What happens when there is a javascript runtime error?
I understand what causes runtime errors. I want to understand how the browser behaves afterwards. Will event handlers attached before the error still work? If a script loaded async finishes after a runtime error will it be able to execute? Basically, how catastrophic is a run-time error?
That depends on browser behavior and you have little or no control over that. If you have a runtime error do you best to resolve it, don’t try to measure how bad it is or rely on current situation that maybe works now, but may not work tomorrow. A runtime error means there’s a bug in the script. You need to fix it.
3 Answers 3
An uncaught runtime error only stops the current execution, which may be
Suppose you have a runtime error while handling an event, the only problem you might have (apart not really handling the event) is a non consistent state of your user variables if your event handler modifies some of them. Other event handlers won’t be impacted besides that.
So it can usually be considered as non catastrophic (I guess I don’t have to remember it’s a good practice to fix errors anyways and that flooding the console with errors isn’t a good thing).
You can use the try/catch/finally block. Using the catch block you can navigate inside the error and it will be running when a run-time error occurred or an illegal operation occurs. Visit this link, here is some more information about try/catch/finally http://www.javascriptkit.com/javatutors/trycatch.shtml
A run-time error can be catastrophic depends on where it happened and what does the piece of code when it occurs. In this way you can crash all the application or just stop some plugin.
Whenever a webpage is opened, all the scripts starts loading. If any of the scripts encounters any runtime error, then the execution of that script stops. That means any further statements in the scripts will not get executed by the browser. You can find these errors on the console window of the browser. However you can make use of try catch blocks to handle the exceptions occuring. For example, consider the following script:
No any further statement after statement var z=x/y; will execute because a runtime error occured.
JavaScript — Errors & Exceptions Handling
There are three types of errors in programming: (a) Syntax Errors, (b) Runtime Errors, and (c) Logical Errors.
Syntax Errors
Syntax errors, also called parsing errors, occur at compile time in traditional programming languages and at interpret time in JavaScript.
For example, the following line causes a syntax error because it is missing a closing parenthesis.
When a syntax error occurs in JavaScript, only the code contained within the same thread as the syntax error is affected and the rest of the code in other threads gets executed assuming nothing in them depends on the code containing the error.
Runtime Errors
Runtime errors, also called exceptions, occur during execution (after compilation/interpretation).
For example, the following line causes a runtime error because here the syntax is correct, but at runtime, it is trying to call a method that does not exist.
Exceptions also affect the thread in which they occur, allowing other JavaScript threads to continue normal execution.
Logical Errors
Logic errors can be the most difficult type of errors to track down. These errors are not the result of a syntax or runtime error. Instead, they occur when you make a mistake in the logic that drives your script and you do not get the result you expected.
You cannot catch those errors, because it depends on your business requirement what type of logic you want to put in your program.
The try. catch. finally Statement
The latest versions of JavaScript added exception handling capabilities. JavaScript implements the try. catch. finally construct as well as the throw operator to handle exceptions.
You can catch programmer-generated and runtime exceptions, but you cannot catch JavaScript syntax errors.
Here is the try. catch. finally block syntax −
The try block must be followed by either exactly one catch block or one finally block (or one of both). When an exception occurs in the try block, the exception is placed in e and the catch block is executed. The optional finally block executes unconditionally after try/catch.
Examples
Here is an example where we are trying to call a non-existing function which in turn is raising an exception. Let us see how it behaves without try. catch−
Click the following to see the result:
Output
Now let us try to catch this exception using try. catch and display a user-friendly message. You can also suppress this message, if you want to hide this error from a user.
Click the following to see the result:
Output
You can use finally block which will always execute unconditionally after the try/catch. Here is an example.
Click the following to see the result:
Output
The throw Statement
You can use throw statement to raise your built-in exceptions or your customized exceptions. Later these exceptions can be captured and you can take an appropriate action.
Example
The following example demonstrates how to use a throw statement.
Click the following to see the result:
Output
You can raise an exception in one function using a string, integer, Boolean, or an object and then you can capture that exception either in the same function as we did above, or in another function using a try. catch block.
The onerror() Method
The onerror event handler was the first feature to facilitate error handling in JavaScript. The error event is fired on the window object whenever an exception occurs on the page.
Click the following to see the result:
Output
The onerror event handler provides three pieces of information to identify the exact nature of the error −
- Error message − The same message that the browser would display for the given error
- URL − The file in which the error occurred
- Line number− The line number in the given URL that caused the error
Here is the example to show how to extract this information.
Example
Click the following to see the result:
Output
You can display extracted information in whatever way you think it is better.
You can use an onerror method, as shown below, to display an error message in case there is any problem in loading an image.
You can use onerror with many HTML tags to display appropriate messages in case of errors.