[SOLVED] Observable.forkJoin() TS2322 error after updating to TypeScript 2.4.1 and Rxjs 5.4.2

Observable.forkJoin() TS2322 error after updating to TypeScript 2.4.1 and Rxjs 5.4.2

In our angular 4.2.4 application, we use RxJS's Observable.forkJoin in a number of places to return heterogeneous types.

For example:

private fleet: Aircraft[];
private contractList: string[];

Observable.forkJoin([
  this.fleetService.getFleet(),
  this.fleetService.getContractList()
]).subscribe(
  next => {
    this.fleet = results[0];
    this.contractList = results[1];
  },
  error => console.error
);

with the following service signatures:

getFleet(): Observable<Aircraft[]> { ... }
getContractList(): Observable<string[]> { ... }

After updating to TypeScript 2.4.1, tsc now complains with the following errors for this.contractList:

ERROR in ./app/fleet/fleetComponent.ts (xx,xx): error TS2322: Type 'Aircraft[]' is not assignable to type 'string[]'.

But I'm not trying to assign an Aircraft[] to a string[].

The same is true for all the other Observable.forkJoin we implement: all elements in result[] are treated as though they were the same type as result[0].

It seems very similar to the problem in Error with Actions observable in @ngrx/effects using TypeScript 2.4.1 but adding "noStrictGenericChecks": true, to tsconfig.js' "compilerOptions" did not solve the problem.

Why is this happening? How can I fix this?

Solution

You are probably seeing the increased enforcement from the Typescript compiler on generics, the side-effect being that forkJoin doesn't have a tuple overloads.

Removing the array notation from the parameter list to forkJoin should work, however:

Observable.forkJoin(
  this.fleetService.getFleet(),
  this.fleetService.getContractList()
).subscribe(
  next => {
    this.fleet = results[0];
    this.contractList = results[1];
  },
  error => console.error
);