Why does a return in `finally` override `try`?

问题:

How does a return statement inside a try/catch block work?

function example() {
    try {
        return true;
    }
    finally {
        return false;
    }
}

I'm expecting the output of this function to be true, but instead it is false!

回答1:

Finally always executes. That's what it's for, which means it's return gets used in your case.

You'll want to change your code so it's more like this:

function example() { 
    var returnState = false; // initialisation value is really up to the design
    try { 
        returnState = true; 
    } 
    catch {
        returnState = false;
    }
    finally { 
        return returnState; 
    } 
} 

Generally speaking you never want to have more than one return statement in a function, things like this are why.

回答2:

According to ECMA-262 (5ed, December 2009), in pp. 96:

The production TryStatement : try Block Finally is evaluated as follows:

1. Let B be the result of evaluating Block.
2. Let F be the result of evaluating Finally.
3. If F.type is normal, return B.
4. Return F.

And from pp. 36:

The Completion type is used to explain the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value, target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript language value or empty, and target is any ECMAScript identifier or empty.

It's clear that return false would set completion type of finally as return, which cause try ... finally to do 4. Return F.

回答3:

When you use finally, any code within that block fires before the method exits. Because you're using a return in the finally block, it calls return false and overrides the previous return true in the try block.

(Terminology might not be quite right.)

回答4:

why you are getting false is you returned in a finally block. finally block should execute always. so your return true changes to return false

function example() {
    try {
        return true;
    }
    catch {
        return false;
    }
}

回答5:

As far as I know, the finally block always executes, irrespective of whether you have a return statement inside try or not. Ergo, you get the value returned by the return statement inside finally block.

I tested this with Firefox 3.6.10 and Chrome 6.0.472.63 both in Ubuntu. It is possible that this code may behave differently in other browsers.

回答6:

Finally is supposed to ALWAYS run at the end of a try catch block so that (by specification) is why you are getting false returned. Keep in mind that it is entirely possible that different browsers have different implementations.

回答7:

What about this?

doubleReturn();

function doubleReturn() {
  let sex = 'boy';

  try {
    return sex;

    console.log('this never gets called...');
  } catch (e) {} finally {
    sex = 'girl'; 

    alert(sex);
  }
}