middlewares.js

function combine() {
  return Array.prototype.slice.call(arguments)
    .reverse()
    .reduce(function(acc, m) {
      return function(left, right, next) {
        return m(left, right, function(l, r) {
          return acc(l, r, next);
        });
      };
    });
}

function set(prop) {
  return function(left, right) {
    return (left[prop] = right);
  }
}

function merge() {
  return function(left, right) {
    return Object.assign(left, right);
  }
}

function exec(prop) {
  return prop ?
    function(left, right) {
      return left[prop](right);
    } :
    function(left, right) {
      return left(right);
    }
}

function _mapLeft(fn) {
  fn = _asFunc(fn);
  return function(left, right, next) {
    return next(fn(left), right);
  }
}

function _mapRight(fn) {
  fn = _asFunc(fn);
  return function(left, right, next) {
    return next(left, fn(right));
  }
}

function _mapMultiple(fn) {
  return function() {
    return combine.apply(undefined, Array.prototype.map.call(arguments, fn));
  }
}

function _asFunc(fn) {
  if (typeof fn === 'string') {
    return function(obj) {
      return obj[fn];
    }
  }
  return fn;
}

module.exports = {
  combine: combine,
  set: set,
  exec: exec,
  map: {
    left: _mapMultiple(_mapLeft),
    right: _mapMultiple(_mapRight)
  }
};