I tried following code, but seems only show "A:1 is expired", "A:2" never be show

var myCache = new node_cache( { stdTTL: 50, checkperiod: 20 } );
myCache.set("A:1", Code, 10);
myCache.set("A:2", Code, 25);

myCache.on("expired", function( key, value ){
console.log(key + " is expired");
// ... do something ...
});

How can make it run the expired check by interval?