Uncaught Typeerror: Cannot Read Property 'length' of Undefined D3.csv
JavaScript Errors and How to Fix Them
JavaScript tin be a nightmare to debug: Some errors it gives can be very difficult to sympathize at beginning, and the line numbers given aren't e'er helpful either. Wouldn't it be useful to have a list where you could look to find out what they mean and how to fix them? Here yous go!
Beneath is a list of the foreign errors in JavaScript. Different browsers tin give you unlike messages for the aforementioned error, and then there are several different examples where applicable.
How to read errors?
Earlier the list, permit'due south quickly expect at the structure of an fault bulletin. Understanding the construction helps sympathize the errors, and yous'll have less trouble if you run into whatsoever 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 message is normally not very useful. Uncaught ways the error was not caught in a
take hold ofargument, andTypeErroris the error's name. - undefined is not a office: This is the message office. With mistake messages, yous take to read them very literally. For example in this case it literally means that the code attempted to use
undefinedsimilar it was a role.
Other webkit-based browsers, like Safari, requite errors in a like format to Chrome. Errors from Firefox are similar, but practice not always include the commencement role, and recent versions of Internet Explorer too give simpler errors than Chrome – just in this case, simpler does non always mean amend.
Now onto the actual errors.
Uncaught TypeError: undefined is not a function
Related errors: number is not a part, object is not a function, cord is not a function, Unhandled Error: 'foo' is not a part, Office Expected
Occurs when attempting to call a value similar a function, where the value is non a office. For example:
var foo = undefined; foo();
This error typically occurs if you are trying to phone call a function in an object, but y'all typed the proper noun wrong.
var ten = document.getElementByID('foo'); Since object properties that don't exist are undefined past default, the higher up would result in this error.
The other variations such as "number is not a office" occur when attempting to call a number like it was a function.
How to fix this error: Ensure the function proper noun is correct. With this fault, the line number will commonly betoken at the correct 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 nearly common example of this error is with if-clauses:
if(doSomething() = 'somevalue')
In this example, the programmer accidentally used a single equals instead of two. The message "left-hand side in assignment" is referring to the part on the left side of the equals sign, then like you can see in the above example, the left-manus side contains something you tin can't assign to, leading to the error.
How to ready this error: Make sure you're not attempting to assign values to function results or to the this keyword.
Uncaught TypeError: Converting round 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 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 above example have a reference to each other, the resulting object cannot be converted into JSON.
How to set up this mistake: 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 listing
The JavaScript interpreter expected something, but information technology 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 set up this error: Sometimes the line number with this error doesn't point to the correct place, making it difficult to ready.
- An error with [ ] { } ( ) is normally caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this case, line number will frequently point to something else than the trouble character
- 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 list of a function 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 endmost quote.
How to ready this error: Ensure all strings have the correct closing quote.
Uncaught TypeError: Cannot read holding 'foo' of nothing, Uncaught TypeError: Cannot read holding 'foo' of undefined
Related errors: TypeError: someVal is null, Unable to get property 'foo' of undefined or aught reference
Attempting to read nix or undefined as if it was an object. For example:
var someVal = aught; console.log(someVal.foo);
How to fix this fault: Normally caused by typos. Check that the variables used well-nigh the line number pointed by the error are correctly named.
Uncaught TypeError: Cannot set up property 'foo' of nix, Uncaught TypeError: Cannot set property 'foo' of undefined
Related errors: TypeError: someVal is undefined, Unable to set property 'foo' of undefined or zilch reference
Attempting to write nothing or undefined equally if it was an object. For example:
var someVal = aught; someVal.foo = i;
How to gear up this error: This too is usually caused by typos. Check the variable names nigh the line the error points to.
Uncaught RangeError: Maximum call stack size exceeded
Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, also much recursion, Stack overflow
Usually acquired by a issues in program logic, causing infinite recursive function calls.
How to set this error: 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 fault: Bank check that the decodeURIComponent call at the mistake's line number gets correct input.
XMLHttpRequest cannot load http://some/url/. No 'Access-Command-Allow-Origin' header is present on the requested resource
Related errors: Cantankerous-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 fix this fault: Ensure the request URL is correct and it respects the aforementioned-origin policy. A good way to notice the offending code is to expect at the URL in the mistake message and notice it from your code.
InvalidStateError: An attempt was made to utilize an object that is not, or is no longer, usable
Related errors: InvalidStateError, DOMException code 11
Means the code called a part that you should non call at the current state. Occurs usually with XMLHttpRequest, when attempting to call functions on it before information technology's ready.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val'); In this case, y'all would get the fault because the setRequestHeader function can only be called afterward calling xhr.open up.
How to fix this mistake: Look at the code on the line pointed by the mistake and make sure it runs at the correct time, or add together any necessary calls before it (such equally xhr.open)
Conclusion
JavaScript has some of the nearly 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 besides help, as they no longer give the completely useless errors they used to.
What'due south the well-nigh confusing mistake you've seen? Share the frustration in the comments!
Want 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 about JavaScript and high quality code on his site.
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Source: https://davidwalsh.name/fix-javascript-errors
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