onEventCase3

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>


<span id="a1">
	<span id="a2">a</span>
	<span id="a3">b</span>
</span>

JavaScript

var $element = '#a2';

$("#a1").on(
		"click",
		$element,		// <-- child Selector
		function() {
	        $("#a1")
	        .find('span')
	        .filter(function() {
	        	return $(this).text() === 'a';
	        })
	        .css('background-color', 'red');
    	}
	);