Jquery Autocomplete Uncaught Typeerror Cannot Read Property 'label' of Null
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 it exist useful to accept a listing where you lot 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. Dissimilar browsers can give you dissimilar letters for the same error, so at that place are several different examples where applicable.
How to read errors?
Before the list, let's quickly look at the structure of an error message. Understanding the construction helps understand the errors, and you'll have less problem if y'all come across whatsoever errors not listed here.
A typical error from Chrome looks like this:
Uncaught TypeError: undefined is not a part
The construction of the error is equally follows:
- Uncaught TypeError: This office of the message is commonly not very useful. Uncaught ways the error was non caught in a
grabstatement, andTypeErroris the fault's proper name. - undefined is not a function: This is the message part. With fault messages, you have to read them very literally. For example in this example it literally ways that the code attempted to employ
undefinedlike it was a function.
Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, just do not always include the first part, and recent versions of Cyberspace Explorer also give simpler errors than Chrome – simply in this instance, simpler does not always mean better.
Now onto the actual errors.
Uncaught TypeError: undefined is non a function
Related errors: number is non a function, object is not a role, string is not a role, Unhandled Mistake: 'foo' is non a function, Office 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 mistake typically occurs if you are trying to telephone call a part in an object, merely y'all typed the name incorrect.
var x = certificate.getElementByID('foo'); Since object properties that don't be are undefined by default, the above would result in this fault.
The other variations such as "number is not a role" occur when attempting to phone call a number like information technology was a office.
How to fix this fault: Ensure the role name is correct. With this mistake, the line number will ordinarily point at the right 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 be assigned to.
The most mutual case 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-paw side in assignment" is referring to the role on the left side of the equals sign, so like you can see in the higher up case, the left-mitt side contains something you lot can't assign to, leading to the error.
How to fix this error: Make certain y'all'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 not supported
Always caused by a round 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 example have a reference to each other, the resulting object cannot be converted into JSON.
How to set up this error: Remove circular references like in the example from any objects you want to convert into JSON.
Unexpected token ;
Related errors: Expected ), missing ) afterward argument list
The JavaScript interpreter expected something, just 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 gear up this mistake: Sometimes the line number with this error doesn't point to the correct place, making it difficult to fix.
- An error with [ ] { } ( ) is usually caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this case, line number will often point to something else than the problem grapheme
- Unexpected / is related to regular expressions. The line number for this will usually be correct.
- Unexpected ; is usually caused past having a ; inside an object or array literal, or inside the argument listing of a function call. The line number will usually be correct for this case as well
Uncaught SyntaxError: Unexpected token ILLEGAL
Related errors: Unterminated String Literal, Invalid Line Terminator
A string literal is missing the closing quote.
How to fix this error: Ensure all strings have the correct endmost quote.
Uncaught TypeError: Cannot read property 'foo' of null, Uncaught TypeError: Cannot read property 'foo' of undefined
Related errors: TypeError: someVal is null, Unable to get property 'foo' of undefined or zilch reference
Attempting to read null or undefined as if it was an object. For example:
var someVal = null; console.log(someVal.foo);
How to fix this mistake: Usually caused by typos. Cheque that the variables used nearly the line number pointed by 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 nil reference
Attempting to write null or undefined as if it was an object. For instance:
var someVal = cypher; someVal.foo = 1;
How to fix this error: This too is usually acquired by typos. Bank check the variable names near the line the error points to.
Uncaught RangeError: Maximum call stack size exceeded
Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, likewise much recursion, Stack overflow
Usually caused by a problems in program logic, causing infinite recursive function calls.
How to set 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 by an invalid decodeURIComponent phone call.
How to fix this error: Check that the decodeURIComponent call at the error'southward line number gets right input.
XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is present on the requested resource
Related errors: Cantankerous-Origin Request Blocked: The Same Origin Policy disallows reading the remote resources at http://some/url/
This mistake is e'er caused past the usage of XMLHttpRequest.
How to prepare this mistake: Ensure the request URL is correct and it respects the same-origin policy. A adept way to find the offending code is to look at the URL in the error message and find it from your code.
InvalidStateError: An effort was fabricated to use an object that is non, or is no longer, usable
Related errors: InvalidStateError, DOMException code xi
Ways the code called a function that y'all should non call at the electric current state. Occurs usually with XMLHttpRequest, when attempting to telephone call functions on it before information technology'due south ready.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val'); In this case, you would become the error considering the setRequestHeader role can simply be chosen after calling xhr.open up.
How to fix this error: Await at the code on the line pointed past the error and make sure it runs at the right time, or add whatsoever necessary calls earlier 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 start to make more sense. Modern browsers also help, equally 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!
Desire to learn 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 spider web applications. His clients include companies like Nokia and hot super secret startups. When not programming or playing games, Jani writes most JavaScript and high quality lawmaking on his site.
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Source: https://davidwalsh.name/fix-javascript-errors
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