I'm working through JavaScript: The Definitive Guide as part of Learn JavaScript Properly, and I'm having trouble reasoning about the filter() method in Chapter 7's section on Array Methods.

Here's the example provided:

The filter() method returns an array containing a subset of the
elements of the array on which it is invoked. The function you pass to
it should be predicate: a function that returns true or false. The
predicate is invoked just as for forEach() and map(). If the return
value is true, or a value that converts to true, then the element
passed to the predicate is a member of the subset and is added to the
array that will become the return value.

Examples:

a = [5, 4, 3, 2, 1];

smallvalues = a.filter(function(x) { return x < 3 });   // [2, 1]

everyother = a.filter(function(x,i) { return i%2==0 }); // [5, 3, 1] 

Where I'm getting confused is how exactly i is applied to x in the the everyother line. Here's what I think is happening:

1. i (the index of a[]) is being passed through the function ,x, which applies the predicate to each element of a[] and returns [4, 2].




2. Then the function says filter [4, 2] out of a[]...I'm real fuzzy on how.

When I mess around in the console, I've tried:

everyother = a.filter(function(i) { return i%2==0 });  // returns [4, 2]

which is what I would expect, but I don't understand what happens internally in how JS handles the parameters when I change the above code to

everyother = a.filter(function(x,i) { return i%2==0 }); // returns [5, 3, 1]

(I do know that the array methods are applied like so: function(element, index, array))

For this particular example, it's obvious to me that I could get the expected result another way:

everyother = a.filter(function(x) { return x%2!=0 }); // returns [5, 3, 1]

But I suspect that line of thinking is precisely missing the point the example is trying to get across...I'm just missing it.