Using Type Guards To Tell TypeScript, If An Object Is Of A Specific Type

[!INFO]-
topic: 🦒 TypeScript
links: Defining union types
source: TypeScript course by the native web
tags: #permanent-note #published

Last Modified: =dateformat(this.file.mtime, "MMM. dd, yyyy - HH:mm")

When working with union types and want to access a function, that only some of the union types support, you have to tell TypeScript, if the object is of the corresponding type. To accomplish this, you can create a function, that returns true or false depending on if the object matches the desired class or interface, but the return type of the function is not boolean, it is a specific type guard syntax variable is class.

In this case, have different types of animals and want to make sure, that the animal is a Cat or a Fish, so we return animal is Cat or animal is Fish.

class Cat {
  public breathe (): void {}
  public meow (): void {}
}

// Type guard
const isCat = function (animal: Cat | Fish): animal is Cat {
  return Boolean((animal as any).meow);
}

class Fish {
  public breathe (): void {}
  public swim (): void {}
}

// Type guard
const isFish = function (animal: Cat | Fish): animal is Fish {
  if ((animal as any).swim) {
    return true;
  }
  return false;
};

const getSomeAnimal = function (): Cat | Fish {
  return new Fish();
}

const animal = getSomeAnimal();

animal.breathe();

if (isFish(animal)) {
  animal.swim();
}

export {};