JavaScript，返回 promise 内箭头 function

Question

I've just seen this code:

const buildSW = () => {
  // This will return a Promise <- that comment was present on the code
  workboxBuild
    .injectManifest({
      swSrc: 'src/sw.js'
      // some code
    })
    .then(({ count, size, warnings }) => {
      // some code
    })
    .catch(console.error);
};
buildSW();

The code seems to be working, has been in production like 1 year, but I get confused with the comment of This will return a Promise because I don't see that there is actually a return statement, and the whole code is inside { }.

Shouldn't be?:

return workboxBuild
    .injectManifest({ ...etc

The code is part from this guides:

https://developers.google.com/web/tools/workbox/guides/generate-service-worker/workbox-build

where the return promise is present there. So how or why is working without return in the code showed above?

Answer 1

The comment is commenting on the line following it, and is redundant. Eg

// a() will return a Promise
a().then(() => () // duh

...because it's equivalent to:

const promise = a();
promise.then(() => ()

Shouldn't be?: return workboxBuild.injectManifest({...etc

It depends on the contract you want buildSW to have.

As written, buildSW returns nothing and handles its own errors, using .catch(console.error) to emit them to console. Appropriate for event handlers — Eg button.onclick = buildSW .

But if you expect to call buildSW from other places, a better contract is to return a promise, and leave error handling to the caller:

const buildSW = () => {
  return workboxBuild.injectManifest({swSrc: 'src/sw.js'})
    .then(({ count, size, warnings }) => {
      // some code that may also fail
    });
};
buildSW().catch(console.error);

Answer 2

I get confused with the comment because I don't see that there is actually a return statement, like it is present in the guide. Shouldn't it have one?

Yes, there should be a return statement. Sure, the function already catches errors and logs them, but it's still a good idea to always return a promise from asynchronous functions.

So how or why is working without return in the code showed above?

It's working only because the buildSW(); call you're doing doesn't try to use the promise that should be returned.