Generics are to types as functions are to values.

generics.ts

function getFirstItem<Type>(list: Type[]): Type {
  return list[0]
}

const item = getFirstItem([1])
const item2 = getFirstItem([''])
const item3 = getFirstItem([new Object()])

type Tree<Type> = {
  value: Type
  left?: Tree<Type>
  right?: Tree<Type>
}

interface Ball {
  name: string
  color: string
}

const ballTree: Tree<Ball> = {
  value: {name: 'basketball', color: 'orange'},
  left: {value: {name: 'football', color: 'white'}},
}

class Basket<Type> {
  constructor(public things: Type[]) {}

  add(thing: Type) {
    this.things.push(thing)
  }

  eat() {
    this.things.pop()
  }
}

class Fruit {
  isFruit: true
  constructor(public name: string) {}
}

class Apple extends Fruit {
  type: 'Apple'
  constructor() {
    super('Apple')
  }
}
class Banana extends Fruit {
  type: 'Banana'
  constructor() {
    super('Banana')
  }
}

class Vegetable {
  isFruit: false
  constructor(public name: string) {}
}

class Cucumber extends Vegetable {
  type: 'Cucumber'
  constructor() {
    super('Cucumber')
  }
}

const fruitBasket = new Basket<Fruit>([])
fruitBasket.add(new Apple())

const appleBasket = new Basket<Apple>([])
appleBasket.add(new Apple())

const bananaBasket = new Basket<Banana>([])
bananaBasket.add(new Banana())

const vegetableBasket = new Basket<Vegetable>([new Cucumber()])