functional js

function id(x) { return x; }

function constant(v) {
  return function () {
    return v;
  }
}

Function.prototype.memoize = function() {
  var f = this;
  var cache = {};
  return function() {
    var key = JSON.stringify(arguments);
    if (!cache.hasOwnProperty(key)) {
      cache[key] = f.apply(null, arguments);
    }
    return cache[key];
  }
}

Function.prototype.compose = function(g) {
  var f = this;
  return function() {
    return f(g.apply(null, arguments));
  }
};