JavaScript: partial application in a useful way, like creating a function that can be passed to filter / forEach / map, etc.

filterify.js

/*
 * filterify
 *
 * Copyright (c) 2012 "Cowboy" Ben Alman
 * Licensed under the MIT license.
 * http://benalman.com/about/license/
 */

Function.prototype.filterify = function filterify() {
  var args = arguments.length > 0 ? [].slice.call(arguments) : [filterify];
  var fn = this;
  return function(value) {
    return fn.apply(this, args.map(function(arg) {
      return arg === filterify ? value : arg;
    }));
  };
};

usage.js

var foo = function(a, b, c) {
  console.log(a, b, c);
};

foo("a", "b", "c");
// a b c

["X", "Y", "Z"].forEach(foo.filterify());
// X undefined undefined
// Y undefined undefined
// Z undefined undefined

["X", "Y", "Z"].forEach(foo.filterify(foo.filterify, "b", "c"));
// X b c
// Y b c
// Z b c

["X", "Y", "Z"].forEach(foo.filterify("a", foo.filterify, "c"));
// a X c
// a Y c
// a Z c

["X", "Y", "Z"].forEach(foo.filterify("a", "b", foo.filterify));
// a b X
// a b Y
// a b Z