reduce implemented recursively, no state

map.js

function compose(f, g) {
  return function(x) {
    return f(g(x));
  }
}

function cons(arr) {
  return function (val) {
    return arr.concat(val);
  }
}

function map(fn, arr) {
  return reduce(arr, compose(cons([]), fn));
}

reducerecur.js

function first(arr) {
  return arr[0];
}

function rest(arr) {
  return Array.prototype.slice.call(arr, 1);
}

function reduce(value, fn, arr) {
  return arr.length ? reduce(fn(value, first(arr), fn, rest(arr))) : value;
}