Jquery Uncaught Typeerror: Cannot Read Property 'tolowercase' of Undefined

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors it gives can exist very hard to empathise at kickoff, and the line numbers given aren't always helpful either. Wouldn't information technology be useful to have a list where you could await to find out what they mean and how to fix them? Here you lot get!

Below is a list of the foreign errors in JavaScript. Different browsers can requite you lot different messages for the same mistake, so there are several different examples where applicable.

How to read errors?

Before the list, allow's apace wait at the structure of an error message. Agreement the structure helps understand the errors, and you'll have less trouble if yous run into whatever errors non listed hither.

A typical error from Chrome looks similar this:

Uncaught TypeError: undefined is not a part

The construction of the error is as follows:

1. Uncaught TypeError: This part of the message is unremarkably not very useful. Uncaught ways the error was not caught in a catch argument, and TypeError is the fault'southward proper noun.
2. undefined is not a part: This is the message role. With error messages, you take to read them very literally. For example in this instance information technology literally means that the lawmaking attempted to use undefined like it was a function.

Other webkit-based browsers, similar Safari, requite errors in a similar format to Chrome. Errors from Firefox are similar, but do not always include the first office, and recent versions of Cyberspace Explorer also requite simpler errors than Chrome – simply in this case, simpler does not always hateful better.

Now onto the bodily errors.

Uncaught TypeError: undefined is non a function

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

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

var foo = undefined; foo();

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

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

Since object properties that don't exist are undefined by default, the in a higher place would outcome in this fault.

The other variations such as "number is non a role" occur when attempting to telephone call a number like it was a role.

How to fix this fault: Ensure the function proper noun is correct. With this error, the line number will usually indicate at the right location.

Uncaught ReferenceError: Invalid left-paw side in assignment

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

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

The most common example of this mistake 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 consignment" is referring to the part on the left side of the equals sign, so like y'all can see in the above example, the left-mitt side contains something y'all tin can't assign to, leading to the error.

How to gear up this error: Make sure yous'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: Non an acyclic Object, TypeError: circadian object value, Circular reference in value argument not supported

Ever acquired by a round 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 accept a reference to each other, the resulting object cannot be converted into JSON.

How to fix this error: Remove circular references like in the instance from any objects you want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) later on argument listing

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

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

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

• An error with [ ] { } ( ) is usually caused by 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 trouble grapheme
• Unexpected / is related to regular expressions. The line number for this will normally be correct.
• Unexpected ; is usually caused by having a ; inside an object or array literal, or within the statement list of a function call. The line number volition unremarkably be right 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 fault: Ensure all strings have the correct closing quote.

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

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

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

var someVal = goose egg; console.log(someVal.foo);

How to fix this error: Commonly caused by typos. Check that the variables used near 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 property 'foo' of undefined or null reference

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

var someVal = null; someVal.foo = 1;

How to fix this error: This as well is usually acquired by typos. Check the variable names near the line the mistake points to.

Uncaught RangeError: Maximum phone call stack size exceeded

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

Usually acquired past a bug in program logic, causing infinite recursive function calls.

How to fix this fault: Cheque recursive functions for bugs that could cause them to continue recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent phone call.

How to set 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-Permit-Origin' header is present on the requested resources

Related errors: Cross-Origin Request Blocked: The Same 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 error: Ensure the request URL is correct and information technology respects the same-origin policy. A skilful way to discover the offending code is to look at the URL in the error message and find information technology from your lawmaking.

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

Related errors: InvalidStateError, DOMException code 11

Means the code chosen a function that you should not call at the current country. Occurs usually with XMLHttpRequest, when attempting to telephone call functions on it before information technology'due south ready.

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

In this case, you lot would go the error considering the setRequestHeader function can just be called later calling xhr.open.

How to fix this mistake: Wait at the lawmaking on the line pointed by the mistake and make sure it runs at the correct 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 offset to make more sense. Modern browsers also assistance, as they no longer give the completely useless errors they used to.

What'south the well-nigh disruptive error you've seen? Share the frustration in the comments!

Want to larn more than nigh these errors and how to prevent them? Notice Issues in JavaScript Automatically with ESLint.

Most Jani Hartikainen

Jani Hartikainen has spent over ten years edifice web applications. His clients include companies like Nokia and hot super surreptitious startups. When not programming or playing games, Jani writes virtually JavaScript and loftier quality code on his site.

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

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