function_jQuery_fn_extended_indexOf.js

jQuery.fn.extend({
	indexOf: function(selector) {
		let thisElement = $(this);
		if ($(this).constructor == $().constructor) {
			if (typeof selector == 'undefined') {
				return $(this).index();
			};
			if (!$(this).is(selector)) {
				console.error('This element is not equal to the selector.');
				return stopError;
			};
			let index = -1;
			$(this).parent().children().each(function() {
				if ($(this).is(selector)) {
					index++;
				};
				if ($(this).is(thisElement)) {
					return false;
				};
			});
			return index;
		} else {
			console.error('The "indexOf" function only accepts jquery elements.');
			return stopError;
		};
	}
});