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JavaScript Errors and How to Fix Them

JavaScript tin can exist 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 information technology be useful to have a listing where yous could look to find out what they mean and how to prepare them? Here you go!

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

How to read errors?

Before the list, let's quickly expect at the construction of an error bulletin. Agreement the structure helps sympathize the errors, and you'll have less problem if you run into any errors non listed hither.

A typical fault from Chrome looks similar this:

Uncaught TypeError: undefined is not a function

The structure of the error is as follows:

1. Uncaught TypeError: This part of the bulletin is usually non very useful. Uncaught means the error was not defenseless in a grab statement, and TypeError is the error'due south name.
2. undefined is not a part: This is the message part. With mistake messages, you have to read them very literally. For instance in this case it literally means that the code attempted to use undefined similar it was a function.

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

At present onto the actual errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

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

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

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

The other variations such every bit "number is not a function" occur when attempting to call a number like information technology was a function.

How to fix this error: Ensure the function proper name is right. With this error, the line number will usually point 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'

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

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

if(doSomething() = 'somevalue')

In this case, the programmer accidentally used a single equals instead of 2. The message "left-paw side in consignment" is referring to the role on the left side of the equals sign, then like you can see in the to a higher place example, the left-hand side contains something you tin can't assign to, leading to the error.

How to gear up this fault: Make sure you lot'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 statement non supported

E'er 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);

Considering both a and b in the in a higher place example take a reference to each other, the resulting object cannot be converted into JSON.

How to set up this error: Remove round references like in the example from any objects you want to convert 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 fault can vary – it might say "Unexpected token ]" or "Expected {" etc.

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

• An error with [ ] { } ( ) is usually acquired by a mismatching pair. Bank check that all your parentheses and brackets have a matching pair. In this case, line number will often bespeak to something else than the trouble character
• Unexpected / is related to regular expressions. The line number for this will usually be correct.
• Unexpected ; is normally caused by 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 instance 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 ready this error: Ensure all strings accept the correct closing quote.

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

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

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

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

How to fix this error: Normally caused by typos. Bank check that the variables used nearly the line number pointed by the error are correctly named.

Uncaught TypeError: Cannot prepare property 'foo' of naught, Uncaught TypeError: Cannot set holding 'foo' of undefined

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

Attempting to write nada or undefined equally if information technology was an object. For example:

var someVal = cipher; someVal.foo = i;

How to fix this fault: This likewise is usually caused by typos. Check the variable names near the line the error points to.

Uncaught RangeError: Maximum call stack size exceeded

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

Usually caused past a bug in program logic, causing infinite recursive office calls.

How to fix 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 telephone call.

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

XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Let-Origin' header is nowadays 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 ever caused by the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is correct and information technology respects the aforementioned-origin policy. A good way to observe the offending code is to wait at the URL in the error message and find 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 xi

Means the code called a role that you should not call at the current state. Occurs normally with XMLHttpRequest, when attempting to call functions on it earlier information technology's prepare.

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

In this case, you would get the mistake because the setRequestHeader function can merely be chosen later calling xhr.open up.

How to prepare this error: Look at the code on the line pointed past the error and make certain information technology runs at the right fourth dimension, or add together whatsoever necessary calls earlier it (such equally 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 make more sense. Modernistic browsers too help, as they no longer give the completely useless errors they used to.

What'south the most confusing error yous've seen? Share the frustration in the comments!

Desire to learn more about these errors and how to prevent them? Find Problems in JavaScript Automatically with ESLint.

About Jani Hartikainen

Jani Hartikainen has spent over ten years building spider web applications. His clients include companies like Nokia and hot super undercover startups. When non 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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