compose.js

const f1 = x => x + 1
const f2 = x => x - 2
const f3 = x => x * 3
const f4 = x => x / 2
const add = (x, y) => x + y

// const g = x => f2(f3(f1(x)))

// Array.prototype.get = function() {
//   return this[0]
// }

// const g = x => [x]
//   .map(f1)
//   .map(f3)
//   .map(f2)
//   .get()
//   // .find(x => true)
  
// function compose(f, g) {
//   return function (...params) {
//     return f(g(...params))
//   }
// }

function compose(...fs) {
  const [g, f, ...rest] = fs.reverse()
  if (g && !f) {
    return g
  }
  if (rest.length === 0) {
    return function(...params) {
      return f(g(...params))
    }
  }
  const [fr, ...rrest] = rest
  return compose(
    ...rrest.reverse(),
    function(...params) {
      return fr(f(g(...params)))
    }
  )
}


const g = compose(f4, f2, f3, f1, add)

console.log(g(8, 2)) // 31