typetheory.js

const productType = { a: 3, b: "hello", c: false } 
// this has the implied type data ProductType = { a :: Number, b :: String, c :: false }
// even if javascript doesn't demand that we think about it, it matters
// as demonstrated by the below function which IMPLICITLY expects that type signature

const repeatWord = ({a, b, c}) => b + (c ? " " : "/n") + (a <= 0 ? "" : repeatWord({a: a - 1, b, c})
// if you give the above function the wrong 'type' you will get unexpected behaviour
// rather than resulting in a type error you will get either a runtime error or an unexpected return value
                                                          
// Similarly you might have this
const sum1 = "hello"
const sum2 = 5
const sum3 = false

// and you might want a function to accept any of these and behave differently depending
// if you use the naive implementation above it can be very hard to distinguish the types
// especially if you throw a javascript null in there which has strange type characteristics
// like being equal to an object etc.
// instead you want to use the discriminated union pattern like below

const goodSum1 = {tag: 'sum1', content: "hello"}
const goodSum2 = {tag: 'sum2', content: 5 }
const goodSum3 = {tag: 'sum3', content: false }

// now it is trivial to check which case you have and handle it