TypeScript Bijection

bijection.ts

class Bijection<T, U> {
	static FORWARD = 0 as const;
	static BACKWARD = 1 as const;

	#maps = [new Map<T, U>(), new Map<U, T>()] as const;

	set(forwardVal: T, backwardVal: U) {
		this.#maps[0].set(forwardVal, backwardVal);
		this.#maps[1].set(backwardVal, forwardVal);
	}

	get<Direction extends typeof Bijection.FORWARD | typeof Bijection.BACKWARD>(
		direction: Direction,
		value: Direction extends typeof Bijection.FORWARD ? T : U
	): (Direction extends typeof Bijection.FORWARD ? U : T) | undefined {
        // @ts-ignore
		return this.#maps[direction].get(value);
	}
}

// Example: 
let numStrBijection = new Bijection<number, string>();
numStrBijection.set(4, "4")
numStrBijection.set(100, "100")

let result1 = numStrBijection.get(Bijection.FORWARD, 4)
let result2 = numStrBijection.get(Bijection.BACKWARD, "4")

let result3 = numStrBijection.get(Bijection.FORWARD, "4") // type error
let result4 = numStrBijection.get(Bijection.BACKWARD, 4) // type error
let result5 = numStrBijection.get(3, 4) // type error