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JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors it gives can be very difficult to empathize at first, and the line numbers given aren't always helpful either. Wouldn't information technology exist useful to take a list where you could expect to discover out what they mean and how to fix them? Here you become!

Below is a list of the strange errors in JavaScript. Unlike browsers tin can requite you different messages for the same error, so there are several unlike examples where applicable.

How to read errors?

Before the list, permit's speedily look at the structure of an error message. Agreement the structure helps understand the errors, and you lot'll have less trouble if you see any errors not listed here.

A typical mistake from Chrome looks like this:

Uncaught TypeError: undefined is not a office

The structure of the error is as follows:

1. Uncaught TypeError: This part of the message is usually non very useful. Uncaught means the error was non caught in a grab statement, and TypeError is the fault's name.
2. undefined is not a part: This is the bulletin part. With error messages, you have to read them very literally. For instance in this instance it literally means that the code attempted to use 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 similar, but exercise not ever include the first part, and contempo versions of Internet Explorer also give simpler errors than Chrome – just in this case, simpler does not always mean better.

Now onto the actual errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

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

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

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

The other variations such as "number is non a role" occur when attempting to phone call a number similar it was a function.

How to fix this fault: Ensure the part name is correct. With this error, the line number volition usually point at the correct 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 exist assigned to.

The virtually common example of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a unmarried equals instead of 2. The message "left-manus side in assignment" is referring to the part on the left side of the equals sign, so like you can see in the above example, the left-mitt side contains something y'all can't assign to, leading to the error.

How to fix this mistake: Make sure you're non attempting to assign values to part 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: circadian object value, Round reference in value statement 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 gear up this error: Remove circular references like in the example from any objects yous want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) later statement list

The JavaScript interpreter expected something, but it wasn't there. Typically acquired past mismatched parentheses or brackets.

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

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

• An error with [ ] { } ( ) is commonly acquired past a mismatching pair. Bank check that all your parentheses and brackets have a matching pair. In this case, line number will frequently point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will ordinarily exist right.
• Unexpected ; is usually caused by having a ; inside an object or array literal, or within the argument listing of a office telephone call. The line number volition usually 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 error: Ensure all strings have the correct closing quote.

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

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

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

var someVal = nothing; console.log(someVal.foo);

How to set up this mistake: Usually acquired past typos. Bank 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 fix property 'foo' of undefined or zippo reference

Attempting to write null or undefined equally if it was an object. For instance:

var someVal = null; someVal.foo = 1;

How to set up this error: This too is normally caused by typos. Cheque the variable names near 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 issues in program logic, causing infinite recursive role 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

Acquired by an invalid decodeURIComponent call.

How to set this fault: Bank check that the decodeURIComponent call at the error'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 Request Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/

This error is ever caused by the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is right and it respects the same-origin policy. A good way to find the offending code is to look at the URL in the mistake message and find information technology from your code.

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

Related errors: InvalidStateError, DOMException code 11

Means the code called a function that y'all should non call at the electric current state. Occurs unremarkably with XMLHttpRequest, when attempting to call functions on it before it's ready.

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

In this case, you would become the fault considering the setRequestHeader function can only exist called after calling xhr.open up.

How to fix this mistake: Look at the code on the line pointed by the error and brand sure it runs at the right time, or add whatever necessary calls before information technology (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 kickoff to make more than sense. Modern browsers also assist, as they no longer give the completely useless errors they used to.

What's the virtually confusing error y'all've seen? Share the frustration in the comments!

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

About Jani Hartikainen

Jani Hartikainen has spent over 10 years building web applications. His clients include companies like Nokia and hot super surreptitious startups. When non programming or playing games, Jani writes nearly JavaScript and loftier quality code on his site.

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