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

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

Using type guards effectively can enhance type safety in TypeScript. Here are some best practices:

1. Use typeof for primitive types: This is the simplest way to check types like string, number, or boolean.
2. Custom type guards: Create functions that return a boolean and use the is keyword to define the type.
3. Avoid excessive type assertions: Rely on type guards instead of using as to assert types, which can lead to runtime errors.
4. Combine with union types: Use type guards to narrow down union types effectively.

Can you give an example of a custom type guard?

Sure! Here's a simple example of a custom type guard:

interface Dog {
  bark(): void;
}

interface Cat {
  meow(): void;
}

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

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

This function checks if the pet has a bark method, thus narrowing the type.

What are the benefits of using type guards?

The benefits of using type guards include:

1. Increased type safety: They help catch errors at compile time rather than runtime.
2. Improved code readability: Type guards make it clear what types are being handled in different branches of code.
3. Better IntelliSense support: IDEs can provide more accurate suggestions and autocompletions when type guards are used.

Are there any performance considerations with type guards?

Yes, while type guards are generally efficient, excessive or complex type checks can impact performance. It's best to use them judiciously, especially in performance-critical code paths. Simple checks like typeof are very fast, while custom type guards may introduce some overhead depending on their complexity.