Arrow style user defined type guard?

Question

The typescript handbook on user defined type guards defines an example type guard as

function isFish(pet: Fish | Bird): pet is Fish {
    return (<Fish>pet).swim !== undefined;
}

Is there a corresponding syntax for arrow functions?

Answer 1

TypeScript supports arrow-function type guards (which may have not been the case in 2017). The following now works as expected. I've used this style frequently in production code:

const isFish = (pet: Fish | Bird): pet is Fish => 
  (pet as Fish).swim !== undefined;