React and Javascript and Uncaught Ype Error: Cannot Read Property Map of Undefined

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors information technology gives can exist 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 lot could look to find out what they hateful and how to fix them? Here you get!

Below is a listing of the strange errors in JavaScript. Different browsers tin requite you lot different messages for the same error, so in that location are several different examples where applicable.

How to read errors?

Before the list, allow's quickly look at the structure of an error message. Agreement the structure helps understand the errors, and you'll have less problem if you run into any errors not listed here.

A typical error from Chrome looks similar this:

Uncaught TypeError: undefined is not a function

The construction of the error is as follows:

1. Uncaught TypeError: This part of the bulletin is unremarkably not very useful. Uncaught means the error was not caught in a grab statement, and TypeError is the fault's name.
2. undefined is not a function: This is the message part. With error messages, yous have to read them very literally. For case in this instance it literally means that the code attempted to use undefined like it was a function.

Other webkit-based browsers, like Safari, requite errors in a similar format to Chrome. Errors from Firefox are similar, but do not e'er include the get-go part, and recent versions of Net Explorer also give simpler errors than Chrome – but in this example, simpler does not e'er hateful amend.

Now onto the actual errors.

Uncaught TypeError: undefined is not a function

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

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

var foo = undefined; foo();

This error typically occurs if you lot 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 result in this mistake.

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

How to fix this error: Ensure the office name is right. With this error, the line number will usually bespeak 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 about mutual instance of this mistake is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a single equals instead of two. The message "left-hand side in assignment" is referring to the part on the left side of the equals sign, so like y'all tin can see in the to a higher place instance, the left-hand side contains something y'all can't assign to, leading to the mistake.

How to fix this fault: Brand sure you're non 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: Non an acyclic Object, TypeError: cyclic object value, Circular reference in value argument not supported

Ever caused past 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 instance have a reference to each other, the resulting object cannot be converted into JSON.

How to prepare this mistake: Remove circular references like in the example from any objects you desire to catechumen into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after statement list

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

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

How to fix this error: Sometimes the line number with this error doesn't indicate to the right place, making it difficult to prepare.

• An error with [ ] { } ( ) is usually acquired by a mismatching pair. Bank check that all your parentheses and brackets have a matching pair. In this instance, line number will often point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this volition usually be correct.
• Unexpected ; is usually caused by having a ; inside an object or array literal, or inside the argument list of a function call. The line number will ordinarily be correct for this case as well

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 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 aught reference

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

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

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

Uncaught TypeError: Cannot set property 'foo' of nil, 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 equally if information technology was an object. For example:

var someVal = null; someVal.foo = i;

How to fix this error: This too is commonly caused by typos. Check the variable names most the line the error points to.

Uncaught RangeError: Maximum telephone 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 ready this mistake: 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 error: Check that the decodeURIComponent telephone call at the fault'southward line number gets correct input.

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

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

This error is always caused past the usage of XMLHttpRequest.

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

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

Related errors: InvalidStateError, DOMException code 11

Means the lawmaking called a function that you should not call at the current state. Occurs normally 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, yous would get the error because the setRequestHeader function can simply exist called after calling xhr.open up.

How to set this error: Look at the code on the line pointed past the fault and brand sure it runs at the correct fourth dimension, or add any necessary calls before it (such as xhr.open)

Conclusion

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, equally 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 learn more than about these errors and how to foreclose them? Notice Problems in JavaScript Automatically with ESLint.

About Jani Hartikainen

Jani Hartikainen has spent over 10 years building web applications. His clients include companies similar Nokia and hot super secret startups. When not programming or playing games, Jani writes about JavaScript and high quality lawmaking on his site.

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

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