Discord Uncaught Exception Type Error Cannot Read Property Is Focused of Null

JavaScript Errors and How to Prepare Them

JavaScript can be a nightmare to debug: Some errors information technology gives can be very hard to empathize at first, and the line numbers given aren't always helpful either. Wouldn't it exist useful to have a list where yous could look to find out what they mean and how to set them? Here y'all go!

Beneath is a list of the strange errors in JavaScript. Dissimilar browsers tin give you different messages for the same error, so there are several dissimilar examples where applicable.

How to read errors?

Before the listing, let'south speedily look at the structure of an fault message. Agreement the structure helps empathize the errors, and you'll have less problem if you run into any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a office

The construction of the fault is as follows:

1. Uncaught TypeError: This office of the message is usually not very useful. Uncaught means the mistake was not defenseless in a catch statement, and TypeError is the error's proper noun.
2. undefined is not a function: This is the message part. With error messages, you have to read them very literally. For example in this instance it literally ways that the code attempted to use undefined like information technology was a function.

Other webkit-based browsers, like Safari, requite errors in a similar format to Chrome. Errors from Firefox are similar, but do not always include the get-go office, and recent versions of Internet Explorer also give simpler errors than Chrome – but in this instance, simpler does not always mean better.

At present onto the actual errors.

Uncaught TypeError: undefined is non a function

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

Occurs when attempting to call a value like a function, where the value is non a function. For example:

var foo = undefined; foo();

This mistake typically occurs if you are trying to phone call a part in an object, simply you lot typed the proper noun incorrect.

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

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

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

How to fix this mistake: Ensure the function proper noun is correct. With this fault, 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'

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

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

if(doSomething() = 'somevalue')

In this example, the developer accidentally used a single equals instead of two. The message "left-mitt side in consignment" is referring to the part on the left side of the equals sign, so like you can run into in the above instance, the left-mitt side contains something y'all can't assign to, leading to the error.

How to fix this error: Make sure y'all're not attempting to assign values to function results or to the this keyword.

Uncaught TypeError: Converting circular structure 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 by a circular reference in an object, which is then passed into JSON.stringify.

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

Considering both a and b in the above example have a reference to each other, the resulting object cannot exist converted into JSON.

How to ready this fault: Remove circular references like in the instance from any objects yous want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) subsequently argument list

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

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

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

• An mistake with [ ] { } ( ) is ordinarily acquired by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this example, line number volition often bespeak to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will commonly exist correct.
• Unexpected ; is normally caused past having a ; inside an object or array literal, or within the statement list of a function call. The line number volition usually be correct for this case too

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated Cord Literal, Invalid Line Terminator

A string literal is missing the closing quote.

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

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

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

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

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

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

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

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

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

var someVal = zippo; someVal.foo = 1;

How to gear up this mistake: This likewise is usually caused 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, as well much recursion, Stack overflow

Normally caused by a bug in program logic, causing infinite 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 telephone call.

How to fix this error: Bank check that the decodeURIComponent phone call at the error'southward 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: Cantankerous-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/

This error is always acquired by the usage of XMLHttpRequest.

How to fix this mistake: Ensure the request URL is correct and it respects the aforementioned-origin policy. A good fashion to discover the offending lawmaking is to await at the URL in the error bulletin and discover information technology from your code.

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

Related errors: InvalidStateError, DOMException code xi

Means the code chosen a office that you should not call at the current land. Occurs usually with XMLHttpRequest, when attempting to call functions on it before it'south fix.

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

In this example, you lot would get the mistake because the setRequestHeader office tin can merely be called afterward calling xhr.open.

How to fix this error: Look at the code on the line pointed past the error and brand sure it runs at the right time, or add any necessary calls before it (such as xhr.open)

Conclusion

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. Modern browsers too assistance, as they no longer give the completely useless errors they used to.

What's the almost confusing error you've seen? Share the frustration in the comments!

Want to larn more about these errors and how to preclude them? Discover Problems in JavaScript Automatically with ESLint.

Most Jani Hartikainen

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

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

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