Uncaught Typeerror: Cannot Read Property 'draw' of Null

JavaScript Errors and How to Set up Them

JavaScript can exist a nightmare to debug: Some errors it gives can be very difficult to understand at offset, and the line numbers given aren't always helpful either. Wouldn't it be useful to take a listing where you could look to detect 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 give you different messages for the same error, and so there are several dissimilar examples where applicative.

How to read errors?

Before the listing, let'south quickly look at the construction of an error message. Understanding the structure helps empathise the errors, and yous'll have less trouble if you run into any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is non a function

The structure of the error is as follows:

1. Uncaught TypeError: This part of the message is commonly not very useful. Uncaught means the fault was not defenseless in a grab statement, and TypeError is the error's name.
2. undefined is not a function: This is the message office. With error messages, you take to read them very literally. For instance in this case 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, only practise not ever include the first function, and recent versions of Net Explorer also give simpler errors than Chrome – but in this instance, simpler does not always hateful meliorate.

At present onto the bodily errors.

Uncaught TypeError: undefined is not a role

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

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

var foo = undefined; foo();

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

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

Since object properties that don't exist are undefined by default, the to a higher place would outcome in this error.

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 name is right. With this error, the line number will usually point at the correct location.

Uncaught ReferenceError: Invalid left-hand side in consignment

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 most common 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-mitt side in assignment" is referring to the part on the left side of the equals sign, and so like you tin encounter in the above example, the left-hand side contains something you tin't assign to, leading to the error.

How to fix this error: Make certain y'all'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: Not an acyclic Object, TypeError: cyclic object value, Circular reference in value argument not supported

E'er acquired 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);

Considering both a and b in the to a higher place case have a reference to each other, the resulting object cannot exist converted into JSON.

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

Unexpected token ;

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

The JavaScript interpreter expected something, but 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 error doesn't signal to the correct place, making it difficult to fix.

• An mistake with [ ] { } ( ) is commonly caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this example, line number will often point to something else than the problem character
• Unexpected / is related to regular expressions. The line number for this will ordinarily be correct.
• Unexpected ; is commonly caused by having a ; inside an object or array literal, or within the statement list of a office call. The line number will usually be correct for this example as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated Cord Literal, Invalid Line Terminator

A string literal is missing the endmost quote.

How to fix this error: Ensure all strings take the correct closing quote.

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

Related errors: TypeError: someVal is goose egg, Unable to get belongings 'foo' of undefined or zilch reference

Attempting to read null or undefined as if it was an object. For case:

var someVal = cypher; panel.log(someVal.foo);

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

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

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

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

var someVal = null; someVal.foo = ane;

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

Uncaught RangeError: Maximum call stack size exceeded

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

Normally caused by a bug in program logic, causing infinite recursive function calls.

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

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent telephone call.

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

XMLHttpRequest cannot load http://some/url/. No 'Access-Command-Allow-Origin' header is nowadays on the requested resources

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

This mistake is always caused by the usage of XMLHttpRequest.

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

InvalidStateError: An attempt was fabricated to use an object that is non, or is no longer, usable

Related errors: InvalidStateError, DOMException code 11

Ways the code called a function that yous should non phone call at the electric current state. Occurs ordinarily with XMLHttpRequest, when attempting to call functions on it before it'southward ready.

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

In this case, you would become the fault considering the setRequestHeader function tin can only be called afterward calling xhr.open.

How to ready this mistake: Look at the code on the line pointed by the error and brand 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 most unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more than familiarity the errors start to make more than sense. Modernistic browsers also help, equally they no longer requite the completely useless errors they used to.

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

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

About Jani Hartikainen

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

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