Cannot Read Property Length of Undefined D3 Time.format
JavaScript Errors and How to Fix Them
JavaScript can exist a nightmare to debug: Some errors it gives can be very hard to understand at first, and the line numbers given aren't always helpful either. Wouldn't information technology be useful to have a list where you could look to find out what they mean and how to fix them? Here you go!
Below is a list of the strange errors in JavaScript. Different browsers can give yous dissimilar messages for the same fault, so at that place are several different examples where applicable.
How to read errors?
Before the listing, let'south chop-chop await at the structure of an error message. Agreement the structure helps understand the errors, and yous'll accept less trouble if y'all run into any errors not listed here.
A typical error from Chrome looks like this:
Uncaught TypeError: undefined is not a role
The structure of the error is as follows:
- Uncaught TypeError: This part of the message is ordinarily not very useful. Uncaught ways the error was non caught in a
catchstatement, andTypeErroris the error's proper noun. - undefined is non a function: This is the message part. With error messages, you take to read them very literally. For example in this example it literally ways that the code attempted to use
undefinedlike it was a part.
Other webkit-based browsers, similar Safari, give errors in a like format to Chrome. Errors from Firefox are similar, but exercise not always include the beginning part, and recent versions of Internet Explorer also requite simpler errors than Chrome – just in this case, simpler does not e'er mean amend.
At present onto the actual errors.
Uncaught TypeError: undefined is not a function
Related errors: number is not a function, object is non a function, string is not a function, Unhandled Fault: 'foo' is non a role, Part Expected
Occurs when attempting to call a value like a part, where the value is not a office. For example:
var foo = undefined; foo();
This error typically occurs if y'all are trying to telephone call a function in an object, merely you lot typed the name wrong.
var ten = document.getElementByID('foo'); Since object properties that don't exist are undefined by default, the higher up would effect in this error.
The other variations such every bit "number is not a function" occur when attempting to call a number like it was a role.
How to fix this error: Ensure the function proper noun is correct. With this fault, the line number will ordinarily 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 past attempting to assign a value to something that cannot be assigned to.
The near common example of this error is with if-clauses:
if(doSomething() = 'somevalue')
In this example, the programmer accidentally used a unmarried equals instead of ii. The message "left-hand side in consignment" is referring to the part on the left side of the equals sign, and then like you can see in the to a higher place instance, the left-manus side contains something you can't assign to, leading to the error.
How to fix this error: Make sure you'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: Not an acyclic Object, TypeError: cyclic object value, Circular reference in value argument non supported
Always 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 above instance take a reference to each other, the resulting object cannot exist converted into JSON.
How to fix this error: Remove circular references similar in the example from any objects y'all want to catechumen into JSON.
Unexpected token ;
Related errors: Expected ), missing ) after statement list
The JavaScript interpreter expected something, only information technology wasn't there. Typically caused past mismatched parentheses or brackets.
The token in this error can vary – information technology might say "Unexpected token ]" or "Expected {" etc.
How to gear up this fault: Sometimes the line number with this error doesn't point to the correct place, making it hard to fix.
- An error with [ ] { } ( ) is usually acquired by a mismatching pair. Cheque that all your parentheses and brackets have a matching pair. In this case, line number volition often signal to something else than the problem character
- Unexpected / is related to regular expressions. The line number for this will usually be correct.
- Unexpected ; is ordinarily caused past having a ; inside an object or assortment literal, or within the argument listing of a function phone call. The line number will usually be correct for this case as well
Uncaught SyntaxError: Unexpected token ILLEGAL
Related errors: Unterminated Cord Literal, Invalid Line Terminator
A string literal is missing the closing quote.
How to set up this mistake: Ensure all strings have 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 goose egg, Unable to become 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; panel.log(someVal.foo);
How to prepare 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 belongings 'foo' of null, Uncaught TypeError: Cannot ready holding 'foo' of undefined
Related errors: TypeError: someVal is undefined, Unable to set up belongings 'foo' of undefined or naught reference
Attempting to write null or undefined as if it was an object. For example:
var someVal = null; someVal.foo = i;
How to fix this mistake: This likewise is normally acquired past typos. Cheque 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, also much recursion, Stack overflow
Usually caused by a problems in program logic, causing infinite recursive function calls.
How to prepare this mistake: Cheque recursive functions for bugs that could cause them to continue recursing forever.
Uncaught URIError: URI malformed
Related errors: URIError: malformed URI sequence
Acquired by an invalid decodeURIComponent telephone call.
How to prepare this fault: Check that the decodeURIComponent call at the error'due south line number gets correct input.
XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is nowadays on the requested resources
Related errors: Cantankerous-Origin Request Blocked: The Aforementioned Origin Policy disallows reading the remote resource at http://some/url/
This error is always acquired 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 proficient style to find the offending code is to look at the URL in the error message and find it from your code.
InvalidStateError: An endeavor was made 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 you should not call at the current state. Occurs normally with XMLHttpRequest, when attempting to phone call functions on it earlier it'south ready.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val'); In this case, you would get the fault considering the setRequestHeader role tin can only exist called after calling xhr.open up.
How to fix this fault: Look at the lawmaking on the line pointed by the mistake and make sure it runs at the correct time, or add together whatsoever 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 familiarity the errors first to brand more sense. Modern browsers also help, as they no longer give the completely useless errors they used to.
What's the most confusing fault you've seen? Share the frustration in the comments!
Want to acquire more 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 secret startups. When not programming or playing games, Jani writes nigh JavaScript and high quality code on his site.
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Source: https://davidwalsh.name/fix-javascript-errors
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