example.js

// In this example we detect if a user clicks a link to travel elsewhere
// but has made changes to a form. If they have changed the form it prompts
// them to confirm before continuing.
jQuery(function ($) {
    var $form = $('form').watchChanges();

    $('a').click(function (e) {
        if ($form.hasChanged() and ! confirm('Continue without saving changes?')) {
            e.preventDefault();
        }
    });
});

watch-has-changed.js

// Written by Luke Morton, licensed under MIT
(function ($) {

	$.fn.watchChanges = function () {
		return this.each(function () {
			$.data(this, 'formHash', $(this).serialize());
		});
	};

	$.fn.hasChanged = function () {
		var hasChanged = false;

		this.each(function () {
			var formHash = $.data(this, 'formHash');

			if (formHash != null && formHash !== $(this).serialize()) {
				hasChanged = true;
				return false;
			}
		});

		return hasChanged;
	};

}).call(this, jQuery);