Uncaught Typeerror: Cannot Read Property 'length' of Undefined Datatables

JavaScript Errors and How to Fix Them

JavaScript can exist a nightmare to debug: Some errors information technology gives can be very difficult to understand at kickoff, and the line numbers given aren't always helpful either. Wouldn't it exist useful to accept a listing where you lot could look to find out what they mean and how to fix them? Here you go!

Beneath is a list of the foreign errors in JavaScript. Different browsers can give you different letters for the same fault, so at that place are several different examples where applicable.

How to read errors?

Before the list, let'southward quickly expect at the structure of an error message. Understanding the structure helps empathise the errors, and you'll have less trouble if you encounter any errors non listed here.

A typical error from Chrome looks similar this:

Uncaught TypeError: undefined is not a function

The structure of the error is as follows:

1. Uncaught TypeError: This function of the message is ordinarily non very useful. Uncaught ways the error was not caught in a catch argument, and TypeError is the mistake's name.
2. undefined is not a function: This is the message office. With error messages, you lot have to read them very literally. For example in this case it literally means that the lawmaking attempted to use undefined like information technology was a function.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, only do not always include the showtime part, and contempo versions of Cyberspace Explorer besides give simpler errors than Chrome – just in this case, simpler does non always mean amend.

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 Mistake: 'foo' is not a function, Function Expected

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

var foo = undefined; foo();

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

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

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

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

How to gear up this error: Ensure the role name is correct. With this error, the line number will usually betoken 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 unmarried equals instead of 2. The message "left-hand side in assignment" is referring to the part on the left side of the equals sign, so like you can come across in the above example, the left-manus side contains something you can't assign to, leading to the error.

How to set up this error: Make sure y'all're not attempting to assign values to function results or to the this keyword.

Uncaught TypeError: Converting circular construction 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 to a higher place example have a reference to each other, the resulting object cannot exist converted into JSON.

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

Unexpected token ;

Related errors: Expected ), missing ) after argument list

The JavaScript interpreter expected something, simply it wasn't there. 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 mistake doesn't point to the right place, making it difficult to prepare.

• 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 oftentimes point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this volition usually exist 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 unremarkably be right for this case besides

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A cord literal is missing the closing quote.

How to fix this error: Ensure all strings take the correct closing quote.

Uncaught TypeError: Cannot read property 'foo' of nothing, Uncaught TypeError: Cannot read belongings 'foo' of undefined

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

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

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

How to fix this fault: Usually acquired by typos. Check that the variables used well-nigh the line number pointed by the fault are correctly named.

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

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

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

var someVal = null; someVal.foo = 1;

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

Uncaught RangeError: Maximum phone call stack size exceeded

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

Usually caused by a issues in program logic, causing infinite recursive function calls.

How to set this mistake: Check recursive functions for bugs that could crusade them to go along recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to fix this error: Bank check that the decodeURIComponent phone call at the error's line number gets correct input.

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

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

This fault is always caused by the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is correct and it respects the same-origin policy. A skillful manner to find the offending lawmaking is to wait at the URL in the fault bulletin and find it from your lawmaking.

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

Related errors: InvalidStateError, DOMException code eleven

Means the code called a role that y'all should non call at the current state. Occurs commonly 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, y'all would get the error because the setRequestHeader office can only be chosen later on calling xhr.open.

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

Conclusion

JavaScript has some of the virtually 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'southward the nearly disruptive error 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 x years building web applications. His clients include companies like Nokia and hot super clandestine startups. When non programming or playing games, Jani writes near JavaScript and high quality code on his site.

codeutopia.netjhartikainenPosts

irwinthinge.blogspot.com

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

Share this post