How do you call the actively selected spread length/width in a document, or page bounds?

In a script I am using

var docWidth = Number(app.activeDocument.documentPreferences.pageWidth);

Also, here’s why it might be a problem to get the width from documentPreferences.pageWidth. In my example above if I set w as this:

var w = doc.documentPreferences.pageWidth;
$.writeln(w)
//returns 8.5 the document‘s default page size

And try the script on a spread with custom page sizes the guides are wrong because the width of page 3 is 4" not 8.5":

But if I get the width from page 3’s bounds the guides are placed where I want them

var w = pg3.bounds[3]-pg3.bounds[1];
$.writeln(w)
//returns 4 for the custom page 3

I appreciate everyone's response but I'm still extremely confused and have no idea what's going on, please don't assume I know anything about coding, I still have absolutely no clue whatsoever what ".bounds" means. It's top of...something, bottom of...something, no one explained of what though, and any documentation is atrociously lacking in any semblance of user-friendly detail or any kind of explanation. 

Below is code that "almost" does what I want.

Hi @dane13860245, every Page is part of a Spread, and every Spread has at least one Page. So you can do:

var mySpread = myPage.parent;
var myPages = mySpread.pages;

I think the only way to get the bounds of a Spread, is to get the bounds of all the spread's Pages, using Page.bounds as Robert mentioned. Here is some example code for getting the bounds of the whole spread:

/**
 * Example of getting bounds of a spread.
 * @author m1b
 * @discussion https://community.adobe.com/t5/indesign-discussions/how-do-you-call-the-actively-selected-spread-length-width-in-a-document-or-page-bounds/m-p/13940663
 */
(function () {

    var doc = app.activeDocument,
        page = doc.layoutWindows[0].activePage,
        spread = page.parent,
        spreadBounds = getExpandedBounds(spread.pages.everyItem().bounds);

    alert('Bounds of active spread:\n'+spreadBounds);

})();

/**
 * Returns the overall bounds, when
 * given an array of bounds ordered
 * as Indesign bounds [TLBR].
 * @author m1b
 * @version 2023-07-17
 * @param {Array<Array<Number>>} arr - an array of bounds arrays.
 * @returns {Array<Number>} - the expanded bounds [TLBR].
 */
function getExpandedBounds(arr) {

    var bounds = [Infinity, Infinity, -Infinity, -Infinity];

    for (var i = 0; i < arr.length; i++) {

        for (var j = 0; j < arr[0].length; j++) {

            if (j < 2)
                bounds[j] = Math.min(bounds[j], arr[i][j])
            else
                bounds[j] = Math.max(bounds[j], arr[i][j])
        }
    }

    return bounds;

};

- Mark

Each page have Bounds property:

https://www.indesignjs.de/extendscriptAPI/indesign-latest/#Page.html