The finally block

JavaScript course · Beginner · Lesson 120/133

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

All lessons in Beginner

1 Variables and data types

The finally block