tailRec.js

const tailRec = (f) => (a) => {
  let v = f(a)
  while (!v.done) {
    v = f(v.value)
  }
  return v.value
}
const done = (v) => ({ value: v, done: true })
const next = (v) => ({ value: v, done: false })

const sum = (n) => {
  const go = ({ i, acc }) => i == 0 ? done(acc) : next({ i: i-1, acc: acc+i })
  return tailRec(go)({i:n, acc:0})
}
console.log(sum(100000))