Typgin NativeScript's untyped Observable

conditional-types.ts

/**
 * Declare a type T of a property K via K of a type X.
 *
 * - Field<{foo: number, bar: string}, 'foo', number> = 'foo'
 * - Field<{foo: number, bar: string}, 'bar', string> = 'bar'
 * - Field<{foo: number, bar: string}, 'foo', string> = never
 */
export type Field<X, K extends string, T> = K extends keyof X
  ? T extends X[K] ? K : never
  : never

Observable.ts

import * as Untyped from 'tns-core-modules/data/observable'
import deprecated from 'deprecated-decorator'  // npm install --save-dev deprecated-decorator
import { Field } from '@/data/conditional-types'

/**
 * Don't dirty your hands.
 * You must use this instead of [[Untyped.Observable]].
 *
 * This description is [here](http://aiya000.github.io/posts/2019-07-04-recover-nativescript-type-unsafe-observable.html).
 */
export default class Observable<X extends object> extends Untyped.Observable {
  constructor() {
    super()
  }

  /**
   * A typed safety set()
   */
  public assign<K extends string, T>(name: Field<X, K, T>, value: T): void {
    super.set(name, value)
  }

  @deprecated('assign')
  public set(name: string, value: any): void {
    super.set(name, value)
  }

  /**
   * A type safety get()
   */
  public take<K extends string, T>(key: Field<X, K, T>): T | null {
    return super.get(key) || null
  }

  @deprecated('take')
  public get(name: string): any {
    super.get(name)
  }
}

using.ts

const p = new Observable<{ x: number, y: string }>()
p.assign('x', 10)
p.assign('y', 'poi')
// 2345: Argument of type '"y"' is not assignable to parameter of type 'never'.
// p.assign('y', 10)

const x: number = p.take('x')
const y: string = p.take('y')

// 2345: Argument of type '"y"' is not assignable to parameter of type 'never'.
// const e: number = p.take('y')

// Use {x?: number, y?: string} if you are careful to forget initializing :D