In this lesson, we'll introduce some of the common types of errors you'll encounter when writing JavaScript code.

1. Recognize common JavaScript errors.
2. Read a stack trace to discern where an error originated.

No one writes perfect code the first time. Or the second time. Or the third time.

We all make mistakes. One of the biggest advantages an experienced programmer has is knowing how to quickly troubleshoot and fix errors they encounter. You will reach this point — it's just a matter of practice. To get you started, let's look at some of the common types of error messages you'll encounter when writing JavaScript code.

This is one of the simplest and most common errors, and it's pretty explicitly telling us what went wrong. We tried to reference a variable or function that doesn't exist in the current scope (or in the scope chain)! For example:

myVar;
// ERROR: Uncaught ReferenceError: myVar is not defined

It can also arise if you forget to put quotation marks around a string:

Hello, world;
// ERROR: Uncaught ReferenceError: Hello is not defined

('Hello, world');
// => "Hello, world"

If you meant to declare the variable in the current scope and simply forgot, declaring the variable should solve the issue:

const myVar = 'Hello, world!';

myVar;
// => "Hello, world!"

The more difficult case is when you expected the variable to exist in the scope chain — that is, you expected the variable to have been declared in an outer scope and therefore to be accessible inside your function. You'll get better at recognizing and debugging this case with practice. For now, double check where the current function is declared. Make sure it's declared at a place where its scope chain should include the outer scope that contains the declaration of the variable you're trying to access.

This one usually indicates that you tried to invoke something that isn't actually a function. For example:

const myVar = 'Hello, world!';

myVar();
// ERROR: Uncaught TypeError: myVar is not a function

A common one that you'll run into when we get into asynchronous programming in JavaScript is Uncaught TypeError: undefined is not a function. The JavaScript engine is telling us that we tried to invoke undefined, which is obviously not invocable. It typically happens when a variable contains undefined instead of a function. The way to debug it is to find where the attempted invocation happened and then figure out why that variable contains undefined.

When you see this error, it means you tried to invoke a function but forgot the closing parenthesis:

console.log('Hello,', 'world!';
// ERROR: Uncaught SyntaxError: missing ) after argument list

function myAdder (num1, num2) {
  return num1 + num2;
}

myAdder(10, 4;
// ERROR: Uncaught SyntaxError: missing ) after argument list

You're probably familiar with this one by now — it means we accidentally tried to assign a new value to a variable declared with the const keyword, which prevents reassignment. However, sometimes you rightfully feel that you didn't try to reassign anything, and it boils down to a small typo:

const snackSelection = 'Pretzels';

if ((snackSelection = 'Pretzels')) {
  console.log("That'll be $1, please!");
}
// ERROR: Uncaught TypeError: Assignment to constant variable.

In this case, we accidentally used the assignment operator, a single = sign, instead of a comparison operator, such as the triple equals (===) strict equality operator.

This is just a selection of some of the many types of errors you might encounter while writing JavaScript code. The main point is to use the information you're given. The JavaScript engine isn't trying to trick you — on the contrary, it's trying to help you debug.

One of the really neat pieces of information provided with the error messages is what's called a stack trace. If you've been following along with the examples in the JS console, you've probably already seen something like this:

const snackSelection = 'Pretzels';

if ((snackSelection = 'Pretzels')) {
  console.log("That'll be $1, please!");
}
// ERROR: Uncaught TypeError: Assignment to constant variable.    VM5412:3
//           at <anonymous>:3:20
//        (anonymous) @ VM5412:3

At first glance, that's the kind of error message that makes a programmer new to JavaScript run screaming in the opposite direction. So many random numbers! What the heck are <anonymous> and (anonymous)?! VM5412:3 looks like the name of R2-D2's cousin!

All of this strange information is telling us the same thing: the exact location of where the error occurred. We're not going to go too deep into it, but VM standards for Virtual Machine, and it's Chrome's way of saying that the script didn't run in a specific file. In this case, the script ran in the JavaScript console, and Chrome arbitrarily assigned an ID of 5412 to the execution of that particular script. If you're coding along — which you should be! — the number in your console is most likely different. If you run the same code again, the number will have changed because Chrome's treating it as a new script execution and will assign it a new ID number.

The :3 piece of VM5412:3 is, however, interesting. It's telling us which line within the script caused the error. In this case, it happened on the third line.

The at <anonymous>:3:20 message elaborates on the :3, indicating that, not only was the error on the third line, it was on the 20th character of the third line. The <anonymous> is, like the VM designation, telling us that the error didn't occur in a specific file.

