Cannot Read Property 'top' of Undefined Fullpage
JavaScript Errors and How to Fix Them
JavaScript tin can be a nightmare to debug: Some errors it gives can be very hard to sympathise at first, and the line numbers given aren't always helpful either. Wouldn't it be useful to have a list where yous could look to find out what they mean and how to fix them? Hither you go!
Below is a list of the strange errors in JavaScript. Dissimilar browsers can give you dissimilar messages for the same error, and then there are several unlike examples where applicative.
How to read errors?
Earlier the list, let'south apace look at the structure of an error message. Agreement the structure helps sympathise the errors, and you'll accept less trouble if you lot run into any errors not listed hither.
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 function of the bulletin is normally not very useful. Uncaught means the error was not caught in a
catchstatement, andTypeErroris the mistake's name. - undefined is not a function: This is the message part. With mistake messages, you have to read them very literally. For example in this instance it literally means that the code attempted to use
undefinedlike information technology was a function.
Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, but practise not e'er include the first part, and contempo versions of Internet Explorer also give simpler errors than Chrome – simply in this case, simpler does non always mean meliorate.
Now onto the bodily errors.
Uncaught TypeError: undefined is not a function
Related errors: number is not a role, object is not a function, string is not a function, Unhandled Mistake: 'foo' is not a function, Office Expected
Occurs when attempting to phone call a value similar a function, where the value is not a part. For instance:
var foo = undefined; foo();
This error typically occurs if you are trying to telephone call a function in an object, merely yous typed the name wrong.
var x = certificate.getElementByID('foo'); Since object properties that don't exist are undefined by default, the above would event in this error.
The other variations such as "number is not a function" occur when attempting to telephone call a number like it was a function.
How to fix this mistake: Ensure the office name is correct. With this mistake, the line number will usually point at the correct location.
Uncaught ReferenceError: Invalid left-paw side in assignment
Related errors: Uncaught exception: ReferenceError: Cannot assign to 'functionCall()', Uncaught exception: ReferenceError: Cannot assign to 'this'
Acquired by attempting to assign a value to something that cannot be assigned to.
The most common example of this fault is with if-clauses:
if(doSomething() = 'somevalue')
In this example, the programmer accidentally used a single equals instead of ii. The bulletin "left-hand side in assignment" is referring to the part on the left side of the equals sign, so like you lot can see in the in a higher place instance, the left-hand side contains something you lot tin can't assign to, leading to the mistake.
How to fix this error: Make sure you're not 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 statement not supported
Always caused past 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); Considering both a and b in the above example accept 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 whatsoever objects you want to catechumen into JSON.
Unexpected token ;
Related errors: Expected ), missing ) after argument list
The JavaScript interpreter expected something, but it wasn't there. Typically acquired 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 bespeak to the correct place, making it hard to gear up.
- An mistake with [ ] { } ( ) is ordinarily caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this case, line number will often signal to something else than the problem character
- Unexpected / is related to regular expressions. The line number for this volition usually be correct.
- Unexpected ; is usually acquired by having a ; inside an object or array literal, or within the argument listing of a function call. The line number will commonly exist correct for this case every bit well
Uncaught SyntaxError: Unexpected token ILLEGAL
Related errors: Unterminated String Literal, Invalid Line Terminator
A cord literal is missing the closing quote.
How to fix this mistake: Ensure all strings accept the correct closing 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 go holding 'foo' of undefined or cipher reference
Attempting to read zip or undefined as if information technology was an object. For example:
var someVal = null; panel.log(someVal.foo);
How to fix this mistake: Ordinarily caused by typos. Check that the variables used near the line number pointed by the error are correctly named.
Uncaught TypeError: Cannot set belongings 'foo' of zippo, Uncaught TypeError: Cannot set belongings 'foo' of undefined
Related errors: TypeError: someVal is undefined, Unable to set holding 'foo' of undefined or null reference
Attempting to write nix or undefined as if it was an object. For example:
var someVal = null; someVal.foo = 1;
How to fix this mistake: This too is normally acquired past typos. Check the variable names near 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 function calls.
How to fix this mistake: Check recursive functions for bugs that could cause them to go along 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 telephone call at the error's line number gets correct input.
XMLHttpRequest cannot load http://some/url/. No 'Admission-Control-Allow-Origin' header is nowadays on the requested resources
Related errors: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/
This fault is always caused by the usage of XMLHttpRequest.
How to gear up this fault: Ensure the asking URL is correct and it respects the same-origin policy. A good style to find the offending lawmaking is to look at the URL in the error bulletin and discover information technology from your lawmaking.
InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable
Related errors: InvalidStateError, DOMException lawmaking 11
Means the code chosen a function that you should not telephone call at the electric current state. Occurs usually with XMLHttpRequest, when attempting to call functions on information technology before it's gear up.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val'); In this case, you would get the error because the setRequestHeader function can only be called later 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 right time, or add together any necessary calls earlier it (such every bit xhr.open up)
Conclusion
JavaScript has some of the about unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors showtime to brand more sense. Modern browsers also help, as they no longer requite the completely useless errors they used to.
What's the near disruptive mistake you've seen? Share the frustration in the comments!
Desire to larn more about these errors and how to prevent them? Find Bug in JavaScript Automatically with ESLint.
About Jani Hartikainen
Jani Hartikainen has spent over ten years building web applications. His clients include companies similar Nokia and hot super secret startups. When not programming or playing games, Jani writes near JavaScript and high quality lawmaking on his site.
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Source: https://davidwalsh.name/fix-javascript-errors
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