Uncaught Exception: Typeerror: Cannot Read Property 'isfocused' of Null

JavaScript Errors and How to Set Them

JavaScript can be a nightmare to debug: Some errors it gives tin be very difficult to empathize at first, and the line numbers given aren't always helpful either. Wouldn't it be useful to have a list where you lot could look to find out what they mean and how to gear up them? Here you go!

Below is a list of the foreign errors in JavaScript. Unlike browsers can give you different letters for the same error, and then there are several unlike examples where applicable.

How to read errors?

Before the list, let'southward quickly look at the structure of an error message. Understanding the structure helps understand the errors, and y'all'll accept less problem if y'all run into whatsoever errors not listed hither.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a function

The construction of the fault is as follows:

1. Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the error was non caught in a catch statement, and TypeError is the error's name.
2. undefined is non a function: This is the message role. With fault letters, you have to read them very literally. For case in this instance it literally means that the code attempted to use undefined like it was a role.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are like, but exercise not e'er include the first office, and contempo versions of Internet Explorer also give simpler errors than Chrome – but in this case, simpler does non e'er mean ameliorate.

Now onto the actual errors.

Uncaught TypeError: undefined is not a role

Related errors: number is not a office, object is not a function, string is not a function, Unhandled Fault: 'foo' is not a function, Function Expected

Occurs when attempting to call a value like a function, where the value is not a function. For example:

var foo = undefined; foo();

This error typically occurs if yous are trying to call a role in an object, but you lot typed the proper noun incorrect.

var 10 = document.getElementByID('foo');

Since object properties that don't exist are undefined by default, the above would result in this error.

The other variations such every bit "number is not a function" occur when attempting to call a number like information technology was a office.

How to fix this error: Ensure the role name is correct. With this error, the line number will commonly point at the correct location.

Uncaught ReferenceError: Invalid left-hand side in consignment

Related errors: Uncaught exception: ReferenceError: Cannot assign to 'functionCall()', Uncaught exception: ReferenceError: Cannot assign to 'this'

Caused by attempting to assign a value to something that cannot be assigned to.

The most common example of this fault is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the developer accidentally used a unmarried equals instead of two. The message "left-hand side in assignment" is referring to the part on the left side of the equals sign, and so like y'all can see in the higher up example, the left-mitt side contains something you can't assign to, leading to the error.

How to set up this error: Make sure you're not attempting to assign values to function results or to the this keyword.

Uncaught TypeError: Converting round structure to JSON

Related errors: Uncaught exception: TypeError: JSON.stringify: Non an acyclic Object, TypeError: cyclic object value, Round reference in value argument non supported

E'er caused by a circular reference in an object, which is so passed into JSON.stringify.

var a = { }; var b = { a: a }; a.b = b; JSON.stringify(a);

Because both a and b in the above example accept a reference to each other, the resulting object cannot be converted into JSON.

How to ready this error: Remove circular references like in the case from whatever objects you desire to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) afterward statement list

The JavaScript interpreter expected something, but information technology wasn't there. Typically acquired past mismatched parentheses or brackets.

The token in this fault can vary – it might say "Unexpected token ]" or "Expected {" etc.

How to set up this error: Sometimes the line number with this error doesn't betoken to the correct place, making it hard to fix.

• An error with [ ] { } ( ) is unremarkably caused by a mismatching pair. Check that all your parentheses and brackets accept a matching pair. In this case, line number will oftentimes point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will normally exist right.
• Unexpected ; is usually caused past having a ; inside an object or array literal, or within the argument listing of a role call. The line number will usually be correct for this example equally well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to fix this error: Ensure all strings have the correct closing quote.

Uncaught TypeError: Cannot read property 'foo' of null, Uncaught TypeError: Cannot read holding 'foo' of undefined

Related errors: TypeError: someVal is null, Unable to get property 'foo' of undefined or zero reference

Attempting to read cipher or undefined as if it was an object. For example:

var someVal = zip; panel.log(someVal.foo);

How to fix this error: Usually caused by typos. Check that the variables used about the line number pointed by the error are correctly named.

Uncaught TypeError: Cannot set belongings 'foo' of null, Uncaught TypeError: Cannot set property 'foo' of undefined

Related errors: TypeError: someVal is undefined, Unable to set property 'foo' of undefined or goose egg reference

Attempting to write null or undefined every bit if it was an object. For example:

var someVal = zero; someVal.foo = one;

How to fix this error: This also is usually caused by typos. Check the variable names near the line the error points to.

Uncaught RangeError: Maximum call stack size exceeded

Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, too much recursion, Stack overflow

Usually caused by a bug in program logic, causing infinite recursive function calls.

How to fix this fault: Bank check recursive functions for bugs that could cause them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to fix this mistake: Check that the decodeURIComponent call at the error's line number gets correct input.

XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is present on the requested resource

Related errors: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/

This mistake is ever caused by the usage of XMLHttpRequest.

How to gear up this error: Ensure the request URL is correct and it respects the same-origin policy. A good way to observe the offending code is to look at the URL in the error bulletin and discover it from your code.

InvalidStateError: An endeavor was made to use an object that is not, or is no longer, usable

Related errors: InvalidStateError, DOMException code xi

Means the lawmaking called a role that you should not call at the current land. Occurs unremarkably with XMLHttpRequest, when attempting to call functions on it earlier it'due south set up.

var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val');

In this case, yous would get the error because the setRequestHeader role can only be chosen subsequently calling xhr.open.

How to prepare this fault: Look at the code on the line pointed by the error and make certain it runs at the correct fourth dimension, or add whatever necessary calls before it (such as xhr.open up)

Decision

JavaScript has some of the most unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors start to make more sense. Mod browsers also help, as they no longer requite the completely useless errors they used to.

What'southward the nearly confusing error you've seen? Share the frustration in the comments!

Want to learn more than well-nigh these errors and how to forbid them? Detect Problems in JavaScript Automatically with ESLint.

About Jani Hartikainen

Jani Hartikainen has spent over 10 years building web applications. His clients include companies similar Nokia and hot super hugger-mugger startups. When not programming or playing games, Jani writes about JavaScript and high quality code on his site.

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Source: https://davidwalsh.name/fix-javascript-errors