Draft: Reference Option implementation for Programming TypeScript

Option.ts

function Option<T>(value?: null | undefined): None
function Option<T>(value: T): Some<T>
function Option<T>(value?: T): Option<T> {
  if (value == null) {
    return new None
  }
  return new Some(value)
}

interface Option<T> {
  flatMap<U>(f: (value: T) => None): None
  flatMap<U>(f: (value: T) => Option<U>): Option<U>
  getOrElse(value: T): T
}

class Some<T> implements Option<T> {
  constructor(protected value?: T) {}
  flatMap<U>(f: (value: T) => None): None
  flatMap<U>(f: (value: T) => Some<U>): Some<U>
  flatMap<U>(f: (value: T) => Option<U>): Option<U> {
    return f(this.value)
  }
  getOrElse(_value: T): T {
    return this.value
  }
}

class None implements Option<never> {
  flatMap<U>(_f: (value: never) => Option<U>): None {
    return new None
  }
  getOrElse<U>(value: U): U {
    return value
  }
}

let a = Option(2)
  .flatMap(n => new Some('a'))
  .flatMap(n => new Some(5 * n))
  .flatMap(() => new None)
  .flatMap(n => new Some(5 * n))
  .flatMap(n => new Some(5 * n))
  .flatMap(() => new None)
  .getOrElse('a')