once方法实现(函数只执行一次)

var once = (function(){
    var memo = {},i = 0;
    return function(fn){
        for(var k in memo){
            if(memo[k].func == fn || memo[k].func.toString() == fn.toString()){
                return memo[k].result;
            }
        }
        var result = fn.apply(this,Array.prototype.slice.apply(arguments,1));
        memo[i++] = {func:fn,result:result};
        return result;
    }
})();