Basic Type Guards

function processValue(value: string | number) {
  if (typeof value === "string") {
    // value is narrowed to string
    return value.toUpperCase();
  }
  // value is narrowed to number
  return value.toFixed(2);
}

class Dog {
  bark() { console.log("Woof!"); }
}

class Cat {
  meow() { console.log("Meow!"); }
}

function makeSound(animal: Dog | Cat) {
  if (animal instanceof Dog) {
    animal.bark();
  } else {
    animal.meow();
  }
}

interface Fish {
  swim: () => void;
}

interface Bird {
  fly: () => void;
}

function move(animal: Fish | Bird) {
  if ("swim" in animal) {
    animal.swim();
  } else {
    animal.fly();
  }
}

function printLength(str: string | null) {
  if (str) {
    // str is narrowed to string
    console.log(str.length);
  } else {
    // str is null
    console.log("No string provided");
  }
}

function example(x: string | number, y: string | boolean) {
  if (x === y) {
    // x and y are both narrowed to string
    x.toUpperCase();
    y.toUpperCase();
  }
}