_, reject) => {
    setTimeout(() => {
      reject(Error("Something went wrong!"));
    }, 1000);
  });
}

With reject we set off a Promise rejection, which carries an error object.

At this point we can handle the exception with catch:

failAfterOneSecond().catch(reason => console.error(reason.message));

Note: it's common to use value as the returning value from a Promise, and reason as the returning object from a rejection.

Node.js has a utility called promisify to ease the "promisification" of old-style callback APIs.

Error handling in Promise.all

The static method Promise.all accepts an array of Promise, and returns an array of results from all resolving Promise:

const promise1 = Promise.resolve("All good!");
const promise2 = Promise.resolve("All good here too!");

Promise.all([promise1, promise2]).then((results) => console.log(results));

// [ 'All good!', 'All good here too!' ]

If any of these Promise rejects, Promise.all rejects with the error from the first rejected Promise.

To handle these situations in Promise.all we use catch, as we did in the previous section:

const promise1 = Promise.resolve("All good!");
const promise2 = Promise.reject(Error("No good, sorry!"));
const promise3 = Promise.reject(Error("Bad day ..."));

Promise.all([promise1, promise2, promise3])
  .then(results => console.log(results))
  .catch(error => console.error(error.message));

To run a function regardless of the outcome of Promise.all, again, we can use finally:

Promise.all([promise1, promise2, promise3])
  .then(results => console.log(results))
  .catch(error => console.error(error.message))
  .finally(() => console.log("Always runs!"));

Error handling in Promise.any

We can consider Promise.any (Firefox > 79, Chrome > 85) as the opposite of Promise.all.

Whereas Promise.all returns a failure even if a single Promise in the array rejects, Promise.any gives us always the first resolved Promise (if present in the array) regardless of any rejection occurred.

In case instead all the Promise passed to Promise.any reject, the resulting error is an AggregateError. Consider the following example:

const promise1 = Promise.reject(Error("No good, sorry!"));
const promise2 = Promise.reject(Error("Bad day ..."));

Promise.any([promise1, promise2])
  .then(result => console.log(result))
  .catch(error => console.error(error))
  .finally(() => console.log("Always runs!"));

Here we handle the error with catch. The output of this code is:

AggregateError: No Promise in Promise.any was resolved
Always runs!

The AggregateError object has the same properties of a basic Error, plus an errors property:

//
  .catch(error => console.error(error.errors))
//

This property is an array of each individual error produced by the reject:

[Error: "No good, sorry!, Error: "Bad day ..."]

Error handling in Promise.race

The static method Promise.race accepts an array of Promise:

const promise1 = Promise.resolve("The first!");
const promise2 = Promise.resolve("The second!");

Promise.race([promise1, promise2]).then(result => console.log(result));

// The first!

The result is the first Promise who wins the "race".

How about rejections then? If the rejecting Promise is not the first to appear in the input array, Promise.race resolves:

const promise1 = Promise.resolve("The first!");
const rejection = Promise.reject(Error("Ouch!"));
const promise2 = Promise.resolve("The second!");

Promise.race([promise1, rejection, promise2]).then(result =>
  console.log(result)
);

// The first!

If the rejection instead appears as the first element of the array, Promise.race rejects, and we must catch the rejection:

const promise1 = Promise.resolve("The first!");
const rejection = Promise.reject(Error("Ouch!"));
const promise2 = Promise.resolve("The second!");

Promise.race([rejection, promise1, promise2])
  .then(result => console.log(result))
  .catch(error => console.error(error.message));

// Ouch!

Error handling in Promise.allSettled

Promise.allSettled is an ECMAScript 2020 addition to the language.

There is not so much to handle with this static method since the result will always be a resolved Promise, even if one or more input Promise rejects.

Consider the following example:

const promise1 = Promise.resolve("Good!");
const promise2 = Promise.reject(Error("No good, sorry!"));

Promise.allSettled([promise1, promise2])
  .then(results => console.log(results))
  .catch(error => console.error(error))
  .finally(() => console.log("Always runs!"));

We pass to Promise.allSettled an array consisting of two Promise: one resolved and another rejected.

In this case catch will never be hit. finally instead runs.

