Что такое javascript error

Top 10 JavaScript errors from 1000+ projects (and how to avoid them)

To give back to our community of developers, we looked at our database of thousands of projects and found the top 10 errors in JavaScript. We’re going to show you what causes them and how to prevent them from happening. If you avoid these «gotchas,» it’ll make you a better developer.

Because data is king, we collected, analyzed, and ranked the top 10 JavaScript errors. Rollbar collects all the errors for each project and summarizes how many times each one occurred. We do this by grouping errors according to their fingerprints. Basically, we group two errors if the second one is just a repeat of the first. This gives users a nice overview instead of an overwhelming big dump like you’d see in a log file.

We focused on the errors most likely to affect you and your users. To do this, we ranked errors by the number of projects experiencing them across different companies. If we looked only at the total number of times each error occurred, then high-volume customers could overwhelm the data set with errors that are not relevant to most readers.

Here are the top 10 JavaScript errors:

Each error has been shortened for easier readability. Let’s dive deeper into each one to determine what can cause it and how you can avoid creating it.

1. Uncaught TypeError: Cannot read property

If you’re a JavaScript developer, you’ve probably seen this error more than you care to admit. This one occurs in Chrome when you read a property or call a method on an undefined object. You can test this very easily in the Chrome Developer Console.

Uncaught TypeError: Cannot read property can occur for many reasons, but a common one is improper initialization of state while rendering the UI components. Let’s look at an example of how this can occur in a real-world app. We’ll pick React, but the same principles of improper initialization also apply to Angular, Vue or any other framework.

There are two important things realize here:

1. A component’s state (e.g. this.state ) begins life as undefined .
2. When you fetch data asynchronously, the component will render at least once before the data is loaded – regardless of whether it’s fetched in the constructor, componentWillMount or componentDidMount . When Quiz first renders, this.state.items is undefined. This, in turn, means ItemList gets items as undefined, and you get an error – «Uncaught TypeError: Cannot read property ‘map’ of undefined» in the console.

This is easy to fix. The simplest way: Initialize state with reasonable default values in the constructor.

The exact code in your app might be different, but we hope we’ve given you enough of a clue to either fix or avoid this problem in your app. If not, keep reading because we’ll cover more examples for related errors below.

2. TypeError: ‘undefined’ is not an object (evaluating

This is an error that occurs in Safari when you read a property or call a method on an undefined object. You can test this very easily in the Safari Developer Console. This is essentially the same as the above error for Chrome, but Safari uses a different error message.

3. TypeError: null is not an object (evaluating

This is an error that occurs in Safari when you read a property or call a method on a null object. You can test this very easily in the Safari Developer Console.

Interestingly, in JavaScript, null and undefined are not the same, which is why we see two different error messages. Undefined is usually a variable that has not been assigned, while null means the value is blank. To verify they are not equal, try using the strict equality operator:

One way this error might occur in a real world example is if you try using a DOM element in your JavaScript before the element is loaded. That’s because the DOM API returns null for object references that are blank.

Any JS code that executes and deals with DOM elements should execute after the DOM elements have been created. JS code is interpreted from top to down as laid out in the HTML. So, if there is a tag before the DOM elements, the JS code within script tag will execute as the browser parses the HTML page. You will get this error if the DOM elements have not been created before loading the script.

In this example, we can resolve the issue by adding an event listener that will notify us when the page is ready. Once the addEventListener is fired, the init() method can make use of the DOM elements.

4. (unknown): Script error

The (unknown): Script error occurs when an uncaught JavaScript error crosses domain boundaries in violation of the cross-origin policy. For example, if you host your JavaScript code on a CDN, any uncaught errors (errors that bubble up to the window.onerror handler, instead of being caught in try-catch) will get reported as simply «Script error» instead of containing useful information. This is a browser security measure intended to prevent passing data across domains that otherwise wouldn’t be allowed to communicate.

To get the real error messages, do the following:

1. Send the Access-Control-Allow-Origin header

Setting the Access-Control-Allow-Origin header to signifies that the resource can be accessed properly from any domain. You can replace with your domain if necessary: for example, Access-Control-Allow-Origin: www.example.com . However, handling multiple domains gets tricky, and may not be worth the effort if you’re using a CDN due to caching issues that may arise. See more here.

Here are some examples on how to set this header in various environments:

Apache

In the folders where your JavaScript files will be served from, create an .htaccess file with the following contents:

Header add Access-Control-Allow-Origin "*"

Nginx

Add the add_header directive to the location block that serves your JavaScript files:

location ~ ^/assets/

HAProxy

Add the following to your asset backend where JavaScript files are served from:

rspadd Access-Control-Allow-Origin:\ *

2. Set crossorigin=»anonymous» on the script tag

In your HTML source, for each of the scripts that you’ve set the Access-Control-Allow-Origin header for, set crossorigin=»anonymous» on the SCRIPT tag. Make sure you verify that the header is being sent for the script file before adding the crossorigin property on the script tag. In Firefox, if the crossorigin attribute is present but the Access-Control-Allow-Origin header is not, the script won’t be executed.

