Upload Error: Typeerror: Cannot Read Property 'imagename' of Undefined

JavaScript Errors and How to Fix Them

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

Below is a list of the strange errors in JavaScript. Different browsers can give y'all dissimilar messages for the same error, so there are several dissimilar examples where applicable.

How to read errors?

Before the list, let's rapidly expect at the structure of an error bulletin. Agreement the construction helps empathize the errors, and you'll accept less trouble if you run into any errors not listed hither.

A typical fault from Chrome looks like this:

Uncaught TypeError: undefined is not a role

The structure of the error is every bit follows:

1. Uncaught TypeError: This part of the bulletin is normally not very useful. Uncaught means the fault was not caught in a take hold of statement, and TypeError is the error's name.
2. undefined is non a part: This is the message part. With mistake messages, you accept to read them very literally. For example in this case it literally means that the code attempted to utilize undefined like it was a role.

Other webkit-based browsers, similar Safari, requite errors in a similar format to Chrome. Errors from Firefox are similar, but practice non ever include the first part, and contempo versions of Internet Explorer also give simpler errors than Chrome – merely in this case, simpler does not ever hateful meliorate.

At present onto the actual errors.

Uncaught TypeError: undefined is non a part

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

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

var foo = undefined; foo();

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

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

Since object properties that don't be are undefined by default, the to a higher place would result in this error.

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

How to fix this error: Ensure the office proper noun is correct. With this error, the line number volition usually betoken at the right location.

Uncaught ReferenceError: Invalid left-manus 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 be assigned to.

The most mutual example 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-hand side in consignment" is referring to the role on the left side of the equals sign, so like you can encounter in the above example, the left-hand side contains something you can't assign to, leading to the error.

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

Uncaught TypeError: Converting round 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 caused 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);

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 fix this error: Remove circular references similar in the example from any objects you desire to catechumen into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after statement listing

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

The token in this error 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 bespeak to the right identify, making it difficult to fix.

• An error with [ ] { } ( ) is commonly caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this case, line number will often indicate to something else than the trouble grapheme
• Unexpected / is related to regular expressions. The line number for this will usually exist correct.
• Unexpected ; is commonly caused past having a ; inside an object or array literal, or within the argument list of a function call. The line number will usually be right for this example every bit 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 cypher, Uncaught TypeError: Cannot read property 'foo' of undefined

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

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

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

How to fix this error: Commonly acquired by typos. Cheque that the variables used nigh the line number pointed past the mistake are correctly named.

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

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

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

var someVal = null; someVal.foo = one;

How to gear up this error: This too is ordinarily caused past typos. Check the variable names most the line the fault points to.

Uncaught RangeError: Maximum telephone call stack size exceeded

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

Usually caused by a bug in programme logic, causing infinite recursive role 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 past an invalid decodeURIComponent call.

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

XMLHttpRequest cannot load http://some/url/. No 'Access-Command-Allow-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 fault is always 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 skillful way to observe the offending lawmaking is to look at the URL in the error message and observe it 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 11

Ways the code called a function that you should non call at the current state. Occurs usually with XMLHttpRequest, when attempting to call functions on it before information technology's set.

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

In this case, you would get the fault because the setRequestHeader function tin just exist called after calling xhr.open.

How to gear up this error: Look at the lawmaking on the line pointed by the mistake and make sure information technology runs at the right fourth dimension, or add whatsoever necessary calls before it (such every bit 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 than familiarity the errors start to make more sense. Modern browsers also help, as they no longer give the completely useless errors they used to.

What's the about disruptive error y'all've seen? Share the frustration in the comments!

Want to larn more about these errors and how to prevent them? Detect Issues in JavaScript Automatically with ESLint.

Nearly Jani Hartikainen

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

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

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