In recent browser, you can use filter

var size = arr.filter(function(value) { return value !== undefined }).length;



console.log(size);

Another method, if the browser supports indexOf for arrays:

var size = arr.slice(0).sort().indexOf(undefined);

If for absurd you have one-digit-only elements in the array, you could use that dirty trick:

console.log(arr.join().length);

There are several methods you can use, but at the end we have to see if it's really worthy doing these instead of a loop.