The result of this code, logged in then is:

[
  { status: 'fulfilled', value: 'Good!' },
  {
    status: 'rejected',
    reason: Error: No good, sorry!
  }
]

Error handling for async/await

async/await in JavaScript denotes asynchronous functions, but from a reader standpoint they benefit from all the readability of synchronous functions.

To keep things simple we'll take our previous synchronous function toUppercase, and we transform it to an asynchronous function by putting async before the function keyword:

async function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

Just by prefixing a function with async we cause the function to return a Promise. That means we can chain then, catch, and finally after the function call:

async function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

toUppercase("abc")
  .then(result => console.log(result))
  .catch(error => console.error(error.message))
  .finally(() => console.log("Always runs!"));

When we throw from an async function the exception becomes cause of rejection for the underlying Promise.

Any error can be intercepted with catch from the outside.

Most important, in addition to this style we can use try/catch/finally, much as we would do with a synchronous function.

In the following example we call toUppercase from another function, consumer, which conveniently wraps the function call with try/catch/finally:

async function toUppercase(string) {
  if (typeof string !== "string") {
    throw TypeError("Wrong type given, expected a string");
  }

  return string.toUpperCase();
}

async function consumer() {
  try {
    await toUppercase(98);
  } catch (error) {
    console.error(error.message);
  } finally {
    console.log("Always runs!");
  }
}

consumer(); // Returning Promise ignored

The output is:

Wrong type given, expected a string
Always runs!

On the same topic: How to Throw Errors From Async Functions in JavaScript?

Error handling for async generators

Async generators in JavaScript are generator functions capable of yielding Promises instead of simple values.

They combine generator functions with async. The result is a generator function whose iterator objects expose a Promise to the consumer.

To create an async generator we declare a generator function with the star *, prefixed with async:

async function* asyncGenerator() {
  yield 33;
  yield 99;
  throw Error("Something went wrong!"); // Promise.reject
}

Being based on Promise, the same rules for error handling apply here. throw inside an async generator causes a Promise rejection, which we intercept with catch.

To pull Promises out an async generators we can use two approaches:

then handlers.
async iteration.

From the above example we know for sure there will be an exception after the first two yield. This means we can do:

const go = asyncGenerator();

go.next().then(value => console.log(value));
go.next().then(value => console.log(value));
go.next().catch(reason => console.error(reason.message));

The output from this code is:

{ value: 33, done: false }
{ value: 99, done: false }
Something went wrong!

The other approach uses async iteration with for await...of. To use async iteration we need to wrap the consumer with an async function.

Here's the complete example:

async function* asyncGenerator() {
  yield 33;
  yield 99;
  throw Error("Something went wrong!"); // Promise.reject
}

async function consumer() {
  for await (const value of asyncGenerator()) {
    console.log(value);
  }
}

consumer();

And as with async/await we handle any potential exception with try/catch:

async function* asyncGenerator() {
  yield 33;
  yield 99;
  throw Error("Something went wrong!"); // Promise.reject
}

async function consumer() {
  try {
    for await (const value of asyncGenerator()) {
      console.log(value);
    }
  } catch (error) {
    console.error(error.message);
  }
}

consumer();

The output of this code is:

33
99
Something went wrong!

The iterator object returned from an asynchronous generator function has also a throw() method, much like its synchronous counterpart.

Calling throw() on the iterator object here won't throw an exception, but a Promise rejection:

async function* asyncGenerator() {
  yield 33;
  yield 99;
  yield 11;
}

const go = asyncGenerator();

go.next().then(value => console.log(value));
go.next().then(value => console.log(value));

go.throw(Error("Let's reject!"));

go.next().then(value => console.log(value)); // value is undefined

To handle this situation from the outside we can do:

go.throw(Error("Let's reject!")).catch(reason => console.error(reason.message));

But let's not forget that iterator objects throw() send the exception inside the generator. This means we can also apply the following pattern:

async function* asyncGenerator() {
  try {
    yield 33;
    yield 99;
    yield 11;
  } catch (error) {
    console.error(error.message);
  }
}

const go = asyncGenerator();

go.next().then(value => console.log(value));
go.next().then(value => console.log(value));

go.throw(Error("Let's reject!"));

go.next().then(value => console.log(value)); // value is undefined

Error handling in Node.js

Synchronous error handling in Node.js

Synchronous error handling in Node.js does not differ too much from what we saw so far.

