TypeScript error Foo only refers to a type but is being used as a value here

Question

interface Foo {
    abcdef: number;
}

let x: Foo | string;

if (x instanceof Foo) {
    // ...
}

But TypeScript gave me this error:

'Foo' only refers to a type, but is being used as a value here.

Answer 1

To do type checking at runtime with an interface is using type guards, if interfaces you wish to check have different properties/functions.

Example

let pet = getSmallPet();

if ((pet as Fish).swim) {
    (pet as Fish).swim();
} else if ((pet as Bird).fly) {
    (pet as Bird).fly();
}