Jest Typeerror: Cannot Read Property 'equal' of Undefined

JavaScript Errors and How to Gear up Them

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

Below is a listing of the strange errors in JavaScript. Different browsers tin give you different messages for the aforementioned error, then there are several different examples where applicative.

How to read errors?

Before the list, permit'south quickly look at the structure of an error bulletin. Understanding the structure helps sympathise the errors, and you'll have less trouble if y'all run across any errors not listed here.

A typical error from Chrome looks similar this:

Uncaught TypeError: undefined is non a part

The structure of the mistake is as follows:

1. Uncaught TypeError: This part of the bulletin is usually not very useful. Uncaught means the fault was not defenseless in a catch statement, and TypeError is the fault'southward proper noun.
2. undefined is not a function: This is the message part. With mistake letters, you have to read them very literally. For example in this case information technology literally ways that the code attempted to utilize undefined like it was a function.

Other webkit-based browsers, similar Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, but do non always include the first part, and contempo versions of Internet Explorer also give simpler errors than Chrome – but in this case, simpler does not always hateful improve.

At present onto the bodily 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 Error: 'foo' is not a function, Function Expected

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

var foo = undefined; foo();

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

var 10 = 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 office" 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 ordinarily bespeak 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 by attempting to assign a value to something that cannot exist assigned to.

The most mutual example of this mistake is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a unmarried equals instead of ii. The message "left-paw side in assignment" is referring to the part on the left side of the equals sign, and then like yous can run into in the higher up example, the left-hand side contains something yous can't assign to, leading to the error.

How to prepare this error: Make sure you're non attempting to assign values to office 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

Always caused by a circular reference in an object, which is 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 example accept a reference to each other, the resulting object cannot be converted into JSON.

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

Unexpected token ;

Related errors: Expected ), missing ) after statement listing

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

The token in this mistake tin vary – information technology might say "Unexpected token ]" or "Expected {" etc.

How to fix this error: Sometimes the line number with this error doesn't point to the correct place, making it difficult to ready.

• An error with [ ] { } ( ) is usually acquired past a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this example, line number will oftentimes point to something else than the problem grapheme
• Unexpected / is related to regular expressions. The line number for this will commonly exist correct.
• Unexpected ; is usually caused past having a ; inside an object or array literal, or inside the argument list of a function call. The line number will usually be correct for this case also

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the endmost quote.

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

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

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

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

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

How to fix this error: Unremarkably caused by typos. Check that the variables used near the line number pointed by the error are correctly named.

Uncaught TypeError: Cannot gear up property 'foo' of nada, 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 nada or undefined as if it was an object. For example:

var someVal = null; someVal.foo = 1;

How to fix this error: This as well is usually caused by typos. Check the variable names nigh the line the mistake 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 part calls.

How to fix this error: Check recursive functions for bugs that could cause them to go on 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 call at the error'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 Asking Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/

This error is ever caused by the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is right and information technology respects the same-origin policy. A good way to notice the offending lawmaking is to look at the URL in the error message and observe it from your code.

InvalidStateError: An attempt was made 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 telephone call at the electric current country. Occurs usually with XMLHttpRequest, when attempting to call functions on it before it'south ready.

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

In this case, yous would become the mistake because the setRequestHeader part can only exist called after calling xhr.open.

How to fix this error: Look at the code on the line pointed by the error and make certain information technology runs at the right fourth dimension, or add any necessary calls before information technology (such as xhr.open up)

Determination

JavaScript has some of the nearly unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors showtime to make more sense. Mod browsers also assistance, equally they no longer give the completely useless errors they used to.

What's the most confusing mistake you've seen? Share the frustration in the comments!

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

About Jani Hartikainen

Jani Hartikainen has spent over 10 years edifice web applications. His clients include companies like Nokia and hot super secret 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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