Common.js:154 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 tin can be very difficult to understand at first, and the line numbers given aren't ever helpful either. Wouldn't it exist useful to have a listing where y'all could look to find out what they hateful and how to fix them? Hither you lot go!

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

How to read errors?

Before the list, permit's quickly look at the structure of an error message. Understanding the structure helps sympathize the errors, and you lot'll accept less problem if you come across any errors not listed hither.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a office

The structure of the error is as follows:

1. Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the fault was not caught in a catch statement, and TypeError is the error'south name.
2. undefined is non a function: This is the message part. With mistake messages, you have to read them very literally. For example in this case it literally means that the lawmaking 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 like, but practice not always include the first part, and recent versions of Internet Explorer likewise give simpler errors than Chrome – but in this case, simpler does non e'er mean amend.

Now onto the bodily errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

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

var x = document.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 similar it was a function.

How to ready this error: Ensure the function proper name is right. With this mistake, the line number will usually bespeak 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 nigh common example of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a single equals instead of 2. The message "left-mitt side in assignment" is referring to the role on the left side of the equals sign, so like you tin run into in the higher up instance, the left-hand side contains something you tin'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: 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 to a higher place example accept a reference to each other, the resulting object cannot exist converted into JSON.

How to prepare this error: Remove circular references like in the example from any objects yous desire to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after statement listing

The JavaScript interpreter expected something, but it 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 fix this error: Sometimes the line number with this fault doesn't betoken to the right place, making it difficult to fix.

• An error with [ ] { } ( ) is usually acquired by a mismatching pair. Check that all your parentheses and brackets take a matching pair. In this case, line number will frequently point to something else than the trouble graphic symbol
• Unexpected / is related to regular expressions. The line number for this will normally exist right.
• Unexpected ; is commonly caused by having a ; inside an object or array literal, or within the argument list of a office call. The line number will unremarkably be correct for this case also

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 have the correct closing quote.

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

Related errors: TypeError: someVal is zilch, Unable to become holding 'foo' of undefined or null reference

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

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

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

Uncaught TypeError: Cannot set up property 'foo' of zip, Uncaught TypeError: Cannot gear up property 'foo' of undefined

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

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

var someVal = zero; someVal.foo = i;

How to gear up this error: This too is usually caused by typos. Cheque 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

Commonly caused past a issues 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 call.

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

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

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

This fault is ever caused by the usage of XMLHttpRequest.

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

InvalidStateError: An attempt 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 function that you should not phone call at the electric current state. Occurs usually with XMLHttpRequest, when attempting to phone call functions on it before information technology's ready.

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

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

How to gear up this error: Look at the lawmaking on the line pointed by the mistake and make sure it runs at the correct time, or add whatever necessary calls before it (such as xhr.open)

Determination

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, as they no longer give the completely useless errors they used to.

What's the most confusing fault you've seen? Share the frustration in the comments!

Want to learn more about 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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