Example of Either implementation in Typescript

Either.ts

export class Left<T> {
  constructor(public value: T) {}

  public isLeft(): this is Left<T> {
    return true;
  }

  public isRight(): this is Right<never> {
    return false;
  }

  public static create<T>(value: T): Left<T> {
    return new this(value);
  }
}

export class Right<R> {
  constructor(public value: R) {}

  public isLeft(): this is Left<never> {
    return false;
  }

  public isRight(): this is Right<R> {
    return true;
  }

  public static create<T>(value: T): Right<T> {
    return new this(value);
  }
}

export type Either<L, R> = Left<L> | Right<R>;

class CustomErrorA extends Error {
  constructor() {
    super('Custom error A.');
    this.name = 'CustomErrorA';
  }
}

class CustomErrorB extends Error {
  constructor() {
    super('Custom error B.');
    this.name = 'CustomErrorB';
  }
}

interface Result {
  random: Number;
}

function checkLucky(): Either<CustomErrorA | CustomErrorB, Result> {
  const random = Math.random();

  if (random < .2) {
    return Left.create(new CustomErrorA());
  }

  if (random < .5) {
    return Left.create(new CustomErrorB());
  }

  return Right.create({ random });
}

setInterval(() => {
  const result = checkLucky();

  if (result.isLeft()) {
    return console.log('🛑 Unlucky:', result.value.message )
  }

  return console.log('🍀 Lucky:', result.value.random.toFixed(2))
}, 1000);

/*
> npx ts-node Either.ts

🍀 Lucky: 0.64
🍀 Lucky: 0.97
🍀 Lucky: 0.63
🛑 Unlucky: Custom error B.
🛑 Unlucky: Custom error B.
🛑 Unlucky: Custom error B.
🍀 Lucky: 0.82
🍀 Lucky: 0.81
🛑 Unlucky: Custom error A.
🍀 Lucky: 0.65
🍀 Lucky: 0.93
🛑 Unlucky: Custom error A.
*/