Transducers are awesome

transducers.js

var foldr = Function.prototype.call.bind(Array.prototype.reduceRight);

var source = [9, 8, 7];
var pipeline = compose(filter(odd), map(add(1)));

var pusher = {
  step: function (arr, value) { arr.push(value); return arr; },
  result: identity
};

console.log(transduce(pipeline, pusher, [], source));

var adder = {
  step: function (a, b) { return a + b },
  result: identity
};

console.log(transduce(pipeline, adder, 0, source));

function odd(value) {
  return value % 2;
}

function add(x) {
  return function (value) {
    return value + x;
  };
}

function identity(value) {
  return value;
}

function compose() {
  var transducers = arguments;
  return function (reducer) {
    return foldr(transducers, function (xf, transducer) {
      return transducer(xf);
    }, reducer);
  };
}

function transduce(transducer, reducer, init, coll) {
  return reduce(transducer(reducer), init, coll);
}

function reduce(xf, init, coll) {
  return xf.result(coll.reduce(xf.step, init));
}

function map(mapper) {
  return function (xf) {
    return {
      init: function () {
        return xf.init();
      },
      step: function (value, item) {
        return xf.step(value, mapper(item));
      },
      result: function (value) {
        return xf.result(value);
      }
    };
  };
}

function filter(predicate) {
  return function (xf) {
    return {
      init: function () {
        return xf.init();
      },
      step: function (value, item) {
        return predicate(item) ? xf.step(value, item) : value;
      },
      result: function (value) {
        return xf.result(value);
      }
    }
  }
}