When using TypeScript with two interfaces combined as a union type, why doesn't it distinguish which type members can be declared?
interface IFish {
swim: () => void;
}
interface ICat {
meow: () => void;
}
type Pet = IFish | ICat;
const pet: Pet = {
meow: () => {
//
},
swim: () => {
//
}
};
When using interfaces, you are specifying the members that an object must have, but not restricting which members it cannot have. This means that an IFish can have a swim method and vice versa.
Let's examine a revised example:
interface IFish {
swim: () => void;
meow: undefined;
}
interface ICat {
meow: () => void;
swim: undefined;
}
type Pet = IFish | ICat;
const pet: Pet = { // error in compilation
meow: () => {
//
},
swim: () => {
//
}
};
In this demonstration, we explicitly declare that an IFish should not have a meow method, while an ICat should not have a swim method. The downside to this approach is that managing multiple interfaces and methods may require adding numerous instances of undefined to your type definitions.
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