React Typeerror: Cannot Read Property 'available' of Undefined

JavaScript Errors and How to Fix Them

JavaScript tin can be a nightmare to debug: Some errors information technology gives can be very difficult to sympathise at offset, and the line numbers given aren't always helpful either. Wouldn't information technology be useful to accept a listing where you could look to find out what they mean and how to gear up them? Here you become!

Beneath is a list of the strange errors in JavaScript. Unlike browsers tin give you different messages for the same mistake, so there are several different examples where applicable.

How to read errors?

Before the list, permit's apace look at the structure of an fault bulletin. Understanding the construction helps understand the errors, and you'll have less trouble if you run into whatever errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is non a function

The construction of the mistake 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 catch statement, and TypeError is the fault'south name.
2. undefined is not a function: This is the message part. With error messages, you have to read them very literally. For case in this case it literally ways that the code attempted to use undefined similar it was a function.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, only practice not always include the beginning part, and contempo versions of Net Explorer also give simpler errors than Chrome – but in this instance, simpler does non always hateful ameliorate.

Now onto the bodily errors.

Uncaught TypeError: undefined is non a role

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

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

var foo = undefined; foo();

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

var 10 = document.getElementByID('foo');

Since object backdrop that don't be are undefined by default, the in a higher place would result in this mistake.

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

How to fix this fault: Ensure the role proper name is right. With this error, the line number will ordinarily betoken at the right 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 past attempting to assign a value to something that cannot be assigned to.

The nearly common example of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this case, the programmer accidentally used a single equals instead of 2. The bulletin "left-paw side in assignment" is referring to the part on the left side of the equals sign, so similar you can come across in the higher up example, the left-manus side contains something you tin't assign to, leading to the error.

How to fix this error: Make sure yous'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: Not an acyclic Object, TypeError: cyclic 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);

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

How to set this mistake: Remove circular references like in the case from any objects you lot want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument listing

The JavaScript interpreter expected something, just 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 gear up this error: Sometimes the line number with this mistake doesn't point to the correct place, making it difficult to fix.

• 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 signal to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will commonly be right.
• Unexpected ; is usually caused past having a ; inside an object or array literal, or inside the statement list of a function call. The line number volition normally be correct for this instance 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 set up this error: Ensure all strings have the correct endmost quote.

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

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

Attempting to read null or undefined every bit if it was an object. For instance:

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

How to fix this mistake: Usually caused by typos. Bank check that the variables used near the line number pointed past the error are correctly named.

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

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

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

var someVal = null; someVal.foo = 1;

How to fix this error: This besides is usually acquired by typos. Check the variable names near 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

Normally acquired by a problems in plan logic, causing infinite recursive function calls.

How to set this fault: Check recursive functions for bugs that could cause them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Acquired by an invalid decodeURIComponent call.

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

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

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

This error is always acquired past the usage of XMLHttpRequest.

How to ready this mistake: Ensure the request URL is correct and it respects the aforementioned-origin policy. A skillful manner to find the offending code is to expect at the URL in the error message and observe it from your code.

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

Related errors: InvalidStateError, DOMException code 11

Means the code called a role that you should not call at the current state. Occurs usually with XMLHttpRequest, when attempting to call functions on it before information technology'due south gear up.

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

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

How to fix this fault: Look at the code on the line pointed by the error and make sure information technology runs at the right time, or add any necessary calls before it (such every bit 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 make more sense. Modern browsers besides help, as they no longer requite the completely useless errors they used to.

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

Want to learn more nearly these errors and how to forestall them? Detect Problems in JavaScript Automatically with ESLint.

Well-nigh Jani Hartikainen

Jani Hartikainen has spent over 10 years building spider web applications. His clients include companies like Nokia and hot super undercover startups. When not 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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