By

Charl Cronje

It would be much better if once we performed the check, we could know the type of pet within each branch.

It just so happens that TypeScript has something called a type guard. A type guard is some expression that performs a runtime check that guarantees the type in some scope.

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

pet is Fish is our type predicate in this example. A predicate takes the form parameterName is Type, where parameterName must be the name of a parameter from the current function signature.

Any time isFish is called with some variable, TypeScript will narrow that variable to that specific type if the original type is compatible.

// Both calls to 'swim' and 'fly' are now okay.
let pet = getSmallPet();
 
if (isFish(pet)) {
  pet.swim();
} else {
  pet.fly();
}

Notice that TypeScript not only knows that pet is a Fish in the if branch; it also knows that in the else branch, you don’t have a Fish, so you must have a Bird.

You may use the type guard isFish to filter an array of Fish | Bird and obtain an array of Fish:

const zoo: (Fish | Bird)[] = [getSmallPet(), getSmallPet(), getSmallPet()];
const underWater1: Fish[] = zoo.filter(isFish);
// or, equivalently
const underWater2: Fish[] = zoo.filter<Fish>(isFish);
const underWater3: Fish[] = zoo.filter<Fish>((pet) => isFish(pet));
Argument of type '(pet: Fish | Bird) => boolean' is not assignable to parameter of type '(value: Fish | Bird, index: number, array: (Fish | Bird)[]) => value is Fish'.
  Signature '(pet: Fish | Bird): boolean' must be a type predicate.

The in operator also acts as a narrowing expression for types.

For a n in x expression, where n is a string literal or string literal type and x is a union type, the “true” branch narrows to types which have an optional or required property n, and the “false” branch narrows to types which have an optional or missing property n.

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