Understanding Type Predicates in TypeScript

In TypeScript, type predicates are special functions that refine the type of a variable within a conditional block.

They enable you to tell the TypeScript compiler that a variable is of a specific type if the function returns true, effectively narrowing the variable’s type for the subsequent code within that block.

This allows for more precise type checking and eliminates the need for manual type assertions in certain situations.

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Example

interface Cat {
  meow(): void;
}

interface Dog {
  bark(): void;
}

function isCat(animal: Cat | Dog): animal is Cat {
  return (animal as Cat).meow !== undefined;
}

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

const myCat: Cat = { meow: () => console.log('Meow!') };
const myDog: Dog = { bark: () => console.log('Woof!') };

makeSound(myCat);
makeSound(myDog);

In this example, isCat is a type predicate. It checks if an animal has a meow method, indicating it’s a Cat. The animal is Cat return type tells TypeScript that if isCat returns true, the animal within the if block is guaranteed to be a Cat.