debounce.js

/*
http://stackoverflow.com/questions/28948383/how-to-implement-debounce-fn-into-jquery-keyup-event
*/

$('input').keyup(debounce(function(){
        var $this=$(this);
        //alert( $this.val() );
        var n1 = $this.val();
        var n2 = $('#n2').val();
        var n3 = $('#n3').val();
        var calc = n1 * n2 * n3;
        alert(calc);
    },500));

//http://davidwalsh.name/javascript-debounce-function
function debounce(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);
	};
};