trampoline.js

const trampoline = (f, ...args) => {
  let r = f(...args);
  while (typeof r === 'function') r = r();
  return r;
};

const f = (x, n) => {
  return x < n ? f(x + 1, n) : 'done';
};

// console.log(f(0, 1000000));

const g = (x, n) => {
  return x < n ? () => g(x + 1, n) : 'done';
};

console.log(trampoline(g, 0, 1000000));