最初の一回だけ実行される関数を生成する関数

地味によく使う

var toExecutableOnce = function(f){
    var called = false, result = undefined;
    return function(){
        if(!called){
            result = f.apply(this, arguments);
            called = true;
        }   
        return result;
    };  
};
var greeting = toExecutableOnce(function(){
    console.log("hello");
    return "world";
});
console.log(greeting());// hello world
console.log(greeting());// world