Typeerror Cannot Read Property 'files' of Undefined

JavaScript Errors and How to Fix Them

JavaScript tin can be a nightmare to debug: Some errors information technology gives tin can exist very difficult to understand at kickoff, and the line numbers given aren't ever helpful either. Wouldn't it exist useful to have a list where you lot could await to discover out what they mean and how to fix them? Here you get!

Beneath is a list of the foreign errors in JavaScript. Different browsers can give y'all different letters for the same error, then there are several different examples where applicable.

How to read errors?

Before the listing, allow'due south quickly look at the structure of an error bulletin. Understanding the construction helps understand the errors, and you'll have less problem if you lot run into any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is non a function

The structure of the error is as follows:

1. Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the error was not caught in a grab statement, and TypeError is the error's name.
2. undefined is not a function: This is the message function. With error letters, you take to read them very literally. For example in this example information technology literally means that the code attempted to use undefined like information technology was a function.

Other webkit-based browsers, like Safari, give errors in a like format to Chrome. Errors from Firefox are similar, but practise not always include the starting time office, and contempo versions of Cyberspace Explorer as well give simpler errors than Chrome – but in this case, simpler does not e'er hateful ameliorate.

Now onto the actual errors.

Uncaught TypeError: undefined is not a part

Related errors: number is not a function, object is non a function, string is not a function, Unhandled Mistake: 'foo' is non a function, Role Expected

Occurs when attempting to call a value like a office, where the value is not a part. For example:

var foo = undefined; foo();

This error typically occurs if you lot are trying to call a function in an object, just you typed the name wrong.

var x = document.getElementByID('foo');

Since object properties that don't be are undefined by default, the higher up would result in this error.

The other variations such as "number is not a office" occur when attempting to phone call a number like it was a function.

How to fix this error: Ensure the function proper name is correct. With this error, the line number will usually bespeak 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 by 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 single equals instead of two. The message "left-manus side in assignment" is referring to the role on the left side of the equals sign, so similar you can see in the above case, the left-hand side contains something y'all 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 construction to JSON

Related errors: Uncaught exception: TypeError: JSON.stringify: Not an acyclic Object, TypeError: cyclic object value, Round reference in value argument not supported

Always caused past a circular reference in an object, which is and then passed into JSON.stringify.

var a = { }; var b = { a: a }; a.b = b; JSON.stringify(a);

Because both a and b in the to a higher place instance have a reference to each other, the resulting object cannot exist converted into JSON.

How to gear up this mistake: Remove round references like in the instance from any objects y'all desire to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument list

The JavaScript interpreter expected something, just information technology 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 fix this fault: Sometimes the line number with this error doesn't point to the correct place, making it difficult to fix.

• An fault with [ ] { } ( ) is ordinarily caused by a mismatching pair. Bank check that all your parentheses and brackets have a matching pair. In this case, line number will oftentimes point to something else than the problem graphic symbol
• Unexpected / is related to regular expressions. The line number for this volition usually be correct.
• Unexpected ; is usually caused by having a ; inside an object or array literal, or within the argument list of a function call. The line number will commonly exist correct for this example also

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to gear up this error: Ensure all strings have the correct closing quote.

Uncaught TypeError: Cannot read holding 'foo' of naught, Uncaught TypeError: Cannot read property 'foo' of undefined

Related errors: TypeError: someVal is null, Unable to get property 'foo' of undefined or cypher reference

Attempting to read cipher or undefined as if information technology was an object. For example:

var someVal = nix; console.log(someVal.foo);

How to fix this fault: Ordinarily caused by typos. Check that the variables used near the line number pointed by the error are correctly named.

Uncaught TypeError: Cannot set holding 'foo' of null, Uncaught TypeError: Cannot set property 'foo' of undefined

Related errors: TypeError: someVal is undefined, Unable to set property 'foo' of undefined or cypher reference

Attempting to write null or undefined as if it was an object. For example:

var someVal = nix; someVal.foo = i;

How to fix this error: This too is usually caused by typos. Check the variable names virtually the line the fault points to.

Uncaught RangeError: Maximum call stack size exceeded

Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, as well much recursion, Stack overflow

Unremarkably caused past a bug in program logic, causing infinite recursive part calls.

How to fix this mistake: Check recursive functions for bugs that could crusade them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused past an invalid decodeURIComponent call.

How to fix this error: Check that the decodeURIComponent call at the error's line number gets right input.

XMLHttpRequest cannot load http://some/url/. No 'Access-Command-Allow-Origin' header is nowadays on the requested resource

Related errors: Cross-Origin Asking Blocked: The Same Origin Policy disallows reading the remote resources at http://some/url/

This error is e'er acquired by the usage of XMLHttpRequest.

How to fix this fault: Ensure the request URL is correct and information technology respects the same-origin policy. A skilful mode to find the offending code is to look at the URL in the error bulletin and find it from your code.

InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

Related errors: InvalidStateError, DOMException code 11

Means the code called a function that you should non call at the current state. Occurs commonly with XMLHttpRequest, when attempting to call functions on it before it's ready.

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 after calling xhr.open.

How to fix this error: Look at the code on the line pointed by the error and make certain it runs at the correct time, or add any necessary calls earlier it (such equally xhr.open up)

Conclusion

JavaScript has some of the near unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors offset to make more sense. Modern browsers too help, every bit they no longer give the completely useless errors they used to.

What'southward the most confusing error you've seen? Share the frustration in the comments!

Want to learn more near these errors and how to forbid them? Observe Problems in JavaScript Automatically with ESLint.

Virtually Jani Hartikainen

Jani Hartikainen has spent over x years edifice web applications. His clients include companies similar Nokia and hot super surreptitious 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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