flow-invariance.js

// @flow
class Animal {}

class Dog extends Animal {
  bark() {}
}
class Cat extends Animal {
  meow() {}
}

const dogs: Dog[] = [new Dog(), new Dog()];
const animals: Animal[] = dogs; // Cannot assign `dogs` to `animals` because `Dog` [1] is incompatible with `Animal` [2] in array element
animals.push(new Cat());

dogs.forEach(d => d.bark());

typescript-covariance.ts

class Animal {}

class Dog extends Animal {
  bark() {}
}
class Cat extends Animal {
  meow() {}
}

const dogs: Dog[] = [new Dog(), new Dog()];
const animals: Animal[] = dogs;
animals.push(new Cat());

dogs.forEach(d => d.bark()); // TypeError: d.bark is not a function