once2.js

function once(callback) {
  var results = {};

  var key = callback.toString();

  return function() {
    if (!results.hasOwnProperty(key)) {
      results[key] = callback();
    }
    return results[key];
  }
}

var hello = once(function() {
    print("building hello...");
    return "Hello, " + "World!";
});

var bye = once(function() {
    print("building bye...");
    return "Good " + "Bye!";
});

print(hello());
print(bye());
print(hello());
print(bye());