functional.md

let arr = [0, 1, 2, 3]

// find the first value in array x that equals to y
// return index, or null if not found
const find1 = (x, y) => {
  for (let i = 0; i < x.length; i++) {
    if (x[i] === y)
      return i
  }
  return null
}

console.log(find1(arr, 2)) 
console.log(find1(arr, 5)) 

// the functional programming version
const find2 = (f => f(f))(f =>  
  (next => (x, y, i = 0) =>  
    (i >= x.length) ? null : 
      (x[i] === y) ? i : 
        next(x, y, i+1))((...args) =>  
          (f(f))(...args)))

console.log(find2(arr, 2)) 
console.log(find2(arr, 5))