Lightsout.js:101 Uncaught Typeerror: Cannot Read Property 'id' of Undefined
JavaScript Errors and How to Set up Them
JavaScript can be a nightmare to debug: Some errors information technology gives tin be very difficult to understand at showtime, and the line numbers given aren't ever helpful either. Wouldn't it exist useful to have a list where you lot could look to find out what they mean and how to fix them? Hither you become!
Beneath is a listing of the foreign errors in JavaScript. Unlike browsers can give you unlike messages for the same error, so there are several different examples where applicable.
How to read errors?
Earlier the list, let'due south chop-chop look at the structure of an mistake message. Understanding the structure helps understand the errors, and you'll have less trouble if y'all run into whatever errors not listed here.
A typical error from Chrome looks like this:
Uncaught TypeError: undefined is not a function
The structure of the error is as follows:
- Uncaught TypeError: This part of the bulletin is ordinarily not very useful. Uncaught means the mistake was non caught in a
take hold of
statement, andTypeError
is the error's proper name. - undefined is not a function: This is the message part. With error messages, you have to read them very literally. For case in this case it literally ways that the lawmaking attempted to use
undefined
like information technology was a function.
Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, merely practice not always include the starting time part, and contempo versions of Net Explorer also give simpler errors than Chrome – only in this case, simpler does not e'er hateful better.
Now onto the bodily errors.
Uncaught TypeError: undefined is not a function
Related errors: number is not a function, object is not a function, string is not a function, Unhandled Mistake: 'foo' is non 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 mistake typically occurs if y'all are trying to phone call a part in an object, merely you typed the proper name wrong.
var ten = certificate.getElementByID('foo');
Since object backdrop that don't exist are undefined
by default, the to a higher place would result in this error.
The other variations such every bit "number is not a function" occur when attempting to telephone call a number like it was a function.
How to set up this mistake: Ensure the function proper noun is correct. With this error, the line number will usually point at the right 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 well-nigh common example of this error is with if-clauses:
if(doSomething() = 'somevalue')
In this case, the programmer accidentally used a single equals instead of two. The bulletin "left-mitt side in assignment" is referring to the part on the left side of the equals sign, so like you can meet in the above example, the left-hand side contains something you can't assign to, leading to the error.
How to fix this error: Make certain yous're not attempting to assign values to role 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, Round reference in value statement non supported
Always caused 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);
Because both a
and b
in the higher up case take a reference to each other, the resulting object cannot exist converted into JSON.
How to fix this fault: Remove circular references like in the example from any objects you want to convert into JSON.
Unexpected token ;
Related errors: Expected ), missing ) later on argument list
The JavaScript interpreter expected something, but it wasn't there. Typically caused by mismatched parentheses or brackets.
The token in this mistake tin vary – it 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 identify, making it difficult to fix.
- An error with [ ] { } ( ) is commonly caused past a mismatching pair. Bank check that all your parentheses and brackets have a matching pair. In this case, line number will ofttimes point to something else than the problem character
- Unexpected / is related to regular expressions. The line number for this will normally be correct.
- Unexpected ; is normally caused by having a ; inside an object or array literal, or within the argument list of a part call. The line number volition usually be right for this case also
Uncaught SyntaxError: Unexpected token ILLEGAL
Related errors: Unterminated Cord Literal, Invalid Line Terminator
A string literal is missing the closing quote.
How to prepare this mistake: Ensure all strings have the right closing quote.
Uncaught TypeError: Cannot read belongings 'foo' of null, Uncaught TypeError: Cannot read property 'foo' of undefined
Related errors: TypeError: someVal is naught, Unable to get belongings 'foo' of undefined or null reference
Attempting to read zippo
or undefined
as if it was an object. For example:
var someVal = null; console.log(someVal.foo);
How to fix this error: Usually caused by typos. Cheque that the variables used near the line number pointed past the error 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 holding 'foo' of undefined or null reference
Attempting to write null
or undefined
as if information technology was an object. For case:
var someVal = goose egg; someVal.foo = 1;
How to ready this error: This too is usually caused past typos. Bank check the variable names near the line the mistake points to.
Uncaught RangeError: Maximum telephone call stack size exceeded
Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, also much recursion, Stack overflow
Usually caused by a bug in programme logic, causing infinite recursive function 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 past an invalid decodeURIComponent phone call.
How to fix this fault: Check that the decodeURIComponent
call at the error'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 Same Origin Policy disallows reading the remote resource at http://some/url/
This fault is always acquired by the usage of XMLHttpRequest.
How to fix this error: Ensure the request URL is right and it respects the same-origin policy. A good way to notice the offending code is to look at the URL in the fault bulletin and find it from your lawmaking.
InvalidStateError: An effort was made to use an object that is not, or is no longer, usable
Related errors: InvalidStateError, DOMException code eleven
Means the code called a role that yous should not call at the current state. Occurs usually with XMLHttpRequest
, when attempting to phone call functions on information technology before it'southward ready.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val');
In this case, you would go the error because the setRequestHeader
part can only be called after calling xhr.open
.
How to fix this fault: Look at the code on the line pointed by the error and make sure it runs at the correct fourth dimension, or add any necessary calls before it (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 too help, equally they no longer give the completely useless errors they used to.
What's the well-nigh confusing error y'all've seen? Share the frustration in the comments!
Want to learn more than about these errors and how to prevent them? Detect Problems in JavaScript Automatically with ESLint.
About Jani Hartikainen
Jani Hartikainen has spent over 10 years building web applications. His clients include companies like Nokia and hot super hush-hush 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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