Error Typeerror: Cannot Read Property 'slice' of Undefined at Safesubscriber._next
JavaScript Errors and How to Set up Them
JavaScript can be a nightmare to debug: Some errors it gives can be very difficult to understand at first, and the line numbers given aren't e'er helpful either. Wouldn't it be useful to accept a list where you could wait to find out what they mean and how to fix them? Hither y'all go!
Below is a list of the strange errors in JavaScript. Different browsers can give you lot different messages for the aforementioned error, so in that location are several different examples where applicative.
How to read errors?
Before the list, let'southward quickly look at the construction of an error bulletin. Understanding the structure helps empathise the errors, and you lot'll have less trouble if you encounter any errors non listed here.
A typical error from Chrome looks similar this:
Uncaught TypeError: undefined is not a function
The structure of the error is equally follows:
- Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the mistake was not defenseless in a
catchstatement, andTypeErroris the error'south name. - undefined is non a role: This is the bulletin function. With error letters, yous have to read them very literally. For instance in this case information technology literally means that the code attempted to use
undefinedlike information technology was a function.
Other webkit-based browsers, similar Safari, requite errors in a similar format to Chrome. Errors from Firefox are similar, but practise not always include the commencement part, and recent versions of Internet Explorer also requite simpler errors than Chrome – but in this case, simpler does not always mean meliorate.
Now onto the actual errors.
Uncaught TypeError: undefined is not a function
Related errors: number is not a function, object is not a function, cord is non a role, Unhandled Mistake: 'foo' is not a function, Function Expected
Occurs when attempting to call a value similar a office, where the value is non a function. For example:
var foo = undefined; foo();
This error typically occurs if you are trying to phone call a part in an object, merely y'all typed the name incorrect.
var x = certificate.getElementByID('foo'); Since object properties that don't exist are undefined by default, the above would outcome in this error.
The other variations such as "number is not a function" occur when attempting to call a number like it was a office.
How to fix this mistake: Ensure the function name is correct. With this error, the line number will usually 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 by attempting to assign a value to something that cannot be assigned to.
The most common case of this error is with if-clauses:
if(doSomething() = 'somevalue')
In this example, the programmer accidentally used a unmarried equals instead of two. The message "left-paw side in assignment" is referring to the function on the left side of the equals sign, so like you can meet in the higher up example, the left-hand side contains something yous tin't assign to, leading to the fault.
How to fix this mistake: Make sure you're not attempting to assign values to part 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 by a round reference in an object, which is and so 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 error: Remove circular references like in the example from any objects you want to convert into JSON.
Unexpected token ;
Related errors: Expected ), missing ) after argument list
The JavaScript interpreter expected something, but it wasn't there. Typically caused past mismatched parentheses or brackets.
The token in this error can vary – it might say "Unexpected token ]" or "Expected {" etc.
How to gear up this fault: Sometimes the line number with this error doesn't point to the right place, making it difficult to gear up.
- An fault with [ ] { } ( ) is usually caused by a mismatching pair. Check that all your parentheses and brackets accept a matching pair. In this example, line number will often indicate to something else than the problem character
- Unexpected / is related to regular expressions. The line number for this volition commonly exist correct.
- Unexpected ; is ordinarily caused by having a ; inside an object or array literal, or inside the argument listing of a office call. The line number volition usually exist correct for this example also
Uncaught SyntaxError: Unexpected token ILLEGAL
Related errors: Unterminated String Literal, Invalid Line Terminator
A cord literal is missing the endmost quote.
How to fix this error: Ensure all strings have the correct closing quote.
Uncaught TypeError: Cannot read belongings 'foo' of aught, 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 null or undefined as if it was an object. For case:
var someVal = null; console.log(someVal.foo);
How to set up this mistake: Normally acquired by typos. Check that the variables used near the line number pointed by the fault are correctly named.
Uncaught TypeError: Cannot set property 'foo' of aught, Uncaught TypeError: Cannot set property 'foo' of undefined
Related errors: TypeError: someVal is undefined, Unable to ready property '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 too is usually caused 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, too much recursion, Stack overflow
Usually caused by a issues in program logic, causing infinite recursive part calls.
How to fix this error: Cheque 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 telephone call.
How to fix this mistake: Check that the decodeURIComponent phone call at the mistake's line number gets correct input.
XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is present on the requested resource
Related errors: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/
This error is ever caused by the usage of XMLHttpRequest.
How to prepare this fault: Ensure the asking URL is correct and it respects the aforementioned-origin policy. A practiced fashion to find the offending code is to look at the URL in the error bulletin and find information technology from your code.
InvalidStateError: An endeavor 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 part that you should non call at the current state. Occurs usually with XMLHttpRequest, when attempting to call functions on information technology before information technology'southward ready.
var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val'); In this case, you would get the error because the setRequestHeader office can merely be called afterward calling xhr.open.
How to fix this error: Look at the code on the line pointed past the error and make sure information technology runs at the correct time, or add any necessary calls before it (such equally xhr.open)
Determination
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 than familiarity the errors start to make more sense. Modern browsers likewise help, every bit they no longer give the completely useless errors they used to.
What's the virtually confusing error you lot've seen? Share the frustration in the comments!
Desire to learn more about these errors and how to preclude them? Detect Problems in JavaScript Automatically with ESLint.
About Jani Hartikainen
Jani Hartikainen has spent over 10 years building 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 code on his site.
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Source: https://davidwalsh.name/fix-javascript-errors
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