Partial Application with Substitution

partialApplication.js

var partial = function () {
  var toArray = function (a) { return [].slice.call(a); },
      appliedArgs = toArray(arguments),
      fn = appliedArgs.shift(),
      placeholderPositions = appliedArgs.map(function (e, i) {
        if (e === '_') { return i; }
      }).join('').split('');
  
  return function () {
    var args = toArray(arguments);

    placeholderPositions.forEach(function (i) { appliedArgs[i] = args.shift(); });
    appliedArgs.concat(args);
    args.reduce(function (p, c) {
      return p.push(c);
    }, appliedArgs);

    return fn.apply(null, appliedArgs);
  };
};

var two = function (a, b, c) { console.log(JSON.stringify(arguments, null, 2)); };
var one = partial(two, '_', 'this should be b');
one('this should be a');

/*
{
  "0": "this should be a",
  "1": "this should be b"
}
*/

var three = function (a, b, c) { console.log(JSON.stringify(arguments, null, 2)); };
var one = partial(three, 'this should be a', '_', 'this should be c');
one('this should be b');

/*
{
  "0": "this should be a",
  "1": "this should be b",
  "2": "this should be c"
}
*/