Understanding JavaScript/jQuery code snippet

Question

I came across the following code and was wondering what the IMG.active refers too. If someone could help, could you go line by line and write comments too?

function slideSwitch() {
    //what does this and the next line do?
    var $active = $('#slideshow IMG.active');
    if ( $active.length == 0 ) $active = $('#slideshow IMG:last');

    //what is going on here?
    var $next =  $active.next().length ? $active.next()
        : $('#slideshow IMG:first');

    // uncomment the 3 lines below to pull the images in random order

    // var $sibs  = $active.siblings();
    // var rndNum = Math.floor(Math.random() * $sibs.length );
    // var $next  = $( $sibs[ rndNum ] );


    //can someone elaborate on these lines?
    $active.addClass('last-active');

    $next.css({opacity: 0.0})
        .addClass('active')
        .animate({opacity: 1.0}, 1000, function() {
            $active.removeClass('active last-active');
        });
}

$(function() {
    setInterval( "slideSwitch()", 5000 );
});

I think this code is trying to pull images from underneath each image?

Looks like carousel code sorgalla.com/projects/jcarousel/examples/static_circular.html — lsl
– lsl, Commented Nov 15, 2011 at 20:03
+1 ... I had fun to figure out and build the gallery using your code! — Roko C. Buljan
– Roko C. Buljan, Commented Nov 15, 2011 at 21:26

Jasper · Accepted Answer · 2011-11-15 20:00:05Z

IMG.active refers to all image tags (<img />) with the active class, so:

<img src="..." class="active" />

--

function slideSwitch() {
    var $active = $('#slideshow IMG.active');//get all `img` elements with the `active` class within the #slideshow element
    if ( $active.length == 0 ) $active = $('#slideshow IMG:last');//if no elements were selected on the last line, then set the active slide (`$active`) to the last image in the slideshow

    var $next =  $active.next().length ? $active.next()
        : $('#slideshow IMG:first');//check to see if there is another image after the current $active element, if not then set the $next variable to the first image in the slideshow

    // uncomment the 3 lines below to pull the images in random order

    // var $sibs  = $active.siblings();
    // var rndNum = Math.floor(Math.random() * $sibs.length );
    // var $next  = $( $sibs[ rndNum ] );


    $active.addClass('last-active');//add the `last-active` class to the current $active element

    $next.css({opacity: 0.0})
        .addClass('active')
        .animate({opacity: 1.0}, 1000, function() {
            $active.removeClass('active last-active');
        });//these lines set the $next element to invisible, adds the `active` class, then animates its opacity to show the image, after the animation is complete it removes the `active` and `last-active` classes from the $next element.
}

//this runs the function above on an interval of 5sec when the `document.ready` event fires
$(function() {
    setInterval( "slideSwitch()", 5000 );
});

Rob W · Accepted Answer · 2011-11-15 20:03:03Z

The comments are added below. Note: $active.next() also selects non-image elements. This is probably not intended. If you want to select the next image element, use $active.nextAll("img:first") instead.

function slideSwitch() {
    // Selects all <img class="active"> elements which are a child of 
    // an element with id="slideshow"
    var $active = $('#slideshow IMG.active');

    // If the collection $active contains no elements, select the last
    // <img> element which is a child of <.. id=slideshow>
    if ( $active.length == 0 ) $active = $('#slideshow IMG:last');

    // If there's another element after <img>, select it. Otherwise, select
    // first <img>
    var $next =  $active.next().length ? $active.next()
        : $('#slideshow IMG:first');

    // Add `class=last-active` to the selected element.
    $active.addClass('last-active');

    $next.css({opacity: 0.0})   // Set opacity 0
        .addClass('active')     // Set class `active`
        .animate({opacity: 1.0}, 1000, function() { //Animate
            $active.removeClass('active last-active');
        });
}

// Create an interval when the document is ready
$(function() {
    setInterval( "slideSwitch()", 5000 );
});

BenM · Accepted Answer · 2011-11-15 20:08:57Z

1

//what is going on here?
var $next =  $active.next().length ? $active.next()
    : $('#slideshow IMG:first');

This is a ternary operator. Basically, if the next() function returns anything other than 0, the next image is assigned as whatever next() returns. Otherwise, it uses the first img element in the #slideshow element.

//can someone elaborate on these lines?
$active.addClass('last-active');

This adds the class of last-active to whichever element currently has a class of active.

edited Nov 15, 2011 at 20:08

answered Nov 15, 2011 at 20:02

BenM

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