index.js

function introduce(fn, memo, dependents) {
  return (function(memo) {
    dependents = dependents || [];
    return function(item) {
      return item === undefined ? memo : memo = fn.apply(undefined, [item, memo].concat(dependents.map(d => d(item))));
    };
  }).bind(undefined, memo);
}

var createSum = module.exports.createSum = introduce((item, sum) => sum + item, 0);

var createCount = module.exports.createCount = introduce((item, count) => ++count, 0);

var createMean = module.exports.createMean = introduce((item, last, sum, count) => sum / count, undefined, [
  createSum(),
  createCount()
]);

var createMin = module.exports.createMin = introduce(
  (item, min) => min === undefined || item < min ? item : min,
  undefined
);

var createMax = module.exports.createMax = introduce(
  (item, max) => max === undefined || item > max ? item : max,
  undefined
);