JQuery.offset () returns undefined

Question

I built this very basic .map () function with jQuery to store the upper offsets of my HTML sections in an array (inside the script tag at the very end of the body):

JQuery

$(window).load(function() {
    var obj = $(this),
        sec = $('section'),
        arr = sec.map(function() {
                  return obj.offset().top
              }).get();
})

It is almost as simple as in the jQuery documentation . And it already worked! ... Until a week ago.
My HTML (jade [compiled with CodeKit]) has a very simple structure:

HTML

body.preload
    section#id1
        whateverContent
    section#id2
        moreContent
    section#id3
        evenMoreContent
    ...

Surprisingly, today the console decided to tell me that top undefined:

Console

Uncaught TypeError: Cannot read property 'top' of undefined

.map() section , :
DOM .
, .

, ?

? - ? , , ? - ? CodeKit? !

, JS ( ):

// Let DOM finish loading
$(window).load(function() {
    // CSS animation hack
    $('body').removeClass('preload')
    // Sticky nav and active class
    var win = $(window),
        doc = $(document),
        obj = $(this),
        sec = $('section'),
        nav = $('nav'),
        anc = $('nav a'),
        pos = nav.offset().top,
        // Fill array with top offsets from sections
        arr = sec.map(function() {
                    return obj.offset().top
              }).get(),
        act = function() {
                    win.scrollTop() > pos ? nav.addClass('sticky')
                    : nav.removeClass('sticky'),
                    $.each(arr, function(i, val) {
                            (win.scrollTop() > val && win.scrollTop() < (val + sec.eq(i).outerHeight(true) + 1) ) ? anc.eq(i).addClass('active')
                            : anc.eq(i).removeClass('active')
                    })
                };
    // Execute sticky function
    win.scroll(act)

Answer 1

. this section, window. $(this) obj map.