How can I use event.key with an else statement?

I want to be able to have an else statement that always default when I hit a key.

var key = event.key;
if(key == "ArrowDown"){
   //do something
} else if(key == "ArrowUp"){
  //do something
} else{
  //do something
}

The problem is that the program stops and waits for event.key, so else only happens when I hit another key, I want else to happen every time through the loop without having to hit a key. I have tried var key = event.key || "asdf" and then switching the else to else if(key == "asdf") but I havent had any luck. This is in javascript. I am not stuck to using event.key, I am open to using something else