Uncaught Typeerror Cannot Read Property 'appendchild' of Undefined

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors it gives can be very difficult to empathise at first, and the line numbers given aren't ever helpful either. Wouldn't it be useful to have a list where you could wait to find out what they mean and how to set them? Here you get!

Below is a list of the strange errors in JavaScript. Unlike browsers can give you different messages for the aforementioned error, so at that place are several different examples where applicable.

How to read errors?

Before the listing, let'south apace look at the structure of an error message. Understanding the structure helps understand the errors, and you'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 not a function

The structure of the error is equally follows:

1. Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the error was non defenseless in a catch statement, and TypeError is the fault'south name.
2. undefined is not a function: This is the bulletin role. With error messages, you lot accept to read them very literally. For example in this case information technology 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 not always include the kickoff role, and recent versions of Internet Explorer too give simpler errors than Chrome – but in this case, simpler does non e'er mean ameliorate.

Now onto the actual errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

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

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

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

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

How to fix this mistake: Ensure the function name is correct. With this error, the line number will usually 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 about 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 message "left-mitt side in consignment" is referring to the office on the left side of the equals sign, so similar y'all can see in the in a higher place example, the left-paw side contains something y'all tin't assign to, leading to the error.

How to fix this fault: Make sure yous're non attempting to assign values to office results or to the this keyword.

Uncaught TypeError: Converting round structure to JSON

Related errors: Uncaught exception: TypeError: JSON.stringify: Not an acyclic Object, TypeError: circadian object value, Circular 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);

Considering both a and b in the in a higher place example have a reference to each other, the resulting object cannot be converted into JSON.

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

Unexpected token ;

Related errors: Expected ), missing ) afterwards statement listing

The JavaScript interpreter expected something, but it wasn't at that place. Typically caused by mismatched parentheses or brackets.

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

How to set this mistake: Sometimes the line number with this error doesn't bespeak to the correct place, making it hard to fix.

• An error with [ ] { } ( ) is usually acquired by a mismatching pair. Check that all your parentheses and brackets accept a matching pair. In this example, line number will frequently 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 commonly acquired by having a ; inside an object or array literal, or within the argument list of a function call. The line number volition usually be correct for this case likewise

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated Cord Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to fix this error: Ensure all strings accept the correct endmost quote.

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

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

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

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

How to fix this fault: Ordinarily acquired by typos. 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 fix property 'foo' of undefined

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

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

var someVal = null; someVal.foo = i;

How to fix this error: This too is usually 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, also much recursion, Stack overflow

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

How to set up this error: Check recursive functions for bugs that could cause them to continue recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to ready this error: Check that the decodeURIComponent call at the mistake's 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: Cantankerous-Origin Request Blocked: The Same Origin Policy disallows reading the remote resources at http://some/url/

This error is ever caused by the usage of XMLHttpRequest.

How to fix this mistake: Ensure the asking 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 error message and find information technology from your lawmaking.

InvalidStateError: An endeavour was made to use an object that is non, or is no longer, usable

Related errors: InvalidStateError, DOMException code 11

Means the code called a office that yous should not call at the current state. Occurs usually with XMLHttpRequest, when attempting to call functions on it before it's gear up.

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

In this case, you would get the fault because the setRequestHeader function can only be called after calling xhr.open.

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

Decision

JavaScript has some of the near 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 help, equally 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 acquire more than about these errors and how to preclude them? Detect Problems in JavaScript Automatically with ESLint.

Well-nigh Jani Hartikainen

Jani Hartikainen has spent over ten 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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