jQuery 1.9 and greater

.attr() versus .prop() (read more)

// Correct if changing the attribute is desired
$(elem).attr("checked", "checked");
// Correct for checking the checkbox
$(elem).prop("checked", true);
// Correct if removing the attribute is desired
// Correct for clearing the checkbox
$(elem).prop("checked", false);

As of jQuery 1.6, the .attr() method returns undefined for attributes that have not been set. To retrieve and change DOM properties such as the checked, selected, or disabled state of form elements, use the .prop() method.

$("input").prop("disabled", false);
$("input").prop("checked", true);

The .prop() method should be used to set disabled and checked instead of the .attr() method. The .val() method should be used for getting and setting value.

In Internet Explorer prior to version 9, using .prop() to set a DOM element property to anything other than a simple primitive value (number, string, or boolean) can cause memory leaks if the property is not removed (using .removeProp()) before the DOM element is removed from the document. To safely set values on DOM objects without memory leaks, use .data().

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