Cleanup with finally

JavaScript course · Intermediate · Lesson 75/134

Learn finally for cleanup that runs whether the promise succeeded or failed.

Cleanup with finally

finally always runs when the promise settles, whether it fulfilled or rejected. It takes no parameter, because it cannot see the value. It is used only for cleanup: hiding a spinner, closing a file and so on.

loadOrder()
  .then(order => console.log(order))
  .catch(error => console.log(error))
  .finally(() => hideSpinner());

finally does not break the chain; the returned value passes to the next step. That is why it is added as the last step.

When do you use finally?

finally is most often used in two cases. First, to hide a loading spinner whether the work succeeded or failed. Second, to reset the interface after a network request finishes.

showSpinner();
loadOrder()
  .finally(() => hideSpinner());

The key point: finally does not catch errors, it only cleans up side effects. Error handling belongs to catch, cleanup belongs to finally.

Exercise A then, catch and finally chain

fetchReport returns a promise. Call fetchReport and build a chain: in then log the report with console.log; in catch log the error with console.log; in finally call hideSpinner.

Show example solution

fetchReport()
  .then(report => console.log(report))
  .catch(error => console.log(error))
  .finally(() => hideSpinner());

Test yourself

What is passed as a parameter to finally?
- No parameter
- The resolved value
- The rejection reason
- The promise object
Show answer

No parameter — Correct! finally takes no parameter; it only does cleanup.

Add finally to the chain

loadOrder()
  .then(order => console.log(order))
  .____(() => hideSpinner());

Show answer

loadOrder()
  .then(order => console.log(order))
  .finally(() => hideSpinner());

Match the three methods

then

Handles success
catch

Handles failure
finally

Runs in both cases

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Cleanup with finally