TS doesn't narrow arrays in a union

[SavvyShah]

Bug Report

🔎 Search Terms

narrowing array union

⏯ Playground Link

Playground link with relevant code

💻 Code

type Point = {
  x: number;
  y:number;
}
type Pair = [number, number]

type Coords = Point | Pair

function print1(a: Coords): number{
  if("x" in a){
    return a.x;
  } else{
    return a[0];
  }
}

function print2(a: Pair[] | Point[]): void{
  if("x" in a[0]){
    console.log(a[0].x)
  } else{
    console.log(a[0][0])
  }
}

🙁 Actual behavior

It should correctly narrow array type as Point[] instead of Point[] | Pair[] in print2().

🙂 Expected behavior

It should have done it because I don't have a mixed type where if a[0] is Point then a[1] could be Pair which is what I mean when I do Point[ ] | Pair[ ]

Also this SO link could help to explain.

[DanielRosenwasser]

This works correctly if you use TypeScript nightly (or 4.4 beta) and use --exactOptionalPropertyTypes. misread the issue

[DanielRosenwasser]

Sorry, misread the issue - maybe I'm missing something but everything seems correctly narrowed in the playground.

[fatcerberus]

Maybe OP was expecting a itself to be narrowed rather than just a[0]? TypeScript doesn't narrow the containing object when you perform a check on one of its properties except in specific situations. Thus why we have discriminated unions.

If I'm right, this is effectively a duplicate of #42384.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.