Cannot Read Property Length of Null in Javscript

JavaScript Errors and How to Set Them

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

Below is a listing of the strange errors in JavaScript. Unlike browsers tin give you dissimilar messages for the aforementioned error, so there are several different examples where applicable.

How to read errors?

Before the list, allow'south quickly look at the structure of an error message. Understanding the construction helps empathize the errors, and you lot'll have less problem if you run across any errors not listed hither.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is non 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 error was not caught in a grab statement, and TypeError is the fault'southward name.
2. undefined is not a function: This is the bulletin part. With error 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 like format to Chrome. Errors from Firefox are similar, simply do not always include the first role, and recent versions of Net Explorer besides give simpler errors than Chrome – but in this example, simpler does not always mean improve.

Now onto the actual errors.

Uncaught TypeError: undefined is not a part

Related errors: number is not a function, object is non a role, cord is not a function, Unhandled Error: 'foo' is not a part, Office Expected

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

var foo = undefined; foo();

This error typically occurs if you are trying to telephone call a part in an object, just you typed the name incorrect.

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

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

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

How to set this error: Ensure the function proper noun is correct. With this error, the line number will usually signal at the right location.

Uncaught ReferenceError: Invalid left-manus 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 developer 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, and then similar y'all can run across in the above example, the left-hand side contains something you lot can't assign to, leading to the error.

How to fix this error: Brand certain you lot're not attempting to assign values to function 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: cyclic object value, Circular reference in value argument non supported

Always acquired 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 exist converted into JSON.

How to set up this fault: Remove circular references like in the example from any objects you want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after statement list

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

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

How to fix this mistake: Sometimes the line number with this error doesn't point to the correct identify, making it difficult to prepare.

• 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 often signal to something else than the problem grapheme
• Unexpected / is related to regular expressions. The line number for this will unremarkably be correct.
• Unexpected ; is unremarkably acquired past having a ; inside an object or array literal, or within the argument list of a function phone call. The line number volition usually exist right for this instance as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A cord literal is missing the endmost quote.

How to prepare this error: Ensure all strings take the correct endmost quote.

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

Related errors: TypeError: someVal is goose egg, Unable to get property 'foo' of undefined or nil reference

Attempting to read zippo or undefined equally if it was an object. For example:

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

How to fix this error: Usually caused 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 zero, Uncaught TypeError: Cannot set 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 it was an object. For example:

var someVal = null; someVal.foo = 1;

How to set this error: This too is usually caused by typos. Check the variable names near the line the error points to.

Uncaught RangeError: Maximum phone call stack size exceeded

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

Usually caused by a bug in program logic, causing space recursive function calls.

How to prepare 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 telephone call at the error'south line number gets right input.

XMLHttpRequest cannot load http://some/url/. No 'Access-Command-Let-Origin' header is present on the requested resources

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

This mistake is always caused by the usage of XMLHttpRequest.

How to set up this mistake: Ensure the request URL is correct and it respects the same-origin policy. A proficient way to find the offending code is to look at the URL in the fault bulletin and find it from your code.

InvalidStateError: An endeavor was made to use 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 call at the current land. Occurs ordinarily with XMLHttpRequest, when attempting to call functions on information technology earlier information technology's set up.

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

In this instance, you would get the mistake because the setRequestHeader function tin but exist called after calling xhr.open.

How to fix this fault: Wait at the lawmaking on the line pointed past the fault and make sure it runs at the correct fourth dimension, or add together any necessary calls before it (such as xhr.open)

Conclusion

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 start to make more than sense. Modern browsers besides help, as they no longer give the completely useless errors they used to.

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

Want to larn more than about these errors and how to foreclose them? Detect Issues 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 hush-hush 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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