JavaScript
The finally block
Learn finally for cleanup code that runs whether or not an error occurred.
The finally block
The finally block comes after try and catch and always runs whether or not an error occurred. This makes it ideal for cleanup: closing a file, clearing a timer or resetting a flag belongs in finally.
Example: try { riskyCode(); } catch (e) { console.error(e); } finally { cleanup(); }. Here cleanup() runs in both the success and error cases. finally cannot be skipped; even a return still runs finally first.
When to use finally?
finally is used especially for resource cleanup: closing files, cutting database connections, stopping timers. Even if return exits the function, finally still runs; so placing important cleanup code here is safe.
Example: you opened a file and an error occurred during processing. catch handles the error, but closing the file is a job for finally. This way the file always closes and no resource leak occurs. finally is optional; you can also write just try...catch.
Exercise Timer management with try/catch/finally
Define a function called runWithTimer. In a try block call console.time("op"), then call console.log("Running"). In a catch (e) block call console.error(e.message). In a finally block call console.timeEnd("op").
Show example solution
function runWithTimer() {
try {
console.time("op");
console.log("Running");
} catch (e) {
console.error(e.message);
} finally {
console.timeEnd("op");
}
}
Test yourself
-
When does the finally block run?
try { work(); } catch (e) { log(e); } finally { done(); }
- Only when try succeeds
- Only when catch ran
- Always, whether or not there is an error
- Only when throw is called
Show answer
Always, whether or not there is an error — Correct. finally always runs; the error state does not matter.
-
Add the finally block
try {
startTimer();
} catch (e) {
console.error(e);
} ____ {
stopTimer();
}
Show answer
try {
startTimer();
} catch (e) {
console.error(e);
} finally {
stopTimer();
}
-
Match finally usage scenarios
- File opening
- close in finally
- Timer start
- stop in finally
- Flag set
- reset in finally
- Temporary change
- undo in finally
Next lesson: Common error types
