Typeerror: Cannot Read Property 'execute' of Undefined

JavaScript Errors and How to Fix Them

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

Below is a listing of the strange errors in JavaScript. Different browsers can give you dissimilar messages for the aforementioned mistake, so at that place are several dissimilar examples where applicative.

How to read errors?

Before the list, let'due south speedily await at the structure of an mistake message. Understanding the construction helps understand the errors, and yous'll have less trouble if you run into any errors not listed here.

A typical fault from Chrome looks like this:

Uncaught TypeError: undefined is non a function

The structure of the error is equally follows:

1. Uncaught TypeError: This role of the message is usually non very useful. Uncaught means the error was not defenseless in a catch argument, and TypeError is the error'southward proper noun.
2. undefined is not a part: This is the message function. With error messages, you lot 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 role.

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

Now onto the actual errors.

Uncaught TypeError: undefined is non a part

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

Occurs when attempting to call a value like a function, where the value is non a office. For case:

var foo = undefined; foo();

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

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

Since object backdrop that don't exist are undefined by default, the higher up would result in this error.

The other variations such every bit "number is non a part" occur when attempting to call a number like information technology was a function.

How to set this error: Ensure the function name is correct. With this fault, the line number will ordinarily bespeak at the correct location.

Uncaught ReferenceError: Invalid left-mitt 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 most common example of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a single equals instead of two. The message "left-manus side in assignment" is referring to the part on the left side of the equals sign, so like you lot tin encounter in the in a higher place example, the left-mitt side contains something you can't assign to, leading to the error.

How to fix this fault: Make 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: Non an acyclic Object, TypeError: cyclic object value, Circular reference in value statement not supported

Always caused by a round 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 higher up example have a reference to each other, the resulting object cannot be converted into JSON.

How to fix this error: Remove round references similar in the example from any objects you desire to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) later argument list

The JavaScript interpreter expected something, just it wasn't in that location. Typically acquired by 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 error doesn't point to the right place, making it difficult to fix.

• An error with [ ] { } ( ) is usually caused by 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 be correct.
• Unexpected ; is usually 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 correct for this example as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated Cord Literal, Invalid Line Terminator

A cord literal is missing the closing quote.

How to prepare this mistake: 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 nix, Unable to go property 'foo' of undefined or null reference

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

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

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

Uncaught TypeError: Cannot ready belongings 'foo' of zilch, Uncaught TypeError: Cannot gear up property 'foo' of undefined

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

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

var someVal = cypher; someVal.foo = 1;

How to gear up this mistake: This too is normally 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 acquired by a problems in programme logic, causing space recursive function calls.

How to ready this fault: Check recursive functions for bugs that could cause 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 call at the fault's line number gets correct input.

XMLHttpRequest cannot load http://some/url/. No 'Admission-Command-Allow-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 always caused past the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is correct and it respects the same-origin policy. A good way to find the offending lawmaking is to look at the URL in the fault message and detect 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 11

Ways the code called a office that you lot should non call at the current state. Occurs usually with XMLHttpRequest, when attempting to telephone call functions on it before it's ready.

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

In this instance, you would go the error because the setRequestHeader part can only be called later on calling xhr.open.

How to fix this mistake: Look at the lawmaking on the line pointed by the error and brand sure it runs at the right fourth dimension, or add together whatsoever necessary calls before it (such as xhr.open)

Determination

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 offset to make more than sense. Modern browsers also assist, as they no longer give the completely useless errors they used to.

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

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

Most Jani Hartikainen

Jani Hartikainen has spent over x years building web applications. His clients include companies like Nokia and hot super cloak-and-dagger startups. When not programming or playing games, Jani writes well-nigh JavaScript and high quality code on his site.

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

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