Count of “Defined” Array Elements

Question

Given following array:

var arr = [undefined, undefined, 2, 5, undefined, undefined];

I\'d like to get the count of elements which are

Answer 1

If the undefined's are implicit then you can do:

var len = 0;
for (var i in arr) { len++ };

undefined's are implicit if you don't set them explicitly

//both are a[0] and a[3] are explicit undefined
var arr = [undefined, 1, 2, undefined];

arr[6] = 3;
//now arr[4] and arr[5] are implicit undefined

delete arr[1]
//now arr[1] is implicit undefined

arr[2] = undefined
//now arr[2] is explicit undefined