Lazyload thing with jQuery

lazyAny.jquery.js

var lazyAny = function(selector, callback, offset){

	offset = isNaN(offset) ? 250 : offset;

	var init = function(){
		debouncedCheck();
		$(window).on('load scroll resize', debouncedCheck);
	};

	var check = function(){
		var $elements = $(selector);


		var $window = $(window);
		$elements.each(function(i, element){
			var $element = $(element),
					top_of_element = $element.offset().top - offset,
					bottom_of_element = $element.offset().top + $element.outerHeight(),
					bottom_of_screen = $window.scrollTop() + window.innerHeight,
					top_of_screen = $window.scrollTop();

			if((bottom_of_screen >= top_of_element) && (top_of_screen <= bottom_of_element)){
				callback($element);
			}
		});
	};

	// https://davidwalsh.name/javascript-debounce-function
	var debounce = function(func, wait, immediate) {
		var timeout;
		return function() {
			var context = this, args = arguments;
			var later = function() {
				timeout = null;
				if (!immediate) func.apply(context, args);
			};
			var callNow = immediate && !timeout;
			clearTimeout(timeout);
			timeout = setTimeout(later, wait);
			if (callNow) func.apply(context, args);
		};
	};

	var debouncedCheck = debounce(check, 100);

	init();
};

usage.js

(function lazySrc(){
	lazyAny('[data-src]', function($element){
		$element.attr('src', $element.data('src')).removeAttr('data-src');
	});
})();