With this error, the line number will usually point at the correct location. In this example, the programmer accidentally used a single equals instead of two. Always caused by a circular reference in an object, which is then passed into JSON.
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 fix this error: Remove circular references like in the example from any objects you want to convert into JSON. Typically caused by mismatched parentheses or brackets.
Attempting to read null or undefined as if it was an object. How to fix this error: Usually caused by typos. Check that the variables used near the line number pointed by the error are correctly named. Attempting to write null or undefined as if it was an object.
How to fix this error: This too is usually caused by typos. Check the variable names near the line the error points to. Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, too much recursion, Stack overflow.
How to fix this error: Check recursive functions for bugs that could cause them to keep recursing forever. How to fix this error: Ensure the request URL is correct and it respects the same-origin policy. A good way to find the offending code is to look at the URL in the error message and find it from your code. Means the code called a function that you should not call at the current state.
In this case, you would get the error because the setRequestHeader function can only be called after calling xhr. How to fix this error: Look at the code on the line pointed by the error and make sure it runs at the correct time, or add any necessary calls before it such as xhr.
With more familiarity the errors start to make more sense. Modern browsers also help, as they no longer give the completely useless errors they used to. Want to learn more about these errors and how to prevent them? About Jani Hartikainen Jani Hartikainen has spent over 10 years building web applications.
Also I'm glad that you also found a solution. Meaning,the element with id iii did not exist. Yes, man, actually, the 'main' bug that was bothering me was in call the "popularCombo" function.
There, I got the exception by passing the line. I don't know if I make myself clear, but that's what happened! Alan Oct 21 :. Ibrahim Diallo Oct 21 :. Hi Alan. You misspelled textnum1 , txtnum2 and textans. Instead you are missing the e. When all this is fixed, you'll realize that your code doesn't work.
If you enter the values 1 and 2, the answer will be The reason is you are adding strings together. You need to change the values to numbers. In your case, use parsefloat. Here is what your code will look like. Schady Oct 21 :. Ibrahima Diallo Oct 21 :. You checked if muterole is valid, but you haven't checked the value of mainrole. Just add the following. Note that you have to try to debug your code. Use console. Fatima Martinez Oct 23 :. I'm new on web developing and i'm very bad on scripts Does anybody knows which is the error?
It looks like the code you posted is not the generated html. Run your code on the browser and check the page source. You can also check on the console Press F12 to open the console. Since I cannot run your code, all I can recommend is to take the time to debug. What the error is telling you is that the form with id frmImport does not exist. There could be too many reasons for this.
But let's rewrite importarDoc like this:. Frank Dec 19 :. I have a problem with mi page, i debugged and got this error: "Uncaught TypeError: Cannot set property 'disabled' of null at ImprimeLista". Ibrahima Diallo Dec 23 :. I redacted your message because it was too much code. The error you are getting is exactly what I have described in the article.
It says it cannot set property disabled of null. Where is disabled used? Example: undefined. Example: undefined in Conditional Expression. Points to Remember :. A null value means absence. An undefined value means lack of value. A null or undefined value evalutes to false in conditional expression. Want to check how much you know JavaScript? Start JavaScript Test. In the App component, wrap the Router component with the Error component.
This permits the Error component to cascade down to any component of the app where the Error component is received as a CascadingParameter. Designate the Error component as a CascadingParameter in the code block. In an example Counter component in an app based on a Blazor project template, add the following Error property:. Call an error processing method in any catch block with an appropriate exception type.
The example Error component only offers a single ProcessError method, but the error processing component can provide any number of error processing methods to address alternative error processing requirements throughout the app. In the following Counter component example, an exception is thrown and trapped when the count is greater than five:. Using the preceding Error component with the preceding changes made to a Counter component, the browser's developer tools console indicates the trapped, logged error:.
If the ProcessError method directly participates in rendering, such as showing a custom error message bar or changing the CSS styles of the rendered elements, call StateHasChanged at the end of the ProcessErrors method to rerender the UI. Because the approaches in this section handle errors with a try-catch statement, a Blazor Server app's SignalR connection between the client and server isn't broken when an error occurs and the circuit remains alive.
Other unhandled exceptions remain fatal to a circuit. For more information, see the preceding section on how a Blazor Server app reacts to unhandled exceptions. If an unhandled exception occurs, the exception is logged to ILogger instances configured in the service container.
By default, Blazor apps log to console output with the Console Logging Provider. Consider logging to a location on the server or backend web API for Blazor WebAssembly apps with a provider that manages log size and log rotation. Native Application Insights features to support Blazor WebAssembly apps and native Blazor framework support for Google Analytics might become available in future releases of these technologies.
During development in a Blazor Server app, the app usually sends the full details of exceptions to the browser's console to aid in debugging.
In production, detailed errors aren't sent to clients, but an exception's full details are logged on the server. You must decide which incidents to log and the level of severity of logged incidents. Hostile users might be able to trigger errors deliberately. For example, don't log an incident from an error where an unknown ProductId is supplied in the URL of a component that displays product details.
Not all errors should be treated as incidents for logging. Blazor WebAssembly apps can trap and send error information on the client to a web API, which logs the error information to a persistent logging provider. Framework and app code may trigger unhandled exceptions in any of the following locations, which are described further in the following sections of this article:.
An error in an executed constructor or a setter for any [Inject] property results in an unhandled exception and stops the framework from instantiating the component. If the app is a Blazor Server app, the circuit fails. If constructor logic may throw exceptions, the app should trap the exceptions using a try-catch statement with error handling and logging.
During the lifetime of a component, Blazor invokes lifecycle methods. If any lifecycle method throws an exception, synchronously or asynchronously, the exception is fatal to a Blazor Server circuit. For components to deal with errors in lifecycle methods, add error handling logic. In the following example where OnParametersSetAsync calls a method to obtain a product:. The declarative markup in a Razor component file. When a component renders, BuildRenderTree executes and builds up a data structure describing the elements, text, and child components of the rendered component.
Rendering logic can throw an exception. An example of this scenario occurs when someObject. PropertyName is evaluated but someObject is null. For Blazor Server apps, an unhandled exception thrown by rendering logic is fatal to the app's circuit. To prevent a NullReferenceException in rendering logic, check for a null object before accessing its members. In the following example, person. Address properties aren't accessed if person. Address is null :.
The preceding code assumes that person isn't null. Often, the structure of the code guarantees that an object exists at the time the component is rendered. In those cases, it isn't necessary to check for null in rendering logic. In the prior example, person might be guaranteed to exist because person is created when the component is instantiated, as the following example shows:.
If the app calls code that could fail for external reasons, trap exceptions using a try-catch statement with error handling and logging. If an event handler throws an unhandled exception for example, a database query fails that isn't trapped and handled by developer code:.
A component may be removed from the UI, for example, because the user has navigated to another page. When a component that implements System. IDisposable is removed from the UI, the framework calls the component's Dispose method.
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