The only new piece of information on the final line is (anonymous), which is slightly different from the <anonymous> above. While <anonymous> indicates that we aren't in a particular file, (anonymous) tells us that we're in the global scope — that the error didn't occur inside of a function.

To get a better sense of the various pieces of information provided in the error message, let's take a look at a couple errors in a JavaScript file. That way, we won't have (most of) this <anonymous> garbage to further confuse us.

Clone this repo to your local machine, and open up index.html in your browser.

When the file's open in your browser, open the JS console, and you should see something similar to the following error:

Whoa, that's so much easier to understand! It's telling us the error occurred on line 5 in errors.js. Let's see what's on line 5 of the file:

if (snackSelection = 'Pretzels') {

Ah, it's the same error from before: we've accidentally used the assignment operator instead of the strict equality operator. Let's fix it:

if (snackSelection === 'Pretzels') {

Save the errors.js file after you've made the fix, and refresh the browser window. You should see two things in the console, the That'll be $1, please! message from the first code snippet and a new error telling us that third is not defined:

Let's see what's on lines 17, 13, and 20 in errors.js:

function first() {
  second(); // Line 13
}

function second() {
  third(); // Line 17
}

first(); // Line 20

Now that we're dealing with a series of function invocations, we can really see the power of the stack trace: it traces the error up through the stack of function calls that led to it. Let's read it backwards and reconstruct the events that led to the error:

1. In the global scope: the JavaScript engine reaches line 20 and invokes first().
2. Inside first(): the engine reaches line 13 and invokes second().
3. Inside second(): the engine reaches line 17 and sees the identifier third, but it can't find a declared variable or function with that name in the current scope (second()) or the outer scope (the global scope).
4. Because it can't find a matching declaration, the JavaScript engine throws an error inside second() that then propagates up the call stack until it reaches the global execution context.

To fix the third is not defined error, let's first try declaring third as the simplest thing we know, a variable:

function first() {
  second();
}

function second() {
  third();
}

const third = 'Declaring a new variable.';

first();

Remember what we learned earlier in the section on common JavaScript errors. If our understanding is correct, this should fix the third is not defined error and, in its place, throw a new error. Can you guess what the new error will be?

Did you correctly deduce what the new error would be?

The new error is telling us that third is not a function. It may have been pretty obvious that our initial solution would just result in another error — after all, the code we're working with isn't all that complicated. However, intentionally breaking your code and seeing whether it breaks in the exact way you predicted is a great technique for improving your debugging and general JavaScript skills. The more you understand the errors and their causes, the easier debugging will become.

The fix, of course, is to declare third() as a function instead of a simple variable:

function first() {
  second();
}

function second() {
  third();
}

function third() {
  console.log("Now I'm a function!");
}

first();

When we save the file and refresh the page again, all of the errors should be gone:

We mentioned earlier that an error thrown in a function "propagates up the call stack until it reaches the global execution context." But why?

The reason thrown errors propagate up the call stack is that the JavaScript engine is looking for something to catch the error. This is getting back to why all of the error messages we've seen so far are uncaught errors.

JavaScript provides a control flow structure called try...catch with which you can try to run some JavaScript statements and catch any errors thrown within the try block. We aren't going to go into any greater depth on the topic because it isn't worth getting sidetracked. If you really want to learn more about handling errors with a try...catch statement, check out the MDN reference. You'll probably encounter try...catch in the wild, but it's often not the best tool for the job. By design, it's a performance nightmare to rely on your code throwing errors as a control flow pattern. It's much better to make your code flexible and properly handle things like bad user input without throwing errors. Errors should be reserved for when something goes seriously wrong.

Arguably the biggest difference between being a novice and an expert developer is how comfortable you feel with reading and debugging error messages. JavaScript — and, indeed, every programming language — is designed by programmers for programmers. The language doesn't intentionally make things more difficult for you. On the contrary, every time you write code that results in some sort of error, JavaScript goes out of its way to provide you with the information you need to find and fix the error. When you see one of those bright red error messages pop up, don't freak out! It's the JavaScript engine starting a friendly dialog with you: "Hey, I tried to do what you asked of me, but I ran into a problem. Here's where the problem occurred, and here's what happened."

As you become more comfortable diagnosing and solving error messages, you'll become a faster, better programmer, and writing JavaScript code will become more and more enjoyable!

• MDN — Errors
  • Uncaught ReferenceError: _____ is not defined
  • Uncaught TypeError: _____ is not a function
  • Uncaught SyntaxError: missing ) after argument list
  • Uncaught TypeError: Assignment to constant variable.
• MDN — try...catch

View Errors and Stack Traces on Learn.co and start learning to code for free.