Transform a strict function in an function with nullified parameters.

TS, mFn

type Null = undefined | null
type MVal<T> = T | Null

declare function mFn<T, R>(f:(a:T) => R): {
  (a: T): R,
  (a: Null): Null
}

declare function mFn<T, T2, R>(f:(a:T, b:T2) => R): {
  (a: T, b:T2): R,
  (a: Null, b: unknown): Null,
  (a: unknown, b: Null): Null,
}

const sum = (a: number, b:number):number => a + b
const mSum = mFn(sum);

const x = mSum(3, 3) // x: number
const y = mSum(3, null) // y: Null
const z = mSum(null, 3) // z: Null

https://www.typescriptlang.org/v2/en/play?#code/C4TwDgpgBAcgrgGwVAvFOA7AJhAZgSwwiygB8oNEEBYAKFEigFkA1AQwQB4AVAPlSjcysKnTo4AxgjYAnaLkwTg+APYYoAWwBiGHgBooAJV4AKXAC4Tbc9wCUqfodvmoAbzpQoVl3ZeG9Hl7WIkjOITS0AL5itJLSclAKGEqq6tq63AbcAEwGxmaW1plQAEY22fYojmHutJ7eggZlOWH+gQ3wSE0umADWGCoA7hhhnQgBdUE9GP1DGN3ho1QT0bQxEmoAzsBQm3AaAg2UGiUQMt3HpzLOl2cOUGxQANSldBsY25oAyvsC6SZ7DS2ADc6y2OwAHn8fhoTABmAxw+wAemRUAhLluMje4KgIGh+3hBkooSgqLxLjGOI+OwAXgTYSTxlAkWS0bTKaJaEA