Trampolines in ES6

trampolines.es6

// See http://raganwald.com/2013/03/28/trampolines-in-javascript.html

function trampoline (fn) {
  return function trampolined (...args) {
    let result = fn.apply(this, args);

    while (result instanceof Function) {
      result = result();
    }

    return result;
  };
}

// Trampolining a singly tail-call recursive function

const evenTCR = trampoline(
  function even (n) {
    if (n === 0) return true;
    else if (n === 1) return false;
    else return () => even(n-2);
  }
);

evenTCR(300000003)
  //=> false

// trampolining mutually tail-call recursive functions

const evenMR = trampoline(
  (n) => {
    function even (n) {
      return n === 0 ? true : () => odd(n-1);
    }
    function odd (n) {
      return n === 0 ? false : () => even(n-1);
    }

    return even(n);
  }
);

evenMR(1000000001)
  //=> false

// Counterexample

const evenStacked = (n) => {
  function even (n) {
    return n == 0 ? true : odd(n-1);
  }
  function odd (n) {
    return n == 0 ? false : even(n-1);
  }

  return even(n);
};

evenStacked(1000000001)
  //=> RangeError: Maximum call stack size exceeded.