TypescriptImmutableDream.js

// Our fruit list
const fruitData = fromJS([
  {
    name: 'blueberry',
    numberOwned: 10,
    best: true,    
  },
  {
    name: 'banana',
    numberOwned: 0,
    best: false,    
  },
])

// Function to retrieve the best fruit (which is always the blueberry, of course)
const getBestFruit = (fruits: typeof fruitData) => fruits.find(fruit => fruit.get('best'))

// Function which calculates the best fruit based on fruit name
const isBestFruit = (fruitName:string) => fruitName === 'blueberry'

// Lets get our best fruit and start inspecting it!
const bestFruit = getBestFruit(fruitData)

// Test Case #1
// Should return true, no errors
isBestFruit(bestFruit.get('name'))

// Test Case #2
// Should return a typescript error, since 'best' is a boolean, not a string
isBestFruit(bestFruit.get('best')) 

// Test Case #3
// Should return a typescript error, since 'worst' does not exist on bestfruit
const isWorst = bestFruit.get('worst')