Typescript won't accept Array with polymorphic elements

polymorhpic-array-elements.ts

class Animal {
    name: string | undefined;
}

class Cat extends Animal {
    food: string | undefined;
}

let animals: Animal[] = [
    { name: "Whiskers", food: "fish" }, //won't go with TS
];

let cat = { name: "Whiskers", food: "fish" }; //won't go with TS
animals.push(cat); // This should work