5. TypeError: Object doesn’t support property

This is an error that occurs in IE when you call an undefined method. You can test this in the IE Developer Console.

This is equivalent to the error «TypeError: ‘undefined’ is not a function» in Chrome. Yes, different browsers can have different error messages for the same logical error.

This is a common problem for IE in web applications that employ JavaScript namespacing. When this is the case, the problem 99.9% of the time is IE’s inability to bind methods within the current namespace to the this keyword. For example, if you have the JS namespace Rollbar with the method isAwesome. Normally, if you are within the Rollbar namespace you can invoke the isAwesome method with the following syntax:

this.isAwesome();

Chrome, Firefox and Opera will happily accept this syntax. IE, on the other hand, will not. Thus, the safest bet when using JS namespacing is to always prefix with the actual namespace.

Rollbar.isAwesome();

6. TypeError: ‘undefined’ is not a function

This is an error that occurs in Chrome when you call an undefined function. You can test this in the Chrome Developer Console and Mozilla Firefox Developer Console.

As JavaScript coding techniques and design patterns have become increasingly sophisticated over the years, there’s been a corresponding increase in the proliferation of self-referencing scopes within callbacks and closures, which are a fairly common source of this/that confusion.

Consider this example code snippet:

function clearBoard() < alert("Cleared"); >document.addEventListener("click", function()< this.clearBoard(); // what is “this” ? >);

If you execute the above code and then click on the page, it results in the following error «Uncaught TypeError: this.clearBoard is not a function». The reason is that the anonymous function being executed is in the context of the document, whereas clearBoard is defined on the window.

A traditional, old-browser-compliant solution is to simply save your reference to this in a variable that can then be inherited by the closure. For example:

var self=this; // save reference to 'this', while it's still this! document.addEventListener("click", function()< self.clearBoard(); >);

Alternatively, in the newer browsers, you can use the bind() method to pass the proper reference:

document.addEventListener("click",this.clearBoard.bind(this));

7. Uncaught RangeError

This is an error that occurs in Chrome under a couple of circumstances. One is when you call a recursive function that does not terminate. You can test this in the Chrome Developer Console.

It may also happen if you pass a value to a function that is out of range. Many functions accept only a specific range of numbers for their input values. For example, Number.toExponential(digits) and Number.toFixed(digits) accept digits from 0 to 100, and Number.toPrecision(digits) accepts digits from 1 to 100.

var a = new Array(4294967295); //OK var b = new Array(-1); //range error var num = 2.555555; document.writeln(num.toExponential(4)); //OK document.writeln(num.toExponential(-2)); //range error! num = 2.9999; document.writeln(num.toFixed(2)); //OK document.writeln(num.toFixed(105)); //range error! num = 2.3456; document.writeln(num.toPrecision(1)); //OK document.writeln(num.toPrecision(0)); //range error!

8. TypeError: Cannot read property ‘length’

This is an error that occurs in Chrome because of reading length property for an undefined variable. You can test this in the Chrome Developer Console.

You normally find length defined on an array, but you might run into this error if the array is not initialized or if the variable name is hidden in another context. Let’s understand this error with the following example.

var testArray= ["Test"]; function testFunction(testArray) < for (var i = 0; i < testArray.length; i++) < console.log(testArray[i]); >> testFunction();

When you declare a function with parameters, these parameters become local ones. This means that even if you have variables with names testArray , parameters with the same names within a function will still be treated as local.

You have two ways to resolve your issue:

Remove parameters in the function declaration statement (it turns out you want to access those variables that are declared outside of the function, so you don’t need parameters for your function):

var testArray = ["Test"]; /* Precondition: defined testArray outside of a function */ function testFunction(/* No params */) < for (var i = 0; i < testArray.length; i++) < console.log(testArray[i]); >> testFunction();

var testArray = ["Test"]; function testFunction(testArray) < for (var i = 0; i < testArray.length; i++) < console.log(testArray[i]); >> testFunction(testArray);

9. Uncaught TypeError: Cannot set property

When we try to access an undefined variable it always returns undefined and we cannot get or set any property of undefined . In that case, an application will throw “Uncaught TypeError cannot set property of undefined.”

For example, in the Chrome browser:

If the test object does not exist, error will throw “Uncaught TypeError cannot set property of undefined.”

10. ReferenceError: event is not defined

This error is thrown when you try to access a variable that is undefined or is outside the current scope. You can test it very easily in Chrome browser.

If you’re getting this error when using the event handling system, make sure you use the event object passed in as a parameter. Older browsers like IE offer a global variable event, and Chrome automatically attaches the event variable to the handler. Firefox will not automatically add it. Libraries like jQuery attempt to normalize this behavior. Nevertheless, it’s best practice to use the one passed into your event handler function.

document.addEventListener("mousemove", function (event) < console.log(event); >)

Conclusion

It turns out a lot of these are null or undefined errors. A good static type checking system like Typescript could help you avoid them if you use the strict compiler option. It can warn you if a type is expected but has not been defined. Even without Typescript, it helps to use guard clauses to check whether objects are undefined before using them.

