partial.js

//http://jsperf.com/partial-speed-test
var strategy = {};
strategy[1] = function() {
  var args = Array.prototype.slice.call(arguments);
  var f = this;
  return function() {
    var inner_args = Array.prototype.slice.call(arguments);
    return f.apply(this, args.concat(inner_args))
  };
};
strategy[2] = function() {
  var args = arguments;
  var f = this;
  return function() {
    var arr = [];
    arr.push.apply(arr, args);
    arr.push.apply(arr, arguments);
    return f.apply(this, arr)
  };
};
strategy[3] = function() {
  var args = arguments;
  var alen = arguments.length;
  var f = this;
  return function() {
    var arr = [];
    for (var i = 0, len = alen + arguments.length; i < len; i += 1) {
        arr[arr.length] = i < alen ? args[i] : arguments[i - alen];
    }
    return f.apply(this, arr);
  };
};
strategy[4] = function() {
  var args = arguments;
  var alen = arguments.length;
  var f = this;
  return function() {
    var arr = [], i, len;
    for (i = 0; i < alen; i += 1) {
        arr[i] = args[i];
    }
    for (; i < alen + arguments.length; i += 1) {
        arr[i] = arguments[i - alen];
    }
    return f.apply(this, arr);
  };
};


var plus_two = function(x,y) { return x+y; };

Function.prototype.partial = strategy[1];
var add_three = plus_two.partial(3);
add_three(4); // 7