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JavaScript Errors and How to Ready Them

JavaScript can exist a nightmare to debug: Some errors it gives can be very difficult to sympathise at first, and the line numbers given aren't always helpful either. Wouldn't it be useful to have a listing where you could wait to find out what they mean and how to fix them? Hither you lot become!

Beneath is a list of the strange errors in JavaScript. Unlike browsers can give y'all different letters for the same error, and then at that place are several dissimilar examples where applicable.

How to read errors?

Before the list, allow'southward rapidly look at the structure of an mistake message. Understanding the structure helps empathise the errors, and you'll have less trouble if yous run into any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a function

The structure of the error is every bit follows:

1. Uncaught TypeError: This part of the message is usually non very useful. Uncaught ways the error was not caught in a catch argument, and TypeError is the error'due south proper name.
2. undefined is non a function: This is the bulletin part. With error messages, you lot have to read them very literally. For example in this case information technology literally ways that the code attempted to use undefined like 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 starting time part, and recent versions of Cyberspace Explorer also requite simpler errors than Chrome – just in this example, simpler does non e'er 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 function, string is non a role, Unhandled Error: 'foo' is not a function, Function Expected

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

var foo = undefined; foo();

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

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

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

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

How to set this mistake: Ensure the part name is right. With this error, the line number will usually betoken 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'

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

The virtually 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 bulletin "left-mitt side in assignment" is referring to the function on the left side of the equals sign, and so similar yous tin see in the in a higher place example, the left-hand side contains something you tin can't assign to, leading to the error.

How to fix this error: Make sure you're not attempting to assign values to role 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 not supported

Always caused 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 have a reference to each other, the resulting object cannot be converted into JSON.

How to set this fault: Remove circular references like in the example from any objects you want to catechumen into JSON.

Unexpected token ;

Related errors: Expected ), missing ) afterward argument list

The JavaScript interpreter expected something, but information technology 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 set up this error: Sometimes the line number with this error doesn't indicate to the right place, making it difficult to set up.

• An error with [ ] { } ( ) is usually caused past a mismatching pair. Cheque that all your parentheses and brackets have a matching pair. In this case, line number will often signal to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will normally be right.
• Unexpected ; is normally caused past having a ; inside an object or array literal, or within the argument list of a part phone call. The line number will normally be correct for this case likewise

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 endmost quote.

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

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

Attempting to read naught or undefined as if it was an object. For example:

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

How to fix this error: Usually caused by typos. Cheque that the variables used near the line number pointed by the error are correctly named.

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

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

Attempting to write naught or undefined as if it was an object. For example:

var someVal = null; someVal.foo = i;

How to fix this error: This besides is usually caused by typos. Cheque the variable names near the line the mistake 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 acquired by a bug in program logic, causing infinite recursive function calls.

How to set this fault: Bank check recursive functions for bugs that could crusade them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to fix this mistake: Check that the decodeURIComponent phone call at the error's line number gets right input.

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

How to set up this error: Ensure the request 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 information technology from your code.

InvalidStateError: An endeavor 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 office that you should non telephone call at the current state. Occurs commonly with XMLHttpRequest, when attempting to call functions on information technology before it'due south ready.

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

In this case, yous would get the error because the setRequestHeader function can only exist called subsequently calling xhr.open.

How to set this error: Look at the code on the line pointed by the error and make certain it runs at the correct fourth dimension, or add any necessary calls before it (such as xhr.open)

Determination

JavaScript has some of the nearly unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors beginning to make more than sense. Mod browsers as well assist, as they no longer give the completely useless errors they used to.

What's the almost disruptive error you've seen? Share the frustration in the comments!

Desire to larn more than about these errors and how to prevent them? Detect Bug in JavaScript Automatically with ESLint.

About Jani Hartikainen

Jani Hartikainen has spent over 10 years building spider web applications. His clients include companies like Nokia and hot super undercover startups. When not programming or playing games, Jani writes about JavaScript and loftier quality code on his site.

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

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