Doesn't remember the class inside a Array.map

[ulrikstrid]

TypeScript Version: nightly (2.0.0-dev.201xxxxx)

Code

class SomeClass {
  constructor(public id: number){}
}

class SomeOtherClass {
  constructor(public name: string){}
}

type SomeUnionType = SomeClass | SomeOtherClass;

function buggyFunction(aArray: string[], bug: SomeUnionType): void {
  if (bug instanceof SomeClass) {
    console.log(bug.id);
    return;
  }

  if (bug instanceof SomeOtherClass) {
    aArray.map((a) => a = bug.name());
    return;
  }

  return;
}

Expected behavior:
Typescript should know that bug is of the type SomeOtherClass inside of the map function.

Actual behavior:
Property 'name' does not exist on type SomeClass | SomeOtherClass.

[kitsonk]

Duplicate of #9263 and #7662 and #9345 as narrowing is currently reset across function boundaries as explained in #7718 (comment).

Though this keeps coming up and it seems logical that anonymous functions, this could be potentially preserved? (or at least arrow functions used as callbacks?) @mhegazy @ahejlsberg @RyanCavanaugh

[ahejlsberg]

The solution here is to guard on a const variable. Type guards are in effect across function expression and arrow function boundaries for const variables. We are considering allowing const as a parameter modifier so you don't have to copy parameters into const locals.

[ulrikstrid]

Yeah, I solved my real issue by assigning the value to a const but it seems wrong to have to that. The behaviour is the same when using interfaces with a .kind property as guard.

[mquandalle]

@ahejlsberg

We are considering allowing const as a parameter modifier so you don't have to copy parameters into const locals.

I’m interested in that, is there an issue to track this solution?