Webpack Polyfills Uncaught Typeerror Cannot Read Property 'call' of Undefined

JavaScript Errors and How to Fix Them

JavaScript tin can be a nightmare to debug: Some errors it gives can be very difficult to empathize 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 detect out what they mean and how to fix them? Here you get!

Below is a list of the foreign errors in JavaScript. Unlike browsers can requite you different messages for the same mistake, and then in that location are several different examples where applicable.

How to read errors?

Before the listing, let's quickly look at the structure of an error message. Understanding the structure helps sympathise the errors, and you'll have less trouble if you come across any errors not listed hither.

A typical fault from Chrome looks similar this:

Uncaught TypeError: undefined is not a function

The structure of the error is as follows:

1. Uncaught TypeError: This part of the bulletin is usually non very useful. Uncaught ways the mistake was non caught in a grab statement, and TypeError is the error's proper noun.
2. undefined is not a part: This is the message office. With error messages, you lot have to read them very literally. For case in this case 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 ever include the first role, and recent versions of Cyberspace Explorer also requite simpler errors than Chrome – but in this case, simpler does not always mean better.

At present onto the actual errors.

Uncaught TypeError: undefined is non a function

Related errors: number is not a office, object is not a function, cord is not a function, Unhandled Error: 'foo' is not a office, Role Expected

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

var foo = undefined; foo();

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

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

Since object backdrop that don't be are undefined by default, the above would result in this fault.

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

How to set up this error: Ensure the role proper noun is correct. With this error, the line number will commonly 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 past attempting to assign a value to something that cannot exist assigned to.

The most common instance of this fault is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a single equals instead of two. The message "left-mitt side in assignment" is referring to the part on the left side of the equals sign, so similar you can see in the above example, the left-mitt side contains something you can't assign to, leading to the error.

How to set this error: Make sure y'all'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: Non an acyclic Object, TypeError: circadian object value, Round reference in value argument not supported

Always 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 example have a reference to each other, the resulting object cannot be converted into JSON.

How to fix this error: Remove circular references similar in the example from whatever objects you lot want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) later on statement list

The JavaScript interpreter expected something, simply 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 error: Sometimes the line number with this mistake doesn't signal to the right place, making it difficult to fix.

• An error with [ ] { } ( ) is normally caused by a mismatching pair. Cheque that all your parentheses and brackets have a matching pair. In this instance, line number will oftentimes indicate to something else than the problem grapheme
• Unexpected / is related to regular expressions. The line number for this will commonly be correct.
• Unexpected ; is ordinarily acquired by having a ; inside an object or array literal, or within the statement list of a role call. The line number volition commonly exist 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 fix this error: Ensure all strings take the correct closing quote.

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

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

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

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

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

Uncaught TypeError: Cannot set belongings 'foo' of zippo, Uncaught TypeError: Cannot set property 'foo' of undefined

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

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

var someVal = nil; someVal.foo = 1;

How to fix this error: This too is unremarkably caused by typos. Cheque the variable names near the line the error points to.

Uncaught RangeError: Maximum call stack size exceeded

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

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

How to fix this error: Bank check recursive functions for bugs that could cause them to go along recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Acquired by an invalid decodeURIComponent call.

How to fix this mistake: Check that the decodeURIComponent telephone call at the fault's 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 Aforementioned Origin Policy disallows reading the remote resources at http://some/url/

This error is always acquired by the usage of XMLHttpRequest.

How to set up this error: Ensure the request URL is correct and it respects the same-origin policy. A good mode to detect the offending code is to look at the URL in the fault message and find it from your code.

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

Related errors: InvalidStateError, DOMException code xi

Means the code called a role that you should non call at the current state. Occurs commonly with XMLHttpRequest, when attempting to call functions on information technology before it's ready.

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

In this case, yous would become the error because the setRequestHeader part can just be called afterward calling xhr.open.

How to fix this error: Await at the lawmaking on the line pointed by the error and make sure it runs at the correct time, or add any necessary calls before information technology (such as xhr.open)

Decision

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 sense. Modern browsers also assistance, as they no longer requite the completely useless errors they used to.

What's the virtually confusing fault you've seen? Share the frustration in the comments!

Want to acquire more well-nigh these errors and how to prevent them? Detect Bug in JavaScript Automatically with ESLint.

Almost Jani Hartikainen

Jani Hartikainen has spent over ten years edifice web applications. His clients include companies similar Nokia and hot super secret startups. When non programming or playing games, Jani writes near JavaScript and high quality code on his site.

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

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