Sum the even values in Fibonacci series which are less than 4000000

README.md

> npm install babel-preset-es2015
> npm install babel-polyfill
> babel-node --presets "es2015" evenfib.js
4613732

evenfib.js

const iterate = function* (init, fn) {
  yield init;
  yield* iterate(fn(init), fn);
}

const map = function (fn, gen) {
  function* inner(gen) {
    for (const x of gen) {
      yield fn(x);
    }
  }
  return (gen === void 0) ? inner : inner(gen);
}

const filter = function (fn, gen) {
  function* inner(gen) {
    for (const x of gen) {
      if (fn(x))
        yield x;
    }
  }
  return (gen === void 0) ? inner : inner(gen);
}

const takeWhile = function (fn, gen) {
  function* inner(gen) {
    for (const x of gen) {
      if (fn(x))
        yield x;
      else
        break;
    }
  }
  return (gen === void 0) ? inner : inner(gen);
}

const reduce = function (fn, gen) {
  function inner(gen) {
    const {value, done} = gen.next();
    if (done) return void 0; 
    
    function rec(acc) {
      const {value, done} = gen.next();
      if (done) return acc;
      return rec(fn(acc, value));
    }
    return rec(value);
  }
  return gen === void 0 ? inner : inner(gen);
}

function* fib() {
  yield* map(x => x[0])(iterate([0,1], ([a,b]) => [b, a+b]));
}

function compose(...fs) {
  return fs.reduce((f,g) => (x => f(g(x))));
}

const add = (a,b) => a + b;
const sum = reduce(add);
const even = x => x%2 === 0;
const lessThan = limit => x => x < limit;

function evenFibonacciSum() {
  return compose(sum, filter(even), takeWhile(lessThan(4000000)))(fib());
}

console.log(evenFibonacciSum());