What is the purpose of the `finally` block?

TL;DR

The finally block in JavaScript is used to execute code after a try and catch block, regardless of whether an error was thrown or caught. It ensures that certain cleanup or finalization code runs no matter what. For example:

try {
  // Code that may throw an error
} catch (error) {
  // Code to handle the error
} finally {
  // Code that will always run
}

Purpose of the finally block

Ensuring cleanup

The finally block is often used to ensure that certain cleanup code runs regardless of whether an error occurred or not. This is useful for tasks like closing files, releasing resources, or resetting states.

try {
  // Attempt to execute code that may throw an error
  let data = fetchData();
} catch (error) {
  // Handle any errors that occur
  console.error('An error occurred:', error);
} finally {
  // Always execute this code
  console.log('Cleanup code runs here');
}

Guaranteeing execution

The finally block guarantees that the code within it will execute after the try and catch blocks have finished. This is true even if a return statement is encountered in the try or catch blocks.

function exampleFunction() {
  try {
    return 'Try block';
  } catch (error) {
    return 'Catch block';
  } finally {
    console.log('Finally block');
  }
}

console.log(exampleFunction()); // Output: 'Finally block' followed by 'Try block'

Handling asynchronous code

When dealing with asynchronous code, the finally block can be used to ensure that certain actions are taken after a promise is settled, regardless of its outcome.

fetch('https://api.example.com/data')
  .then((response) => response.json())
  .catch((error) => console.error('Fetch error:', error))
  .finally(() => console.log('Fetch attempt finished'));

Further reading

• MDN Web Docs: try...catch
• JavaScript.info: try...catch
• MDN Web Docs: Promise.prototype.finally()