Socket.io Typeerror: Cannot Read Property 'prototype' of Undefined

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors information technology gives can exist very hard to understand at first, and the line numbers given aren't e'er helpful either. Wouldn't it be useful to take a listing where you could look to find out what they mean and how to prepare them? Here you go!

Below is a listing of the strange errors in JavaScript. Different browsers can requite you dissimilar messages for the same error, and then there are several different examples where applicable.

How to read errors?

Before the listing, let's quickly expect at the structure of an fault bulletin. Understanding the construction helps sympathise the errors, and you'll have less trouble if you run into any errors non listed here.

A typical mistake from Chrome looks similar this:

Uncaught TypeError: undefined is non a function

The construction of the error is every bit follows:

1. Uncaught TypeError: This part of the message is ordinarily not very useful. Uncaught means the error was not caught in a catch statement, and TypeError is the error'due south name.
2. undefined is not a office: This is the message part. With fault messages, you have to read them very literally. For example in this instance 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 similar format to Chrome. Errors from Firefox are like, but do not always include the first role, and recent versions of Internet Explorer also give simpler errors than Chrome – merely in this case, simpler does not always mean better.

Now onto the actual errors.

Uncaught TypeError: undefined is not a function

Related errors: number is not a function, object is not a function, cord is non a function, Unhandled Mistake: '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 are trying to call a function in an object, but you typed the proper name wrong.

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

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

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

How to fix this error: Ensure the function proper noun is correct. With this error, the line number will commonly point at the correct 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'

Caused past attempting to assign a value to something that cannot exist assigned to.

The near common example of this fault is with if-clauses:

if(doSomething() = 'somevalue')

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

How to fix this error: Brand sure yous'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: Non an acyclic Object, TypeError: circadian object value, Round reference in value argument not supported

Always caused by a circular reference in an object, which is and so 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 prepare this mistake: Remove circular references similar in the example from whatsoever objects you want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument list

The JavaScript interpreter expected something, but it wasn't in that location. 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 fault doesn't point to the right place, making it difficult to ready.

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

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to set this error: Ensure all strings accept the correct closing quote.

Uncaught TypeError: Cannot read property 'foo' of zip, Uncaught TypeError: Cannot read holding 'foo' of undefined

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

Attempting to read zip or undefined as if information technology was an object. For instance:

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

How to gear up this error: Usually caused by typos. Check that the variables used near the line number pointed by the fault are correctly named.

Uncaught TypeError: Cannot set property 'foo' of null, 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 goose egg 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 caused 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

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

How to fix this error: Check recursive functions for bugs that could crusade them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused past an invalid decodeURIComponent call.

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

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

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

This fault is always acquired by the usage of XMLHttpRequest.

How to ready this error: Ensure the asking URL is correct and information technology respects the same-origin policy. A good manner to find the offending code is to look at the URL in the error bulletin and find it from your code.

InvalidStateError: An effort was made to employ an object that is non, or is no longer, usable

Related errors: InvalidStateError, DOMException code 11

Means the code called a function that you should non call at the current country. Occurs commonly with XMLHttpRequest, when attempting to call functions on it before it's prepare.

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

In this case, you would get the fault considering the setRequestHeader function can only be called afterwards calling xhr.open.

How to ready this error: 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 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 than familiarity the errors first to make more sense. Modern browsers also help, as they no longer requite the completely useless errors they used to.

What's the virtually disruptive error you've seen? Share the frustration in the comments!

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

Virtually Jani Hartikainen

Jani Hartikainen has spent over x years edifice web applications. His clients include companies like Nokia and hot super underground 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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