Perhaps create a shared Fruit interface that provides isDecayed. fruits is now of type Fruit[] so the type can be explicit. Like this:
interface Fruit {
isDecayed: boolean;
}
interface Apple extends Fruit {
color: string;
}
interface Pear extends Fruit {
weight: number;
}
interface FruitBasket {
apples: Apple[];
pears: Pear[];
}
const fruitBasket: FruitBasket = { apples: [], pears: [] };
const key: keyof FruitBasket = Math.random() > 0.5 ? 'apples': 'pears';
const fruits: Fruit[] = fruitBasket[key];
const freshFruits = fruits.filter((fruit) => !fruit.isDecayed);