Propositions in conditionals

gistfile1.txt

// is :: (U String Constructor) ---> T --(Predicate for `object` being a `test`)--> Bool
const is = test => object =>
  typeof test === 'string'
  ? typeof object === string // object is assured to be a string
  : object instanceof test // Object can be anything but a string

const x = [1, 2, 3]
if ( is(Array)(x) ) {
  // x is assured to be an array
  x.push(5)
} else {
  // x can be anything but an array
}