Error Typeerror: Cannot Read Property 'length' of Null

JavaScript Errors and How to Fix Them

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

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

How to read errors?

Before the list, let's rapidly look at the structure of an error message. Understanding the structure helps sympathize the errors, and you lot'll have less trouble if yous run into any errors not listed here.

A typical error from Chrome looks similar this:

Uncaught TypeError: undefined is non a office

The structure of the fault is as follows:

1. Uncaught TypeError: This part of the message is usually non very useful. Uncaught means the error was not defenseless in a grab statement, and TypeError is the mistake's name.
2. undefined is not a function: This is the message part. With error messages, you have to read them very literally. For case in this case information technology literally means that the code attempted to employ undefined like information technology was a function.

Other webkit-based browsers, similar Safari, give errors in a similar format to Chrome. Errors from Firefox are like, but practice not always include the start part, and recent versions of Cyberspace Explorer too requite simpler errors than Chrome – but in this case, simpler does not ever hateful improve.

At present onto the actual errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

This error typically occurs if you are trying to call a function in an object, just you lot typed the proper noun wrong.

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

Since object properties that don't exist are undefined past default, the above would upshot in this fault.

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

How to fix this error: Ensure the function name is correct. With this mistake, the line number volition usually bespeak at the correct 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 by attempting to assign a value to something that cannot exist assigned to.

The most common example 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 consignment" is referring to the part on the left side of the equals sign, so similar you tin can come across in the above example, the left-mitt side contains something yous can't assign to, leading to the mistake.

How to fix this fault: Make certain you'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 non supported

Ever acquired 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);

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

How to fix this error: Remove circular references like in the example from any objects y'all desire to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after statement list

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

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

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

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

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 mistake: Ensure all strings have the right closing quote.

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

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

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

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

How to fix this mistake: Commonly caused by typos. Bank check that the variables used near the line number pointed past the mistake are correctly named.

Uncaught TypeError: Cannot set up holding 'foo' of zip, 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 null or undefined as if it was an object. For example:

var someVal = cypher; someVal.foo = one;

How to set up this fault: This also is usually acquired past typos. Check the variable names near the line the mistake points to.

Uncaught RangeError: Maximum call stack size exceeded

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

Ordinarily caused by a bug in program logic, causing space recursive function calls.

How to prepare this error: 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 ready this error: Cheque that the decodeURIComponent telephone call at the error'due south line number gets correct input.

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

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

This error is e'er caused past the usage of XMLHttpRequest.

How to set up this error: Ensure the asking URL is correct and information technology respects the same-origin policy. A good way to observe the offending code is to look at the URL in the error message and find it from your code.

InvalidStateError: An attempt was fabricated to utilise an object that is non, 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 usually with XMLHttpRequest, when attempting to call functions on information technology before information technology's ready.

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

In this case, you lot would get the mistake because the setRequestHeader part tin can only be called after calling xhr.open.

How to set up this error: Expect at the code on the line pointed by the fault and make sure information technology runs at the correct time, or add any necessary calls earlier it (such as xhr.open)

Conclusion

JavaScript has some of the almost unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more than familiarity the errors start to make more sense. Mod browsers as well aid, 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 learn more almost these errors and how to prevent them? Observe Problems in JavaScript Automatically with ESLint.

Near Jani Hartikainen

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

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

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