css opacity in javascript

I want register JavaScript events to toggle CSS opacity on selectable images.

For example, given a div with a list of images like the following,

<div id="foo_images">
   <img src="foo.jpg" />
   <img src="bar.jpg" />
   <img src="baz.jpg" />
   ...
</div>

I would like these images to be 50% transparent but 80% visible during a mouseover. I would also like these images to be selectable, i.e., an image should become fully visible after clicking it.

I would like to do this in JavaScript without having to modify the html image list.

We could add transparency in the stylesheet, as well as the mouseover effect, e.g.,

#foo_images img {
   filter: alpha(opacity=50);
   opacity: 0.5;
}
#foo_images img:hover {
   filter: alpha(opacity=80);
   opacity: 0.8;
}

In practice, it is highly recommended to keep all presentation elements in the stylesheet, but for the sake of example we can adjust an objects transparency using JavaScript, e.g.,

  //set obj transparency to 50%
  obj.style.opacity = 0.5; //non-IE browsers
  obj.style.filter = 'alpha(opacity=50)'; //IE

Using this approach, we can register events with JavaScript so that the transparency will change as the cursor hovers over, e.g.,

obj.onmouseover = function() {
  obj.style.opacity = 0.8;
  obj.style.filter = 'alpha(opacity=80)';
}
obj.onmouseout = function() {
  obj.style.opacity = 0.5;
  obj.style.filter = 'alpha(opacity=50)';
}

This would obviously be a lot of repeated code so we can use JavaScript closures, e.g.,

function opacityC(obj, value) {return function() {
  obj.style.opacity = value/100;
  obj.style.filter = 'alpha(opacity=' + value + ')';
}}
obj.onmouseover = opacityC(obj,80);
obj.onmouseout = opacityC(obj,50);

Next, to make the images selectable we can assign onclick events. We'll need to toggle between selected and not-selected, so we'll dynamically add a selected attribute to the image object. We can use the same closure approach to assign a function to the onclick event, e.g.,

function toggleSelectC(obj) {return function() {
  if (obj.selected != 'selected') {
    opacityC(obj, 100)();
    obj.onmouseout = opacityC(obj, 100);
    obj.style.border = '1px solid black';
    obj.selected = 'selected';
  } else {
    obj.onmouseout = opacityC(obj, 50);
    obj.style.border = '1px solid white';
    obj.selected = '';
  }
}}

image1.onclick = toggleSelectC(image1);
image2.onclick = toggleSelectC(image2);

Finally, we will loop through all of the images in the div, set the initial transparency and add event handlers, e.g.,

  var obj = document.getElementById('foo_images');
  var images = obj.getElementsByTagName("img");
  for (var i = 0; i < images.length; i++) {
    var img = images[i];
    opacityC(img,50)();
    img.onmouseover = opacityC(img, 80);
    img.onmouseout = opacityC(img, 50);
    img.onclick = toggleSelectC(img);
  }

Putting this all together, you would have the following HTML and JavaScript:

<div id="foo_images">
   <img src="foo.jpg" />
   <img src="bar.jpg" />
   <img src="baz.jpg" />
</div>
<script type="text/javascript" defer><!--

/**
 * closure to adjust an objects transparency
 */
function opacityC(obj, value) {return function() {
  obj.style.opacity = value/100;
  obj.style.filter = 'alpha(opacity=' + value + ')';
}}

/**
 * closure to toggle selected/non-selected style
 */
function toggleSelectC(obj) {return function() {
  if (obj.selected != 'selected') {
    opacityC(obj, 100)();
    obj.onmouseout = opacityC(obj, 100);
    obj.style.border = '1px solid black';
    obj.selected = 'selected';
  } else {
    opacityC(obj, 50)();
    obj.onmouseout = opacityC(obj, 50);
    obj.style.border = '1px solid white';
    obj.selected = '';
  }
}}

/**
 * register all selectable images
 */
function registerSelectableImages(obj) {
  var images = obj.getElementsByTagName("img");
  for (var i = 0; i < images.length; i++) {
    var img = images[i];
    opacityC(img,50)();
    img.style.border = '1px solid white';
    img.onmouseover = opacityC(img, 80);
    img.onmouseout = opacityC(img, 50);
    img.onclick = toggleSelectC(img);
  }
}
registerSelectableImages(document.getElementById('foo_images'));
--></script>

Applying the above code to the three sample images will produce the following results:


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