How to know if an array has holes?

In javascript it is easy to write an array using literals

var arreglo = [1, 2, 3];

This will create an array of size 3 where I can iterate through all its elements with a for loop.

for (var i = 0; i < arreglo.lenght; i++) {
    console.log(arreglo[i]);
}

But there's a problem

I can declare an array like this

var arreglo = new Array(3);

or do something like this

delete arreglo[1];

which will create a sparse array with holes

Now when I go to test if the array has a value at that position I can do something like

for (var i = 0; i < arreglo.lenght; i++) {
    if (arreglo[i]) {
        // haz algo
    }
    // o también
    if (arreglo[i] !== undefined) {
        // haz algo
    }        
}

but then if i do

arreglo[2] = undefined;

the above will fail catastrophically even though the array has a value at that position

How can I detect if there is actually a gap or has been assigned the value of undefinedat that position?