PartitionOn need help to explain this task
 

Есть задачка на codewars:
Write a function which partitions a list of items based on a given predicate.

After the partition function is run, the list should be of the form [ F, F, F, T, T, T ] where the Fs (resp. Ts) are items for which the predicate function returned false (resp. true).

NOTE: the partitioning should be stable; in other words: the ordering of the Fs (resp. Ts) should be preserved relative to each other.

For convenience and utility, the partition function should return the boundary index. In other words: the index of the first T value in items.

For example:

var items = [1, 2, 3, 4, 5, 6];
function isEven(n) {return n % 2 == 0}
var i = partitionOn(isEven, items);
// items should now be [1, 3, 5, 2, 4, 6]
// i     should now be 3

Sample tests:

var items = [1, 2, 3, 4, 5, 6];
function isEven(n) {return n % 2 == 0}
var i = partitionOn(isEven, items);
Test.assertEquals( i, 3, 'partioned at 3' );
Test.assertSimilar( items.slice(0, i), [1, 3, 5] );
Test.assertSimilar( items.slice(i),    [2, 4, 6] );

Мой код:

// partition the items array so that all values for which pred returns true are
// at the end, returning the index of the first true value
function partitionOn(pred, items) {
    var pTrue = [];
    var pFalse = [];
    items.forEach( function(value) {
    if (pred(value)) { pTrue.push(value);}
    else {pFalse.push(value);}});
    items = pFalse.concat(pTrue);
    return pTrue.length, items;
}