Inspect Uncaught Typeerror: Cannot Read Property '0' of Undefined

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors information technology gives can be very difficult to understand at beginning, and the line numbers given aren't e'er helpful either. Wouldn't it be useful to take a list where you could look to find out what they hateful and how to set up them? Hither you go!

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

How to read errors?

Before the listing, allow's quickly await at the structure of an mistake message. Understanding the construction helps sympathize the errors, and you'll take less trouble if you run into any errors not listed here.

A typical fault from Chrome looks like this:

Uncaught TypeError: undefined is not a part

The construction of the error is as follows:

1. Uncaught TypeError: This part of the bulletin is usually not very useful. Uncaught means the error was non defenseless in a catch argument, and TypeError is the error'south name.
2. undefined is non a function: This is the message part. With error messages, y'all have to read them very literally. For example in this case it literally means that the code attempted to use undefined like it was a office.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, simply do non ever include the first part, and recent versions of Cyberspace Explorer also give simpler errors than Chrome – merely in this case, simpler does non always hateful better.

Now onto the actual errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

This fault typically occurs if you are trying to phone call a office in an object, only you typed the name wrong.

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

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

The other variations such equally "number is not a office" occur when attempting to telephone call a number like it was a role.

How to set this error: Ensure the office proper name is correct. With this error, the line number volition normally point at the correct location.

Uncaught ReferenceError: Invalid left-hand side in consignment

Related errors: Uncaught exception: ReferenceError: Cannot assign to 'functionCall()', Uncaught exception: ReferenceError: Cannot assign to 'this'

Acquired past 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 unmarried equals instead of ii. The message "left-hand side in consignment" is referring to the function on the left side of the equals sign, and so like you can meet in the above example, the left-mitt side contains something y'all can't assign to, leading to the error.

How to fix this fault: Make sure you're non 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: Non 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 case have a reference to each other, the resulting object cannot be converted into JSON.

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

Unexpected token ;

Related errors: Expected ), missing ) after argument listing

The JavaScript interpreter expected something, just information technology wasn't in that location. Typically caused past 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 correct identify, making it difficult to set.

• An error with [ ] { } ( ) is usually caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this instance, line number volition oft signal to something else than the trouble character
• Unexpected / is related to regular expressions. The line number for this will usually be right.
• Unexpected ; is usually caused by having a ; inside an object or array literal, or within the statement listing of a function call. The line number volition usually be correct for this instance as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the endmost quote.

How to prepare this mistake: Ensure all strings take the correct closing quote.

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

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

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

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

How to fix this fault: Usually caused past typos. Bank check that the variables used well-nigh the line number pointed by the error are correctly named.

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

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

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

var someVal = null; someVal.foo = 1;

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

Uncaught RangeError: Maximum call stack size exceeded

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

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

How to fix this mistake: Check recursive functions for bugs that could cause them to proceed recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Acquired by an invalid decodeURIComponent call.

How to gear up this error: Check that the decodeURIComponent telephone call at the mistake'due south line number gets correct input.

XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is present on the requested resource

Related errors: Cross-Origin Request Blocked: The Aforementioned Origin Policy disallows reading the remote resources at http://some/url/

This error is e'er caused past the usage of XMLHttpRequest.

How to set this error: Ensure the request URL is correct and it respects the same-origin policy. A good way to observe the offending code is to await at the URL in the error message and observe it from your lawmaking.

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

Related errors: InvalidStateError, DOMException code xi

Means the code called a function that you lot should not call at the electric current country. Occurs normally with XMLHttpRequest, when attempting to phone call functions on it before it'southward gear up.

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

In this case, you would become the error considering the setRequestHeader office tin but be called later 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 correct time, or add together whatsoever necessary calls before it (such equally xhr.open)

Conclusion

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 start to brand more sense. Modern browsers also aid, as they no longer give the completely useless errors they used to.

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

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

Almost Jani Hartikainen

Jani Hartikainen has spent over 10 years edifice web applications. His clients include companies like Nokia and hot super secret startups. When not 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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