Document.getelementbyid Cannot Read Property 'classlist' of Undefined

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors it gives can exist very difficult to understand at first, and the line numbers given aren't always helpful either. Wouldn't it be useful to take a list where you could look to observe out what they mean and how to set them? Here you get!

Beneath is a list of the foreign errors in JavaScript. Different browsers can give yous unlike messages for the same error, so at that place are several different examples where applicative.

How to read errors?

Before the list, permit's quickly look at the structure of an error message. Understanding the structure helps sympathize the errors, and you'll accept less trouble if you encounter whatever errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a role

The structure of the error is as follows:

1. Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the error was not caught in a take hold of argument, and TypeError is the error'southward proper name.
2. undefined is not a function: This is the message part. With error messages, yous have to read them very literally. For example in this example it literally means that the code attempted to use undefined like information technology was a function.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, but practise non always include the first function, and recent versions of Internet Explorer also give simpler errors than Chrome – but in this instance, simpler does non always mean better.

Now onto the bodily errors.

Uncaught TypeError: undefined is not a role

Related errors: number is not a role, object is non a part, string is not a part, Unhandled Error: 'foo' is non a function, Office Expected

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

var foo = undefined; foo();

This error typically occurs if you are trying to call a function in an object, but you typed the name wrong.

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

Since object backdrop that don't be are undefined by default, the above would upshot in this mistake.

The other variations such as "number is not a function" occur when attempting to call a number like it was a function.

How to fix this mistake: Ensure the part name is correct. With this error, the line number will usually betoken at the right location.

Uncaught ReferenceError: Invalid left-hand side in assignment

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

Caused past attempting to assign a value to something that cannot exist assigned to.

The most common case of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a single equals instead of two. The message "left-hand side in assignment" is referring to the function on the left side of the equals sign, so similar you lot can see in the in a higher place example, the left-hand side contains something you tin't assign to, leading to the error.

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

Uncaught TypeError: Converting circular construction to JSON

Related errors: Uncaught exception: TypeError: JSON.stringify: Not an acyclic Object, TypeError: cyclic object value, Circular reference in value argument not supported

Always acquired past a circular reference in an object, which is and then 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 have a reference to each other, the resulting object cannot be converted into JSON.

How to set up this mistake: Remove circular references similar in the case from whatever objects you want to catechumen into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument list

The JavaScript interpreter expected something, but it wasn't there. Typically caused by mismatched parentheses or brackets.

The token in this error can vary – information technology might say "Unexpected token ]" or "Expected {" etc.

How to set this fault: Sometimes the line number with this fault doesn't point to the correct place, making it difficult to fix.

• An error with [ ] { } ( ) is usually caused past a mismatching pair. Check that all your parentheses and brackets take a matching pair. In this case, line number will often point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this volition usually be correct.
• Unexpected ; is usually caused by having a ; within an object or assortment literal, or within the argument list of a office call. The line number volition usually be correct for this case as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A cord literal is missing the closing quote.

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

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

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

Attempting to read null or undefined as if information technology was an object. For example:

var someVal = null; console.log(someVal.foo);

How to gear up this error: Ordinarily caused by typos. Check that the variables used near the line number pointed past the error are correctly named.

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

Related errors: TypeError: someVal is undefined, Unable to prepare belongings 'foo' of undefined or null reference

Attempting to write null or undefined as if it was an object. For case:

var someVal = null; someVal.foo = 1;

How to fix this error: This too is ordinarily acquired by typos. Check the variable names nearly the line the error points to.

Uncaught RangeError: Maximum call stack size exceeded

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

Usually caused by a issues in plan logic, causing space recursive function calls.

How to fix this error: Check recursive functions for bugs that could cause them to go on recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to gear up this error: Bank check that the decodeURIComponent call at the error'south line number gets right input.

XMLHttpRequest cannot load http://some/url/. No 'Admission-Command-Allow-Origin' header is nowadays on the requested resource

Related errors: Cantankerous-Origin Asking Blocked: The Aforementioned Origin Policy disallows reading the remote resource at http://some/url/

This error is always caused past the usage of XMLHttpRequest.

How to fix this mistake: Ensure the request URL is correct and it respects the aforementioned-origin policy. A good way to notice the offending code is to wait at the URL in the error message and find it from your lawmaking.

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

Related errors: InvalidStateError, DOMException code 11

Means the lawmaking called a function that you should not call at the current state. Occurs normally with XMLHttpRequest, when attempting to call functions on it before it's ready.

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

In this case, yous would get the error because the setRequestHeader function tin can only exist chosen subsequently calling xhr.open.

How to fix this error: Look at the code on the line pointed by the error and make sure it runs at the correct time, or add whatever necessary calls before it (such as xhr.open)

Conclusion

JavaScript has some of the well-nigh unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors start to brand more sense. Modern browsers also help, as they no longer give the completely useless errors they used to.

What'due south the almost confusing fault yous've seen? Share the frustration in the comments!

Want to learn more than virtually these errors and how to forbid them? Detect Bug in JavaScript Automatically with ESLint.

About Jani Hartikainen

Jani Hartikainen has spent over 10 years building web applications. His clients include companies like Nokia and hot super secret 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

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