为什么 try..catch..finally 块的 finally 节在 catch 之前运行？

Question

try {
  throw new Error('yikes!');
} catch (e) {
  throw e;
} finally {
  console.log('caught error!')
}

Prints out:

caught error!
Error: yikes!

So is the finally block running before the throw statement?

It only looks like the finally block ran first, but we can see this isn't actually the case:

let failed;

try {
  throw new Error('yikes!');
  failed = false;
} catch (e) {
  failed = true;
  throw e;
} finally {
  console.log(`caught error! failed: ${failed}`)
}

prints

caught error! failed: true
Error: yikes!

So why is the printing of the error out of band? is there some asynchronous behaviour here that I'm not seeing?

Answer 1

1. Try runs - Your try runs and throws the error

2. Catch runs - Your catch catches the error and simply re-throws it.

3. Finally runs - You print out your string

4. The rethrown error is now uncaught - And your browser logs details about the error

Answer 2

The finally clause runs before the overall try block finishes. The error that's logged happens when the runtime intercepts the exception, which is after the try block finishes. (Note that your catch code does not console.log() anything. It throws a new exception, but the finally clause will still run before the rest of the world sees that.)

All languages with try catch finally that I know of behave in exactly the same way. The point of finally blocks is to provide a place to put code that is guaranteed to run whether the try succeeds or fails.

Answer 3

The throw statement doesn't print anything. The error is printed later when the browser catches the thrown error.