Typescript issues with types

Question

How is possible that this code compiles, it should fail with the type thrown after the first if

class Optional<T> {}

class Some<T> extends Optional<T> {
  constructor(public t: T) {
      super()
  }
}

function div(n: number, d: number): Optional<number> {
  if (d === 0.0) {
    return "IMPOSSIBLE"
  }
  return new Some(d / n)
}

console.log(div(1, 0))

Austaras · Accepted Answer · 2020-05-25 21:52:29Z

2

Because TypeScript has a structual type system which means it judges type compatibility not by name but by its properties, so an empty class is essentially any but for few types like null or undefined, since it requires none property

answered May 25, 2020 at 21:52

Austaras

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