Transducers in JavaScript

transducers.js

function compose(f, g) {
  if (arguments.length === 2) {
    return function() {
      return f(g.apply(null, arguments));
    };
  }

  var args = [];
  for (var i = 0, len = arguments.length; i < len; i++) {
    args[i] = arguments[i];
  }

  return function() {
    var i = args.length - 1;
    var result = args[i].apply(this, arguments);
    while (i--) {
      result = args[i].call(this, result);
    }
    return result;
  };
}

function map(f) {
  return function(reduce) {
    return function(acc, x) {
      return reduce(acc, f(x));
    };
  };
}

function filter(p) {
  return function(reduce) {
    return function(acc, x) {
      console.log('filter=>' + x + ' ' + p(x));
      return p(x) ? reduce(acc, x) : acc;
    };
  };
}

function collect(xs, x) {
  xs.push(x);
  return xs;
}

function transduce(reducer, xs, val) {
  var res = val || [];
  return xs.reduce(reducer, res);
}

var even = function(x) {
  return x % 2 === 0;
};

var gt = function(x) {
  return function(v) {
    return v > x;
  };
};

function multi(m) {
  return function(x) {
    return x * m;
  };
}
var filterEven = filter(even);
var mapMulti = map(multi(10));
var filterLessThan40 = filter(gt(40));

var filterMap = compose(filterEven, mapMulti, filterLessThan40)(collect);
var mapFilter = compose(map(multi(10)), filter(even))(collect);
var chained = filter(even)(map(multi(10))(collect));
var acc = [];

console.log("filterMap reducer");
var result = transduce(filterMap, xs, acc);
console.log(result);
console.log(acc === result);

console.log("mapFilter reducer");
acc = [];
result = transduce(mapFilter, xs, acc);
console.log(result);
console.log(acc === result);

console.log("chained reducer");
acc = [];
result = transduce(chained, xs, acc);
console.log(result);
console.log(acc === result);