A try without a catch clause sends its error to the next higher catch, or the window, if there is no catch defined within that try.
If you do not have a catch, a try expression requires a finally clause.
try {
// whatever;
} finally {
// always runs
}
A try without a catch clause sends its error to the next higher catch, or the window, if there is no catch defined within that try.
If you do not have a catch, a try expression requires a finally clause.
try {
// whatever;
} finally {
// always runs
}
It's possible to have an empty catch block, without an error variable, starting with ES2019. This is called optional catch binding and was implemented in V8 v6.6, released in June 2018. The feature has been available since Node 10, Chrome 66, Firefox 58, Opera 53 and Safari 11.1.
The syntax is shown below:
try {
throw new Error("This won't show anything");
} catch { };You still need a catch block, but it can be empty and you don't need to pass any variable. If you don't want a catch block at all, you can use the try/finally, but note that it won't swallow errors as an empty catch does.
try {
throw new Error("This WILL get logged");
} finally {
console.log("This syntax does not swallow errors");
}I want to try action A, and if it fails try action B, and if it fails, try action C. I need to do that without having to specify a "catch". Is that possible? Thanks!
language agnostic - Why use try … finally without a catch clause? - Software Engineering Stack Exchange
[AskJS] I just found out that "finally" block runs even when "try" / "catch" block returns
javascript - What is the point of finally in a try..catch? - Stack Overflow
Using `try`/`finally` without `catch` - Question - Scala Users
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It depends on whether you can deal with the exceptions that can be raised at this point or not.
If you can handle the exceptions locally you should, and it is better to handle the error as close to where it is raised as possible.
If you can't handle them locally then just having a try / finally block is perfectly reasonable - assuming there's some code you need to execute regardless of whether the method succeeded or not. For example (from Neil's comment), opening a stream and then passing that stream to an inner method to be loaded is an excellent example of when you'd need try { } finally { }, using the finally clause to ensure that the stream is closed regardless of the success or failure of the read.
However, you will still need an exception handler somewhere in your code - unless you want your application to crash completely of course. It depends on the architecture of your application exactly where that handler is.
The finally block is used for code that must always run, whether an error condition (exception) occurred or not.
The code in the finally block is run after the try block completes and, if a caught exception occurred, after the corresponding catch block completes. It is always run, even if an uncaught exception occurred in the try or catch block.
The finally block is typically used for closing files, network connections, etc. that were opened in the try block. The reason is that the file or network connection must be closed, whether the operation using that file or network connection succeeded or whether it failed.
Care should be taken in the finally block to ensure that it does not itself throw an exception. For example, be doubly sure to check all variables for null, etc.
Even though I've been writing JS for years, I never realized that "finally" block always runs within the function, even when "try" / "catch" block returns a value
function testFinally() {
try {
console.log("try block");
return "return value";
} finally {
console.log("finally called");
}
}
console.log(testFinally());Running this will log:
try block finally called return value
Very useful in case you have to run a callback that may fail, but you have to do some required setup / teardown around the callback. Suppose you wanna do something like this:
let isCallbackRunning = false;
function runCallback(callback) {
if (!isCallbackRunning) {
isCallbackRunning = true;
const result = callback();
isCallbackRunning = false;
return result;
}
}
Well, the problem is, if callback() throws, isCallbackRunning = false will not run, which means when you call runCallback next time, it will permanently get stuck and not actually run the callback.
So we really need to maintain isCallbackRunning correct at all times, regardless of whether callback function throws or not. So the way to do this is this:
function runCallback(callback) {
if (!isCallbackRunning) {
isCallbackRunning = true;
try {
return callback();
} finally {
isCallbackRunning = false;
}
}
}
What's curious is that, imagine callback() doesn't throw. Even though the result is immediately returned in the try block, finally block will actually run right before that, ensuring our teardown still gets called and making code simpler.
finally basically runs even if you have an early-return from try-catch or even if you don't handle the error in the try-catch. Here is an example I like:
function myFunction() {
try {
console.log('inside "try"');
return
} finally {
console.log('inside "finally"');
}
console.log("after try-finally");
}
myFunction()When you run myFunction(), it will print the following:
inside "try"
inside "finally"
Since you returned from try, it didn't execute any instructions after the try-finally block. But, Javascript did execute the finally block.
Thies are example codes in MDN web docs to explain the important of finally block and it behaviour.
openMyFile();
try {
// tie up a resource
writeMyFile(theData);
} finally {
closeMyFile(); // always close the resource
}
Accordig to the MDN documentation finally execute no matter what the logic fail or not inside the try block.
In most cases inside a function there are code blocks must execute before workflow end.
Ex: no matter what if file is get opened it must close before exit from workflow.
function doIt() {
try {
return 1;
} finally {
return 2;
}
}
doIt(); // returns 2
In above code whether it should return 1. It will return 2 because it guarantee the execution of code inside finally block.But according to MDN this is a bad implementation.
Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/try...catch