Implementation of optional types in TypeScript. Just replace `null` with `nil` in your app

optional.d.ts

declare interface Optional<T> {
  isNil: boolean
  valueOf(): T
}

declare const nil: Optional<any>

interface Object {
  isNil: boolean
}

optional.js

var nil = { isNil: true, valueOf: function() { return undefined } };
Object.prototype.isNil = false;

sample.ts

/// <reference path="./optional.d.ts" />


// Passing in an Optional
// ======================

function expectsAnOptional(optional: Optional<number>) {
  if (optional.isNil) {
    // Nil was provided
  } else {
    const value = optional.valueOf() // Unwrap
  }
}

function expectsNonOptional(concrete: number) {}

// expectsAnOptional("string") <-- Error, expects number
expectsAnOptional(nil) // <-- Works, is optional
expectsAnOptional(123) // <-- Works

// expectsNonOptional("string") <-- Error, expects number
// expectsNonOptional(nil) <-- Error, doesn't allow optional
expectsNonOptional(123) // <-- Works


// Returning an Optional
// =====================

function returnsAnOptional(): Optional<number> {
  if (someCondition) {
    return nil // <-- Works, returns optional
  } else {
    // return "string" <-- Error, can't return string
    return 123 // <-- Works, returns number
  }
}

function returnsNonOptional(): number {
  if (someCondition) {
    // return nil <-- Error, can't return optional
  } else {
    // return "string" <-- Error, can't return string
    return 123 // <-- Works, returns number
  }
}