Get an element by index in jQuery

$(...)[index]      // gives you the DOM element at index
$(...).get(index)  // gives you the DOM element at index
$(...).eq(index)   // gives you the jQuery object of element at index

DOM objects don’t have css function, use the last…

$('ul li').eq(index).css({'background-color':'#343434'});

docs:

.get(index) Returns: Element

• Description: Retrieve the DOM elements matched by the jQuery object.
• See: https://api.jquery.com/get/

.eq(index) Returns: jQuery

• Description: Reduce the set of matched elements to the one at the specified index.
• See: https://api.jquery.com/eq/