What are the best practices for using type guards in TypeScript?

What are the best practices for using type guards in TypeScript?

Type guards are essential for ensuring type safety in TypeScript. Best practices include using the typeof operator for primitive types, creating user-defined type guards for complex types, and leveraging the instanceof operator for class instances.

Can you give me an example of a user-defined type guard?

Sure! Here's a simple example:

interface Dog {
  bark(): void;
}

interface Cat {
  meow(): void;
}

function isDog(pet: Dog | Cat): pet is Dog {
  return (pet as Dog).bark !== undefined;
}

This function checks if the pet is a Dog by verifying if it has a bark method.

What about using type guards with union types?

When using type guards with union types, you can narrow down the type by checking specific properties. For example:

function handlePet(pet: Dog | Cat) {
  if (isDog(pet)) {
    pet.bark(); // TypeScript knows pet is a Dog here
  } else {
    pet.meow(); // TypeScript knows pet is a Cat here
  }
}

Are there any performance considerations with type guards?

Yes, while type guards are powerful, excessive use can lead to performance overhead, especially in large applications. It's best to use them judiciously and only when necessary.