getElementById() within $(document).ready() returns null

Question

I am "new" to JavaScript, making my first image- and media-slider for a website. I have searched for answers in the web and here in SO, but they did not work for me.

My last big change was to divide my script into two parts. One outside of $(document).ready() and one inside. I want the user to be able to call a function via a button in the HTML. To make this possible this function has to be global and can not be located inside the $(document).ready(). Am I right? Before I divided my script everything was inside the $(document).ready() area and it worked properly. But of cause I could not call the function via a button.

But now the part inside my $(document).ready(), which has to build the slider when the page is loaded, is not waiting for it. All my getElementById()'s are producing the error: "Can't set property 'style' of null." So the slider will not be built. This is telling me getElementById() returns "null" even if it is inside $(document).ready().

HTML:

  <!-- very content -->

  <link href="...css" />
  <script src="...js" />
  <div id="slider"></div>

  <!-- more content -->

  <div id="changeContent"></div>

  <!-- more content -->

</body>
</html>

JavaScript:

    // Configuration
var slidersParentId = 'imslider';
var slideShowTitle = '';
var thumbnailWidth = 20;
var slidesWidth = 281.25;
var slidesHeight = 144.5625;
var currentSlideWidth = 500;
var currentSlideHeight = 257;
var nextSlideWidth = 375;
var nextSlideHeight = 192.75;
var prevSlideWidth = nextSlideWidth;
var prevSlideHeight = nextSlideHeight;
var resizeDifference = currentSlideWidth - nextSlideWidth;
var slidesMargin = 20;
var animationDistance = slidesWidth + slidesMargin;
var animationSpeed = 2000;
var intervalSpeed = 7000;
var contentDiv = "descriptif_site_spip";

// Variables from slides.json
var numberOfSlides;
var jsonSlides = {};
var bgImgUrl;

// Cache the DOM
var $slideShow = $('#'+slidersParentId);
var $slideInner;
var $slides;
var $slideNav;
var $navThumb;
var $thumb;
var $hovers;
var $content = document.getElementById(contentDiv);

// Other global Variables
var interval;
var currentSlide = 1;
var nextSlide = currentSlide + 1;
var prevSlide = currentSlide - 1;
var lastSlide = currentSlide;
var lastCurrentDif;
var lastNextSlide = lastSlide + 1;
var lastPrevSlide = lastSlide - 1;
var thumbImgs = [];
var navTo;
var interval;
var i;

// Global Variables for dragging
var dragStartPosition;
var dragStopPosition;
var draggedDistance;
var slidesDragged;
var posSlidesDragged;
var negSlidesDragged;

// IRIS-MEDIA-MODUS ?!
var iris_mode = 1;

//global functions...
//global warming...


$(document).ready(function(){

  $.ajax({}

    //getting some json...

  });

  // Building the slideshow windows
  if (slideShowTitle !== 0) {
    $slideShow.append('<h2 class="slideShowTitle">'+slideShowTitle+'</h2>');
  }

  $slideShow.append('<div id="outerWindow"><div id="innerWindowPositioner"<div id="innerWindow"></div></div</div>');
  $slideInner = $slideShow.find('#innerWindow');
  $slideInner.css({'width': numberOfSlides*slidesWidth+numberOfSlides*slidesMargin*currentSlideWidth*3});


// Building the slides & hovers
for (i=1; i<=numberOfSlides; i++) {

    var idSlides = "slide_Nr"+i;
    alert(idSlides);
    var idHover = "hover_Nr"+i;
    var j = i-1;
        bgImgUrl = "url('" +jsonSlides[j].mediaUrl+ "')";
    var title = jsonSlides[j].title;
    var artUrl = jsonSlides[j].articleUrl;
    var subtitle = jsonSlides[j].subtitle;
    var text = jsonSlides[j].text;
    var date = jsonSlides[j].date;

    $slideInner.append('<div class="slide" id="'+idSlides+'"><div class="hover" id="'+idHover+'"></div></div>');
    $('#'+idSlides).css('top', '56.21875px');
    document.getElementById(idSlides).style.backgroundImage = bgImgUrl;

    $('#'+idHover).append('<div class="hover-title"><h3><a href="'+artUrl+'">'+title+'</a></h3><span class="hover-subtitle">'+subtitle+'</span></div><span class="hover-date">'+date+'</span><br clear="all" /><p class="hover-text">'+text+'</p>');
}

$slides = $slideInner.find('.slide');
$hovers = $slides.find('.hover');


// Building the thumbnail navigation
$slideShow.append('<div id="slideShowNavigation"><ul id="navigationThumbnails"></ul></div>');
$slideNav = $slideShow.find('#slideShowNavigation');
$navThumb = $slideNav.find('#navigationThumbnails');

for (i=0; i<numberOfSlides; i++) {
    thumbImgs[i] = jsonSlides[i].mediaUrl;
//  alert(thumbImgs);
}

for (i=1; i<=numberOfSlides; i++) {

        j = i-1;
    var idThumbs = "thumb_Nr"+i;
        bgImgUrl = "url('" +jsonSlides[j].mediaUrl+ "')";

    $navThumb.append('<li class="thumb" id="'+idThumbs+'"></li>');
    document.getElementById(idThumbs).style.backgroundImage = bgImgUrl;
    }

$thumb = $navThumb.find('.thumb');
$( '<li class="year">&nbsp;>&nbsp;2000&nbsp;>&nbsp;</li>' ).insertBefore( "#thumb_Nr62" );
$navThumb.prepend('<li class="year">1980&nbsp;>>&nbsp;</li>');
$navThumb.append('<li class="year">&nbsp;&nbsp;2020&nbsp>></li>');
$navThumb.append('<br clear="both" />');

// ...some
// ...more
// ...functions

});

With this HTML-structure it is not working. When I put the script-inclusion at the end of my HTML it works correctly and the slider will be built.

So why is my $(document).ready() firing too early? I also tried $(window).load() but it hat no effect.

Or is there any why to make function inside $(document).ready() globally available without removing it from $(document).ready()?

Answer 1

Probably because your script is running before the rest of the page loads. Try using $(window).ready().

EDIT:

Had wrong function .load() I meant to have .ready()... Whoops!

Answer 2

My last big change was to divide my script into two parts. One outside of $(document).ready() and one inside. I want the user to be able to call a function via a button in the HTML. To make this possible this function has to be global and can not be located inside the $(document).ready().

Actualy, it can:

All global JavaScript objects, functions, and variables automatically become members of the window object.

I suppose you need something like this:

window.addEventListener('DOMContentLoaded', function () {
    window.my_magic_function = function (/* my magic params */) {
        /* doing my magic */
    };
});

<button onclick="my_magic_function(/* my magic params */)">Do magic</button>

window.addEventListener('DOMContentLoaded', ...) equivalent to $(document).ready(...)