Typeerror Cannot Read Property 'bind' of Undefined Unit Tests
JavaScript Errors and How to Set up Them
JavaScript can be a nightmare to debug: Some errors it gives tin exist very hard to sympathize at first, and the line numbers given aren't always helpful either. Wouldn't it exist useful to have a list where you could wait to discover out what they mean and how to fix them? Hither you go!
Beneath is a list of the strange errors in JavaScript. Different browsers can requite you unlike letters for the aforementioned error, then in that location are several dissimilar examples where applicable.
How to read errors?
Before the list, let's apace look at the structure of an mistake message. Understanding the structure helps empathise the errors, and you'll accept less problem if you run into any errors non listed hither.
A typical error from Chrome looks similar this:
Uncaught TypeError: undefined is not a role
The structure of the mistake is equally follows:
- Uncaught TypeError: This part of the message is unremarkably not very useful. Uncaught means the error was non defenseless in a
catchstatement, andTypeErroris the error's proper name. - undefined is not a function: This is the message part. With mistake messages, y'all take to read them very literally. For example in this case it literally ways that the lawmaking attempted to apply
undefinedlike information technology was a function.
Other webkit-based browsers, like Safari, give errors in a like format to Chrome. Errors from Firefox are like, but practise non always include the first function, and contempo versions of Net Explorer also give simpler errors than Chrome – only in this case, simpler does not always mean better.
Now onto the actual errors.
Uncaught TypeError: undefined is not a function
Related errors: number is non a function, object is not a role, string is not a function, Unhandled Error: 'foo' is non a part, Function Expected
Occurs when attempting to phone call a value like a function, where the value is not a office. For example:
var foo = undefined; foo();
This fault typically occurs if you are trying to phone call a office in an object, simply you typed the proper noun wrong.
var x = certificate.getElementByID('foo'); Since object backdrop that don't exist are undefined by default, the in a higher place would result in this mistake.
The other variations such as "number is not a role" occur when attempting to call a number like information technology was a function.
How to fix this fault: Ensure the office name is correct. With this error, the line number will unremarkably point at the right 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'
Caused by attempting to assign a value to something that cannot be assigned to.
The most common instance of this fault is with if-clauses:
if(doSomething() = 'somevalue')
In this instance, the programmer accidentally used a single equals instead of two. The bulletin "left-hand side in consignment" is referring to the part on the left side of the equals sign, and so like y'all tin see in the above example, the left-mitt side contains something you can't assign to, leading to the error.
How to set up this error: Make certain you're not attempting to assign values to part 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, Round reference in value argument 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 exist converted into JSON.
How to gear up this error: Remove circular references like in the example from any objects you want to catechumen into JSON.
Unexpected token ;
Related errors: Expected ), missing ) after statement listing
The JavaScript interpreter expected something, but information technology wasn't there. Typically caused by mismatched parentheses or brackets.
The token in this mistake can vary – it might say "Unexpected token ]" or "Expected {" etc.
How to ready this fault: Sometimes the line number with this error doesn't point to the correct identify, making it difficult to gear up.
- An error 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 oftentimes point to something else than the problem character
- Unexpected / is related to regular expressions. The line number for this will usually be right.
- Unexpected ; is commonly caused by having a ; within an object or array literal, or within the statement list of a function call. The line number will commonly be correct for this case also
Uncaught SyntaxError: Unexpected token ILLEGAL
Related errors: Unterminated Cord Literal, Invalid Line Terminator
A cord literal is missing the endmost quote.
How to prepare 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 null reference
Attempting to read nada or undefined as if it was an object. For example:
var someVal = nada; console.log(someVal.foo);
How to set up this error: Usually caused past typos. Check that the variables used most the line number pointed by the error are correctly named.
Uncaught TypeError: Cannot set property 'foo' of null, Uncaught TypeError: Cannot ready property 'foo' of undefined
Related errors: TypeError: someVal is undefined, Unable to set belongings 'foo' of undefined or null reference
Attempting to write null or undefined as if it was an object. For example:
var someVal = null; someVal.foo = 1;
How to fix this error: This also 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 by a bug in program logic, causing infinite recursive function calls.
How to prepare this fault: Bank check recursive functions for bugs that could cause 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 mistake: Check that the decodeURIComponent phone call at the error's line number gets correct input.
XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is present on the requested resources
Related errors: Cantankerous-Origin Request Blocked: The Same 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 mistake: Ensure the request URL is correct and it respects the aforementioned-origin policy. A skilful way to find the offending code is to await at the URL in the fault message and find it from your lawmaking.
InvalidStateError: An attempt was fabricated to use an object that is non, or is no longer, usable
Related errors: InvalidStateError, DOMException code 11
Means the code called a function that you should not phone call at the current state. Occurs commonly with XMLHttpRequest, when attempting to telephone call functions on information technology before it's set.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val'); In this case, y'all would go the error because the setRequestHeader function can just be called subsequently calling xhr.open up.
How to gear up this mistake: Await at the code on the line pointed by the mistake and make sure it runs at the correct time, or add any necessary calls before it (such as xhr.open)
Conclusion
JavaScript has some of the nigh unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more than familiarity the errors start to make more than sense. Modern browsers besides assist, as they no longer requite the completely useless errors they used to.
What'south the most confusing mistake you've seen? Share the frustration in the comments!
Want to larn more about these errors and how to prevent them? Detect Bug in JavaScript Automatically with ESLint.
About Jani Hartikainen
Jani Hartikainen has spent over 10 years edifice web applications. His clients include companies like Nokia and hot super secret startups. When non programming or playing games, Jani writes well-nigh JavaScript and loftier quality lawmaking on his site.
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Source: https://davidwalsh.name/fix-javascript-errors
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