Typescript does a lot to automatically figure out types through regular Javascript code, but type predicates offer more direct control over types. A type predicate is a return type which takes the form of parameterName is Type. When the function is called with a variable, that variable will be narrowed to that specific type if it's compatible.
function isFish(pet: Fish | Bird): pet is Fish {
return (pet as Fish).swim !== undefined;
}
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