Uncaught Typeerror: Cannot Read Property '' of Null Throws at Lightning Salesforce

JavaScript Errors and How to Fix Them

JavaScript can exist a nightmare to debug: Some errors it gives can exist very hard to empathize at offset, and the line numbers given aren't always helpful either. Wouldn't it be useful to take a list where you could look to detect out what they mean and how to fix them? Here y'all become!

Beneath is a listing of the foreign errors in JavaScript. Different browsers can give you lot different messages for the same error, so there are several dissimilar examples where applicative.

How to read errors?

Before the list, let'south apace look at the structure of an error message. Agreement the construction helps understand the errors, and you'll have less trouble if you lot run into whatever errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is non a function

The structure of the error is as follows:

1. Uncaught TypeError: This part of the bulletin is normally not very useful. Uncaught means the error was not caught in a catch argument, and TypeError is the error'due south name.
2. undefined is non a function: This is the message part. With error messages, you accept to read them very literally. For instance in this instance it literally means that the code 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 similar, but do not always include the first part, and recent versions of Internet Explorer as well give simpler errors than Chrome – but in this case, simpler does non ever hateful better.

Now onto the bodily errors.

Uncaught TypeError: undefined is non a function

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

Occurs when attempting to call a value similar a office, where the value is non a part. For example:

var foo = undefined; foo();

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

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

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

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

How to fix this error: Ensure the function name is right. With this error, the line number volition usually signal at the correct location.

Uncaught ReferenceError: Invalid left-manus side in consignment

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 exist assigned to.

The virtually common case of this fault is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a unmarried equals instead of two. The bulletin "left-hand side in consignment" is referring to the office on the left side of the equals sign, and so like you can see in the above example, the left-hand side contains something you tin't assign to, leading to the error.

How to fix this error: Make certain 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

E'er 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 higher up case take a reference to each other, the resulting object cannot be converted into JSON.

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

Unexpected token ;

Related errors: Expected ), missing ) after statement listing

The JavaScript interpreter expected something, just it wasn't there. Typically acquired past 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 mistake doesn't point to the correct identify, making information technology difficult to set up.

• An error with [ ] { } ( ) is unremarkably acquired by a mismatching pair. Check that all your parentheses and brackets take a matching pair. In this instance, line number will ofttimes point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will normally 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 right for this case as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to fix this fault: Ensure all strings have 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 null, Unable to go property 'foo' of undefined or null reference

Attempting to read naught or undefined as if information technology was an object. For example:

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

How to set up this error: Normally acquired past typos. Check that the variables used virtually the line number pointed by the error are correctly named.

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

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

Attempting to write zip or undefined as if it was an object. For example:

var someVal = aught; someVal.foo = 1;

How to fix this error: This too is usually caused by typos. Cheque the variable names nearly the line the fault points to.

Uncaught RangeError: Maximum call stack size exceeded

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

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

How to gear up this error: Cheque 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 telephone call.

How to fix this mistake: Cheque that the decodeURIComponent phone call at the mistake'due south line number gets right input.

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

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

This error is always caused by the usage of XMLHttpRequest.

How to set up this fault: Ensure the request URL is right and information technology respects the aforementioned-origin policy. A proficient way to find the offending code is to look at the URL in the error bulletin and find it from your code.

InvalidStateError: An endeavour was made to use an object that is not, or is no longer, usable

Related errors: InvalidStateError, DOMException code xi

Means the code called a function that you should not call at the current land. Occurs usually with XMLHttpRequest, when attempting to call functions on it before it's ready.

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

In this case, y'all would get the error because the setRequestHeader office can only be called after calling xhr.open.

How to ready this error: Expect 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 up)

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 than familiarity the errors first to make more sense. Modern browsers too aid, as they no longer give the completely useless errors they used to.

What'south the near confusing error you've seen? Share the frustration in the comments!

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

About Jani Hartikainen

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

codeutopia.netjhartikainenPosts

ricocomplefro.blogspot.com

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

Share this post