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JavaScript Errors and How to Fix Them

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

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

How to read errors?

Before the list, permit's rapidly look at the structure of an error message. Understanding the structure helps understand the errors, and you'll have less trouble if y'all come across any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a office

The construction of the error is every bit follows:

1. Uncaught TypeError: This office of the message is usually not very useful. Uncaught ways the error was not defenseless in a take hold of argument, and TypeError is the error's name.
2. undefined is not a function: This is the message office. With fault messages, you accept to read them very literally. For example in this instance information technology literally means that the code attempted to use undefined like it was a role.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, but do not e'er include the offset part, and contempo versions of Internet Explorer too requite simpler errors than Chrome – but in this case, simpler does not always mean ameliorate.

Now onto the bodily errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

This error typically occurs if y'all are trying to phone call a function in an object, merely you typed the proper name incorrect.

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

Since object properties that don't be are undefined by default, the above would outcome in this mistake.

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

How to set up this error: Ensure the part proper noun is correct. With this error, the line number will commonly point 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 be assigned to.

The nigh mutual example of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this case, the developer accidentally used a unmarried equals instead of two. The bulletin "left-hand side in assignment" is referring to the role on the left side of the equals sign, and then like you can encounter in the higher up example, the left-hand side contains something you can't assign to, leading to the error.

How to fix this mistake: Brand sure 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

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

How to fix this fault: Remove round references like in the example from any objects you desire to catechumen into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument list

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 this error: Sometimes the line number with this mistake doesn't point to the correct place, making information technology difficult to set up.

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

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the endmost quote.

How to fix this mistake: Ensure all strings have the correct closing quote.

Uncaught TypeError: Cannot read holding 'foo' of nil, Uncaught TypeError: Cannot read property 'foo' of undefined

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

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

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

How to set this mistake: Usually acquired past typos. Bank check that the variables used nigh the line number pointed by the fault are correctly named.

Uncaught TypeError: Cannot ready property 'foo' of goose egg, Uncaught TypeError: Cannot set up property 'foo' of undefined

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

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

var someVal = null; someVal.foo = 1;

How to fix this fault: This too is usually caused by typos. Check the variable names almost the line the mistake points to.

Uncaught RangeError: Maximum call stack size exceeded

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

Usually acquired past a bug in program logic, causing infinite recursive function calls.

How to fix this fault: 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 phone call.

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

XMLHttpRequest cannot load http://some/url/. No 'Admission-Control-Allow-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 ever caused by the usage of XMLHttpRequest.

How to set up this mistake: Ensure the request URL is right and it respects the same-origin policy. A skillful way to discover the offending code is to look at the URL in the error message and find it from your code.

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

Related errors: InvalidStateError, DOMException code eleven

Means the lawmaking called a function that yous should not call at the current land. Occurs ordinarily with XMLHttpRequest, when attempting to phone call functions on information technology earlier it's gear up.

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

In this case, y'all would get the error considering the setRequestHeader function can only be called afterwards calling xhr.open.

How to fix this error: Await at the lawmaking on the line pointed by the error and make certain information technology runs at the right time, or add whatsoever necessary calls before it (such as 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 familiarity the errors start to make more sense. Mod browsers besides help, as they no longer give the completely useless errors they used to.

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

Want to learn more nigh these errors and how to prevent them? Observe Bug 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 secret startups. When non programming or playing games, Jani writes about JavaScript and high quality code on his site.

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

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