We hope you learned something new and can avoid errors in the future, or that this guide helped you solve a head scratcher. Nevertheless, even with the best practices, unexpected errors do pop up in production. It’s important to have visibility into errors that affect your users, and to have good tools to solve them quickly.

Rollbar gives you visibility to production JavaScript errors and gives you more context to solve them quickly. For example, it offers additional debugging features like telemetry which tells you what happened on the user’s browser leading up to the error. That’s insight you don’t have outside of your local developer console. Learn more in Rollbar’s full list of features for JavaScript applications.

If you haven’t already, signup for a 14-day free trial of Rollbar and let us help you take control of impactful JavaScript errors. ��

«Rollbar allows us to go from alerting to impact analysis and resolution in a matter of minutes. Without it we would be flying blind.»

What went wrong? Troubleshooting JavaScript

When you built up the «Guess the number» game in the previous article, you may have found that it didn’t work. Never fear — this article aims to save you from tearing your hair out over such problems by providing you with some tips on how to find and fix errors in JavaScript programs.

Types of error

Generally speaking, when you do something wrong in code, there are two main types of error that you’ll come across:

• Syntax errors: These are spelling errors in your code that actually cause the program not to run at all, or stop working part way through — you will usually be provided with some error messages too. These are usually okay to fix, as long as you are familiar with the right tools and know what the error messages mean!
• Logic errors: These are errors where the syntax is actually correct but the code is not what you intended it to be, meaning that program runs successfully but gives incorrect results. These are often harder to fix than syntax errors, as there usually isn’t an error message to direct you to the source of the error.

Okay, so it’s not quite that simple — there are some other differentiators as you drill down deeper. But the above classifications will do at this early stage in your career. We’ll look at both of these types going forward.

An erroneous example

To get started, let’s return to our number guessing game — except this time we’ll be exploring a version that has some deliberate errors introduced. Go to GitHub and make yourself a local copy of number-game-errors.html (see it running live here).

1. To get started, open the local copy inside your favorite text editor, and your browser.
2. Try playing the game — you’ll notice that when you press the «Submit guess» button, it doesn’t work!

Note: You might well have your own version of the game example that doesn’t work, which you might want to fix! We’d still like you to work through the article with our version, so that you can learn the techniques we are teaching here. Then you can go back and try to fix your example.

At this point, let’s consult the developer console to see if it reports any syntax errors, then try to fix them. You’ll learn how below.

Fixing syntax errors

Earlier on in the course we got you to type some simple JavaScript commands into the developer tools JavaScript console (if you can’t remember how to open this in your browser, follow the previous link to find out how). What’s even more useful is that the console gives you error messages whenever a syntax error exists inside the JavaScript being fed into the browser’s JavaScript engine. Now let’s go hunting.

1. Go to the tab that you’ve got number-game-errors.html open in, and open your JavaScript console. You should see an error message along the following lines:
2. The first line of the error message is:

Uncaught TypeError: guessSubmit.addeventListener is not a function number-game-errors.html:86:15

• The first part, Uncaught TypeError: guessSubmit.addeventListener is not a function , is telling us something about what went wrong.
• The second part, number-game-errors.html:86:15 , is telling us where in the code the error came from: line 86, character 15 of the file «number-game-errors.html».

Warning: Error message may not be on line 86. If you are using any code editor with an extension that launches a live server on your local machine, this will cause extra code to be injected. Because of this, the developer tools will list the error as occurring on a line that is not 86.

.addeventListener("click", checkGuess); 

Note: See our TypeError: «x» is not a function reference page for more details about this error.

Syntax errors round two

1. Save your page and refresh, and you should see the error has gone.
2. Now if you try to enter a guess and press the Submit guess button, you’ll see another error!
3. This time the error being reported is:

Uncaught TypeError: can't access property "textContent", lowOrHi is null

Depending on the browser you are using, you might see a different message here. The message above is what Firefox will show you, but Chrome, for example, will show you this:

Uncaught TypeError: Cannot set properties of null (setting 'textContent')

It’s the same error, but different browsers describe it in a different way.

Note: This error didn’t come up as soon as the page was loaded because this error occurred inside a function (inside the checkGuess() < >block). As you’ll learn in more detail in our later functions article, code inside functions runs in a separate scope than code outside functions. In this case, the code was not run and the error was not thrown until the checkGuess() function was run by line 86.

.textContent = "Last guess was too high!"; 

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

.log(lowOrHi); 

This code will print the value of lowOrHi to the console after we tried to set it in line 49. See console.log() for more information.

p class="lowOrHi">p> 

let randomNumber = Math.floor(Math.random()) + 1; 

= Math.floor(Math.random()) + 1; 

.log(randomNumber); 

.random(); 

.floor(Math.random()) + 1; 

.floor(Math.random() * 100); 

.floor(Math.random() * 100) + 1; 

const userGuess = Number(guessField.value); 

const userGuess === Number(guessField.value); 

if (userGuess === randomNumber)  

if (userGuess = randomNumber)  

function checkGuess()  

function checkGuess(  

document.addEventListener('buttonClick', function (event) < console.log("Button Clicked!"); >, false);

document.addEventListener('buttonClick', function (event) < alert("I am in a box with a " str + "!"); >, false);