[Old Pedant]

Originally Posted by DaveyErwin

function addOne(element, index, array) { return ++array[index]; }
var Aarr = [0,1,2,3,4,5,6,7,8,9];
Aarr.filter(addOne);
alert(Aarr);

Well, yes. That will increment each element of the array by one.

But you would get the same results using

function addOne(element, index, array) 
{ 
    ++array[index]; 
    return false; // or return Math.random() < 0.5; even 
}

The return value from filter has no impact on what happens in the *original* array.

If you were to do this:

function addOne(element, index, array) { return ++array[index]; }
var Aarr = [0,1,2,3,4,5,6,7,8,9];
var Barr = Aarr.filter(addOne);
alert(Aarr + "\n" + Barr);

You would find that Barr will contain a copy of the *original* array (0 through 9) while Aaar has every element incremented by one.

But, again, if you started with (say)

var Aarr = [0,1,-1,3,-1,7,0];

Then you would end up with

Aarr == [ 1,2,0,4,0,8,1 ]
Barr == [ 0,1,3,7,0 ]

Because the -1s on being increment become zero and so the return from filter is, effectively, false and those -1s get filtered out.