Typescript: static return might fail with switch statement

[ORESoftware]

I have this:

const onUpdate = function (v: string, o: any) : Promise<any> {

  const finalName = String(v || '').split('.')[1]; // collection name
  const _id = o && o.o2 && o.o2._id;
  let key;

  switch (finalName) {

    case 'categories':      
      return Category.findOne({_id}).exec().then(function (cat: any) {
         // ....
      });

    case 'acquisitions':
        return Acquisition.findOne({_id}).exec().then(function (acq: any) {
         // ...
      });

    case 'functionalTeams':
      return FunctionalGroup.findOne({_id}).exec().then(function (ft: any) {
           // ...
      });

    default:
      log.error('document collection did not match a pre-defined name');
      // nothing is returned here, but can TS check to see if it does/doesn't
  }

};

can TypeScript check to see if a Promise fails to be returned in the default or after the switch statement? Right now it's compiling even though a Promise is not returned in all cases.

[andy-ms]

You probably just need --strictNullChecks enabled.

[MartinJohns]

Or alternatively --noImplicitReturns. Enabling --strictNullChecks is often very difficult to add in old code bases, but --noImplicitReturns is a no-brainer.

[ORESoftware]

@andy-ms yeah that's fine, I guess I would expect the default behavior to always catch this one tho?

[MartinJohns]

The default behaviour is to align with JavaScript. If a function returns "nothing", it implicitly returns undefined. That is what is happening here. And with strictNullChecks disabled, the value of undefined is compatible with the type Promise<any>, the value is compatible with any type.

strictNullChecks will change the semantics of undefined from a mere value to a type, so the value undefined will be of the type undefined, which is not compatible with Promise<any> anymore. You will get an error.

noImplicitReturns will enforce what the name implies: A method may not have an implicit return value. That means you have to explicitly write return undefined;.

[ORESoftware]

ok understood, to move this in a more productive direction, is there a way to never allow null/undefined in the place of a defined value? In Java, all those NullPointerException's are caused by allowing null to exist in far too many places.

Returning null from a function/method instead of an array or Promise, or anything, it's just crazy imo.

[MartinJohns]

Yes... That is pretty much what strictNullChecks is for. And related to that the strictPropertyInitialization feature (works only together with strictNullChecks.

[ORESoftware]

@MartinJohns thanks for the info...I still think strictNullChecks should be the default behavior, and that users should have to specifically set it to false, but nbd I guess