Tutorial Check if jQuery is Loaded

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if (typeof jQuery == 'undefined') {

    // jQuery IS NOT loaded, do stuff here.

}

Tutorial Check if Event was Triggered or Native

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$('button').click(function(event, wasTriggered) {
    if (wasTriggered) {
        alert('triggered in code');
    } else {
        alert('triggered by mouse');
    }
});

$('button').trigger('click', true);

Tutorial Check if Element is inside Another Specific Element

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Replace the first selector with the child you are testing and the second selector with the parent you are testing for.

if ( $(".child-element").parents("#main-nav").length == 1 ) { 

   // YES, the child element is inside the parent

} else {
 
   // NO, it is not inside

}

Tutorial Check if Element Exists

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if ($('#myElement').length > 0) { 
    // it exists 
}

Or to make it a fancy function with a callback:

// Tiny jQuery Plugin
// by Chris Goodchild
$.fn.exists = function(callback) {
  var args = [].slice.call(arguments, 1);

  if (this.length) {
    callback.call(this, args);
  }

  return this;
};

// Usage
$('div.test').exists(function() {
  this.append('<p>I exist!</p>');
});
​

Tutorial Check if Checkbox is Checked

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Say that 10 times fast =).

Find out if a single checkbox is checked or not, returns true or false:

$('#checkBox').attr('checked');

Find all checked checkboxes:

$('input[type=checkbox]:checked');

Reference URL

Tutorial Check for Empty Elements

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Do something for each empty element found:

$('*').each(function() {
         if ($(this).text() == "") {
                   //Do Something
         }
});

TRUE or FALSE if element is empty:

var emptyTest = $('#myDiv').is(':empty');

Tutorial Change WMode with jQuery

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If you don't set the wmode on a flash embed it will float over the top of an overlay which can be a pretty big deal. This is ideal in environment with lots of legacy video code or where users will be posting new code and teaching them about wmode is a lost cause. This should work in all browsers.

$("embed").attr("wmode", "opaque");
var embedTag;
$("embed").each(function(i) {
       embedTag = $(this).attr("outerHTML");
       if ((embedTag != null) && (embedTag.length > 0)) {
               embedTag = embedTag.replace(/embed /gi, "embed wmode="opaque" ");
               $(this).attr("outerHTML", embedTag);
       } else {
               $(this).wrap("<div></div>");
       }
});

Reference URL