For synchronous code, try/catch/finally works fine.

However, things get interesting if we take a glance at the asynchronous world.

Asynchronous error handling in Node.js: the callback pattern

For asynchronous code, Node.js strongly relies on two idioms:

the callback pattern.
event emitters.

In the callback pattern, asynchronous Node.js APIs accept a function which is handled through the event loop and executed as soon as the call stack is empty.

Consider the following code:

const { readFile } = require("fs");

function readDataset(path) {
  readFile(path, { encoding: "utf8" }, function(error, data) {
    if (error) console.error(error);
    // do stuff with the data
  });
}

If we extract the callback from this listing, we can see how it is supposed to deal with errors:

//
function(error, data) {
    if (error) console.error(error);
    // do stuff with the data
  }
//

If any errors arises from reading the given path with fs.readFile, we get an error object.

At this point we can:

simply log the error object as we did.
throw an exception.
pass the error to another callback.

To throw an exception we can do:

const { readFile } = require("fs");

function readDataset(path) {
  readFile(path, { encoding: "utf8" }, function(error, data) {
    if (error) throw Error(error.message);
    // do stuff with the data
  });
}

However, as with events and timers in the DOM, this exception will crash the program. The following attempt to stop it with try/catch won't work:

const { readFile } = require("fs");

function readDataset(path) {
  readFile(path, { encoding: "utf8" }, function(error, data) {
    if (error) throw Error(error.message);
    // do stuff with the data
  });
}

try {
  readDataset("not-here.txt");
} catch (error) {
  console.error(error.message);
}

Passing the error to another callback is the preferred option, if we don't want to crash the program:

const { readFile } = require("fs");

function readDataset(path) {
  readFile(path, { encoding: "utf8" }, function(error, data) {
    if (error) return errorHandler(error);
    // do stuff with the data
  });
}

Here errorHandler is what the name suggests, a simple function for error handling:

function errorHandler(error) {
  console.error(error.message);
  // do something with the error:
  // - write to a log.
  // - send to an external logger.
}

Asynchronous error handling in Node.js: event emitters

Much of what you do in Node.js is based on events. Most of the times you interact with an emitter object and some observers listening for messages.

Any event-driven module (like net for example) in Node.js extends a root class named EventEmitter.

EventEmitter in Node.js has two fundamental methods: on and emit.

Consider this simple HTTP server:

const net = require("net");

const server = net.createServer().listen(8081, "127.0.0.1");

server.on("listening", function () {
  console.log("Server listening!");
});

server.on("connection", function (socket) {
  console.log("Client connected!");
  socket.end("Hello client!");
});

Here we listen for two events: listening and connection.

In addition to these events, event emitters expose also an error event, fired in case of errors.

If you run this code listening on port 80 instead of the previous example, you'll get an exception:

const net = require("net");

const server = net.createServer().listen(80, "127.0.0.1");

server.on("listening", function () {
  console.log("Server listening!");
});

server.on("connection", function (socket) {
  console.log("Client connected!");
  socket.end("Hello client!");
});

Output:

events.js:291
      throw er; // Unhandled 'error' event
      ^

Error: listen EACCES: permission denied 127.0.0.1:80
Emitted 'error' event on Server instance at: ...

To catch it we can register an event handler for error:

server.on("error", function(error) {
  console.error(error.message);
});

This will print:

listen EACCES: permission denied 127.0.0.1:80

In addition, the program won't crash.

To learn more on the topic, consider also reading "Error handling in Node.js".

Wrapping up

In this guide we covered error handling in JavaScript for the whole spectrum, from simple synchronous code, to advanced asynchronous primitives.

There are many ways in which an exception can manifest in our JavaScript programs.

Exceptions from synchronous code are the most straightforward to catch. Exceptions from asynchronous code paths instead can be tricky to deal with.

In the meantime, new JavaScript APIs in the browser are almost all headed toward Promise. This pervasive pattern makes easier to handle exceptions with then/catch/finally, or with try/catch for async/await.

After reading this guide you should be able to recognize all the different situations that may arise in your programs, and catch your exceptions correctly.

Thanks for reading and stay tuned!